Adding upstream version 86.0.1.upstream/86.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

29 files changed, 2727 insertions, 0 deletions

diff --git a/js/src/devtools/gc-ubench/argparse.js b/js/src/devtools/gc-ubench/argparse.js
new file mode 100644
index 0000000000..454b53fb71
--- /dev/null
+++ b/js/src/devtools/gc-ubench/argparse.js
@@ -0,0 +1,127 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Command-line argument parser, modeled after but not identical to Python's
+// argparse.
+
+var ArgParser = class {
+  constructor(desc) {
+    this._params = [];
+    this._doc = desc;
+
+    this.add_argument("--help", {
+      help: "display this help message",
+    });
+  }
+
+  // name is '--foo', '-f', or an array of aliases.
+  //
+  // spec is an options object with keys:
+  //   dest: key name to store the result in (optional for long options)
+  //   default: value to use if not passed on command line (optional)
+  //   help: description of the option to show in --help
+  //   options: array of valid choices
+  //
+  // Prefixes of long option names are allowed. If a prefix is ambiguous, it
+  // will match the first parameter added to the ArgParser.
+  add_argument(name, spec) {
+    const names = Array.isArray(name) ? name : [name];
+
+    spec = Object.assign({}, spec);
+    spec.aliases = names;
+    for (const name of names) {
+      if (!name.startsWith("-")) {
+        throw new Error(`unhandled argument syntax '${name}'`);
+      }
+      if (name.startsWith("--")) {
+        spec.dest = spec.dest || name.substr(2);
+      }
+      this._params.push({ name, spec });
+    }
+  }
+
+  parse_args(args) {
+    const opts = {};
+    const rest = [];
+
+    for (const { spec } of this._params) {
+      if (spec.default !== undefined) {
+        opts[spec.dest] = spec.default;
+      }
+    }
+
+    const seen = new Set();
+    for (let i = 0; i < args.length; i++) {
+      const arg = args[i];
+      if (!arg.startsWith("-")) {
+        rest.push(arg);
+        continue;
+      } else if (arg === "--") {
+        rest.push(args.slice(i+1));
+        break;
+      }
+
+      if (arg == "--help" || arg == "-h") {
+        this.help();
+      }
+
+      let parameter;
+      let [passed, value] = arg.split("=");
+      for (const { name, spec } of this._params) {
+        if (passed.startsWith("--")) {
+          if (name.startsWith(passed)) {
+            parameter = spec;
+          }
+        } else if (passed.startsWith("-") && passed === name) {
+          parameter = spec;
+        }
+        if (parameter) {
+          if (value === undefined) {
+            value = args[++i];
+          }
+          opts[parameter.dest] = value;
+          break;
+        }
+      }
+
+      if (parameter) {
+        if (seen.has(parameter)) {
+          throw new Error(`${parameter.aliases[0]} given multiple times`);
+        }
+        seen.add(parameter);
+      } else {
+        throw new Error(`invalid command-line argument '${arg}'`);
+      }
+    }
+
+    for (const { name, spec } of this._params) {
+      if (spec.options && !spec.options.includes(opts[spec.dest])) {
+        throw new Error(`invalid ${name} value '${opts[spec.dest]}'`);
+        opts[spec.dest] = spec.default;
+      }
+    }
+
+    return { opts, rest };
+  }
+
+  help() {
+    print(`Usage: ${this._doc}`);
+    const specs = new Set(this._params.map(p => p.spec));
+    const optstrs = [...specs].map(p => p.aliases.join(", "));
+    let maxlen = Math.max(...optstrs.map(s => s.length));
+    for (const spec of specs) {
+      const name = spec.aliases[0];
+      let helptext = spec.help ?? "undocumented";
+      if ("options" in spec) {
+        helptext += ` (one of ${spec.options.map(x => `'${x}'`).join(", ")})`;
+      }
+      if ("default" in spec) {
+        helptext += ` (default '${spec.default}')`;
+      }
+      const optstr = spec.aliases.join(", ");
+      print(`  ${optstr.padEnd(maxlen)}  ${helptext}`);
+    }
+    quit(0);
+  }
+};
diff --git a/js/src/devtools/gc-ubench/benchmarks/bigTextNodes.js b/js/src/devtools/gc-ubench/benchmarks/bigTextNodes.js
new file mode 100644
index 0000000000..0a66be03d3
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/bigTextNodes.js
@@ -0,0 +1,42 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "bigTextNodes",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var foo = [ textNode, textNode, ... ]",
+
+      enabled: "document" in globalThis,
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+
+      defaultGarbagePerFrame: "8",
+      defaultGarbagePiles: "8",
+
+      makeGarbage: N => {
+        var a = [];
+        var s = "x";
+        for (var i = 0; i < 16; i++) {
+          s = s + s;
+        }
+        for (var i = 0; i < N; i++) {
+          a.push(document.createTextNode(s));
+        }
+        garbage[garbageIndex++] = a;
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/deepWeakMap.js b/js/src/devtools/gc-ubench/benchmarks/deepWeakMap.js
new file mode 100644
index 0000000000..bf46d1d97b
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/deepWeakMap.js
@@ -0,0 +1,40 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "deepWeakMap",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "o={wm,k}; w.mk[k]=o2={wm2,k2}; wm2[k2]=....",
+
+      defaultGarbagePerFrame: "1K",
+      defaultGarbagePiles: "1K",
+
+      load: N => {
+        garbage = new Array(N);
+      },
+
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+
+      makeGarbage: M => {
+        var initial = {};
+        var prev = initial;
+        for (var i = 0; i < M; i++) {
+          var obj = [new WeakMap(), Object.create(null)];
+          obj[0].set(obj[1], prev);
+          prev = obj;
+        }
+        garbage[garbageIndex++] = initial;
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/events.js b/js/src/devtools/gc-ubench/benchmarks/events.js
new file mode 100644
index 0000000000..0fe91b7596
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/events.js
@@ -0,0 +1,40 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "events",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var foo = [ textNode, textNode, ... ]",
+
+      enabled: "document" in globalThis,
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+
+      defaultGarbagePerFrame: "100K",
+      defaultGarbagePiles: "8",
+
+      makeGarbage: N => {
+        var a = [];
+        for (var i = 0; i < N; i++) {
+          var e = document.createEvent("Events");
+          e.initEvent("TestEvent", true, true);
+          a.push(e);
+        }
+        garbage[garbageIndex++] = a;
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/expandoEvents.js b/js/src/devtools/gc-ubench/benchmarks/expandoEvents.js
new file mode 100644
index 0000000000..4c6a53c8fe
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/expandoEvents.js
@@ -0,0 +1,41 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "expandoEvents",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var foo = [ textNode, textNode, ... ]",
+
+      enabled: "document" in globalThis,
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+
+      defaultGarbagePerFrame: "100K",
+      defaultGarbagePiles: "8",
+
+      makeGarbage: N => {
+        var a = [];
+        for (var i = 0; i < N; i++) {
+          var e = document.createEvent("Events");
+          e.initEvent("TestEvent", true, true);
+          e.color = ["tuna"];
+          a.push(e);
+        }
+        garbage[garbageIndex++] = a;
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/globalArrayArrayLiteral.js b/js/src/devtools/gc-ubench/benchmarks/globalArrayArrayLiteral.js
new file mode 100644
index 0000000000..a9003d8be6
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/globalArrayArrayLiteral.js
@@ -0,0 +1,32 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "globalArrayArrayLiteral",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var foo = [[], ....]",
+      defaultGarbagePerFrame: "1M",
+      defaultGarbagePiles: "1K",
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+      makeGarbage: N => {
+        for (var i = 0; i < N; i++) {
+          garbage[garbageIndex++] = ["foo", "bar", "baz", "baa"];
+          if (garbageIndex == garbage.length) {
+            garbageIndex = 0;
+          }
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/globalArrayBuffer.js b/js/src/devtools/gc-ubench/benchmarks/globalArrayBuffer.js
new file mode 100644
index 0000000000..06bf73eea8
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/globalArrayBuffer.js
@@ -0,0 +1,36 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "globalArrayBuffer",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var foo = ArrayBuffer(N); # (large malloc data)",
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+
+      defaultGarbagePerFrame: "4M",
+      defaultGarbagePiles: "8K",
+
+      makeGarbage: N => {
+        var ab = new ArrayBuffer(N);
+        var view = new Uint8Array(ab);
+        view[0] = 1;
+        view[N - 1] = 2;
+        garbage[garbageIndex++] = ab;
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/globalArrayFgFinalized.js b/js/src/devtools/gc-ubench/benchmarks/globalArrayFgFinalized.js
new file mode 100644
index 0000000000..d61d56b81c
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/globalArrayFgFinalized.js
@@ -0,0 +1,37 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "globalArrayFgFinalized",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description:
+        "var foo = [ new Map, new Map, ... ]; # (foreground finalized)",
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+
+      defaultGarbagePiles: "8K",
+      defaultGarbagePerFrame: "48K",
+
+      makeGarbage: N => {
+        var arr = [];
+        for (var i = 0; i < N; i++) {
+          arr.push(new Map());
+        }
+        garbage[garbageIndex++] = arr;
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/globalArrayLargeArray.js b/js/src/devtools/gc-ubench/benchmarks/globalArrayLargeArray.js
new file mode 100644
index 0000000000..38e4f2a85b
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/globalArrayLargeArray.js
@@ -0,0 +1,34 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "globalArrayLargeArray",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var foo = [[...], ....]",
+      defaultGarbagePerFrame: "3M",
+      defaultGarbagePiles: "1K",
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+      makeGarbage: N => {
+        var a = new Array(N);
+        for (var i = 0; i < N; i++) {
+          a[i] = N - i;
+        }
+        garbage[garbageIndex++] = a;
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/globalArrayLargeObject.js b/js/src/devtools/gc-ubench/benchmarks/globalArrayLargeObject.js
new file mode 100644
index 0000000000..ed1c38b271
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/globalArrayLargeObject.js
@@ -0,0 +1,36 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "globalArrayLargeObject",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var foo = { LARGE }; # (large slots)",
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+
+      defaultGarbagePiles: "8K",
+      defaultGarbagePerFrame: "64K",
+
+      makeGarbage: N => {
+        var obj = {};
+        for (var i = 0; i < N; i++) {
+          obj["key" + i] = i;
+        }
+        garbage[garbageIndex++] = obj;
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/globalArrayNewObject.js b/js/src/devtools/gc-ubench/benchmarks/globalArrayNewObject.js
new file mode 100644
index 0000000000..04487aac20
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/globalArrayNewObject.js
@@ -0,0 +1,33 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "globalArrayNewObject",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var foo = [new Object(), ....]",
+      defaultGarbagePerFrame: "128K",
+      defaultGarbagePiles: "1K",
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+
+      makeGarbage: N => {
+        for (var i = 0; i < N; i++) {
+          garbage[garbageIndex++] = new Object();
+          if (garbageIndex == garbage.length) {
+            garbageIndex = 0;
+          }
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/globalArrayObjectLiteral.js b/js/src/devtools/gc-ubench/benchmarks/globalArrayObjectLiteral.js
new file mode 100644
index 0000000000..a31e76bdc6
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/globalArrayObjectLiteral.js
@@ -0,0 +1,32 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "globalArrayObjectLiteral",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var foo = [{}, ....]",
+      defaultGarbagePerFrame: "384K",
+      defaultGarbagePiles: "1K",
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+      makeGarbage: N => {
+        for (var i = 0; i < N; i++) {
+          garbage[garbageIndex++] = { a: "foo", b: "bar", 0: "foo", 1: "bar" };
+          if (garbageIndex == garbage.length) {
+            garbageIndex = 0;
+          }
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/globalArrayReallocArray.js b/js/src/devtools/gc-ubench/benchmarks/globalArrayReallocArray.js
new file mode 100644
index 0000000000..54316736e1
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/globalArrayReallocArray.js
@@ -0,0 +1,34 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "globalArrayReallocArray",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var foo = [[,,,], ....]",
+      defaultGarbagePerFrame: "2M",
+      defaultGarbagePiles: "1K",
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+      makeGarbage: N => {
+        var a = [];
+        for (var i = 0; i < N; i++) {
+          a[i] = N - i;
+        }
+        garbage[garbageIndex++] = a;
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/largeArrayPropertyAndElements.js b/js/src/devtools/gc-ubench/benchmarks/largeArrayPropertyAndElements.js
new file mode 100644
index 0000000000..0202f56e40
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/largeArrayPropertyAndElements.js
@@ -0,0 +1,40 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "largeArrayPropertyAndElements",
+  (function() {
+    var garbage;
+    var index;
+
+    return {
+      description: "Large array with both properties and elements",
+
+      load: n => {
+        garbage = new Array(n);
+        garbage.fill(null);
+        index = 0;
+      },
+
+      unload: () => {
+        garbage = null;
+        index = 0;
+      },
+
+      defaultGarbagePiles: "100K",
+      defaultGarbagePerFrame: "48K",
+
+      makeGarbage: n => {
+        for (var i = 0; i < n; i++) {
+          index++;
+          index %= garbage.length;
+
+          var obj = {};
+          garbage[index] = obj;
+          garbage["key-" + index] = obj;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/noAllocation.js b/js/src/devtools/gc-ubench/benchmarks/noAllocation.js
new file mode 100644
index 0000000000..8e6ba53943
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/noAllocation.js
@@ -0,0 +1,10 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set("noAllocation", {
+  description: "Do not generate any garbage.",
+  load: N => {},
+  unload: () => {},
+  makeGarbage: N => {},
+});
diff --git a/js/src/devtools/gc-ubench/benchmarks/pairCyclicWeakMap.js b/js/src/devtools/gc-ubench/benchmarks/pairCyclicWeakMap.js
new file mode 100644
index 0000000000..ac43325b6f
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/pairCyclicWeakMap.js
@@ -0,0 +1,46 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "pairCyclicWeakMap",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "wm1[k1] = k2; wm2[k2] = k3; wm1[k3] = k4; wm2[k4] = ...",
+
+      defaultGarbagePerFrame: "10K",
+      defaultGarbagePiles: "1K",
+
+      load: N => {
+        garbage = new Array(N);
+      },
+
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+
+      makeGarbage: M => {
+        var wm1 = new WeakMap();
+        var wm2 = new WeakMap();
+        var initialKey = {};
+        var key = initialKey;
+        var value = {};
+        for (var i = 0; i < M / 2; i++) {
+          wm1.set(key, value);
+          key = value;
+          value = {};
+          wm2.set(key, value);
+          key = value;
+          value = {};
+        }
+        garbage[garbageIndex++] = [initialKey, wm1, wm2];
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/propertyTreeSplitting.js b/js/src/devtools/gc-ubench/benchmarks/propertyTreeSplitting.js
new file mode 100644
index 0000000000..9fc62b29e2
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/propertyTreeSplitting.js
@@ -0,0 +1,24 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "propertyTreeSplitting",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    var obj = {};
+    return {
+      description: "use delete to generate Shape garbage (piles are unused)",
+      load: N => {},
+      unload: () => {},
+      makeGarbage: N => {
+        for (var a = 0; a < N; ++a) {
+          obj.x = 1;
+          obj.y = 2;
+          delete obj.x;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/selfCyclicWeakMap.js b/js/src/devtools/gc-ubench/benchmarks/selfCyclicWeakMap.js
new file mode 100644
index 0000000000..c7736c72b9
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/selfCyclicWeakMap.js
@@ -0,0 +1,42 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "selfCyclicWeakMap",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var wm = new WeakMap(); wm[k1] = k2; wm[k2] = k3; ...",
+
+      defaultGarbagePerFrame: "10K",
+      defaultGarbagePiles: "1K",
+
+      load: N => {
+        garbage = new Array(N);
+      },
+
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+
+      makeGarbage: M => {
+        var wm = new WeakMap();
+        var initialKey = {};
+        var key = initialKey;
+        var value = {};
+        for (var i = 0; i < M; i++) {
+          wm.set(key, value);
+          key = value;
+          value = {};
+        }
+        garbage[garbageIndex++] = [initialKey, wm];
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/benchmarks/textNodes.js b/js/src/devtools/gc-ubench/benchmarks/textNodes.js
new file mode 100644
index 0000000000..07fd07e7b7
--- /dev/null
+++ b/js/src/devtools/gc-ubench/benchmarks/textNodes.js
@@ -0,0 +1,38 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+tests.set(
+  "textNodes",
+  (function() {
+    var garbage = [];
+    var garbageIndex = 0;
+    return {
+      description: "var foo = [ textNode, textNode, ... ]",
+
+      enabled: "document" in globalThis,
+
+      load: N => {
+        garbage = new Array(N);
+      },
+      unload: () => {
+        garbage = [];
+        garbageIndex = 0;
+      },
+
+      defaultGarbagePerFrame: "100K",
+      defaultGarbagePiles: "8",
+
+      makeGarbage: N => {
+        var a = [];
+        for (var i = 0; i < N; i++) {
+          a.push(document.createTextNode("t" + i));
+        }
+        garbage[garbageIndex++] = a;
+        if (garbageIndex == garbage.length) {
+          garbageIndex = 0;
+        }
+      },
+    };
+  })()
+);
diff --git a/js/src/devtools/gc-ubench/harness.js b/js/src/devtools/gc-ubench/harness.js
new file mode 100644
index 0000000000..db7fa06d63
--- /dev/null
+++ b/js/src/devtools/gc-ubench/harness.js
@@ -0,0 +1,328 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Global defaults
+
+// Allocate this much "garbage" per frame. This might correspond exactly to a
+// number of objects/values, or it might be some set of objects, depending on
+// the mutator in question.
+var gDefaultGarbagePerFrame = "8K";
+
+// In order to avoid a performance cliff between when the per-frame garbage
+// fits in the nursery and when it doesn't, most mutators will collect multiple
+// "piles" of garbage and round-robin through them, so that the per-frame
+// garbage stays alive for some number of frames. There will still be some
+// internal temporary allocations that don't end up in the piles; presumably,
+// the nursery will take care of those.
+//
+// If the per-frame garbage is K and the number of piles is P, then some of the
+// garbage will start getting tenured as long as P*K > size(nursery).
+var gDefaultGarbagePiles = "8";
+
+var gDefaultTestDuration = 8.0;
+
+// The Host interface that provides functionality needed by the test harnesses
+// (web + various shells). Subclasses should override with the appropriate
+// functionality. The methods that throw an error must be implemented. The ones
+// that return undefined are optional.
+//
+// Note that currently the web UI doesn't really use the scheduling pieces of
+// this.
+var Host = class {
+  constructor() {}
+  start_turn() {
+    throw new Error("unimplemented");
+  }
+  end_turn() {
+    throw new Error("unimplemented");
+  }
+  suspend(duration) {
+    throw new Error("unimplemented");
+  } // Shell driver only
+  now() {
+    return performance.now();
+  }
+
+  minorGCCount() {
+    return undefined;
+  }
+  majorGCCount() {
+    return undefined;
+  }
+  GCSliceCount() {
+    return undefined;
+  }
+
+  features = {
+    haveMemorySizes: false,
+    haveGCCounts: false,
+  };
+};
+
+function percent(x) {
+  return `${(x*100).toFixed(2)}%`;
+}
+
+function parse_units(v) {
+  if (!v.length) {
+    return NaN;
+  }
+  var lastChar = v[v.length - 1].toLowerCase();
+  if (!isNaN(parseFloat(lastChar))) {
+    return parseFloat(v);
+  }
+  var units = parseFloat(v.substr(0, v.length - 1));
+  if (lastChar == "k") {
+    return units * 1e3;
+  }
+  if (lastChar == "m") {
+    return units * 1e6;
+  }
+  if (lastChar == "g") {
+    return units * 1e9;
+  }
+  return NaN;
+}
+
+var AllocationLoad = class {
+  constructor(info, name) {
+    this.load = info;
+    this.load.name = this.load.name ?? name;
+
+    this._garbagePerFrame =
+      info.garbagePerFrame ||
+      parse_units(info.defaultGarbagePerFrame || gDefaultGarbagePerFrame);
+    this._garbagePiles =
+      info.garbagePiles ||
+      parse_units(info.defaultGarbagePiles || gDefaultGarbagePiles);
+  }
+
+  get name() {
+    return this.load.name;
+  }
+  get description() {
+    return this.load.description;
+  }
+  get garbagePerFrame() {
+    return this._garbagePerFrame;
+  }
+  set garbagePerFrame(amount) {
+    this._garbagePerFrame = amount;
+  }
+  get garbagePiles() {
+    return this._garbagePiles;
+  }
+  set garbagePiles(amount) {
+    this._garbagePiles = amount;
+  }
+
+  start() {
+    this.load.load(this._garbagePiles);
+  }
+
+  stop() {
+    this.load.unload();
+  }
+
+  reload() {
+    this.stop();
+    this.start();
+  }
+
+  tick() {
+    this.load.makeGarbage(this._garbagePerFrame);
+  }
+
+  is_dummy_load() {
+    return this.load.name == "noAllocation";
+  }
+};
+
+var AllocationLoadManager = class {
+  constructor(tests) {
+    this._loads = new Map();
+    for (const [name, info] of tests.entries()) {
+      this._loads.set(name, new AllocationLoad(info, name));
+    }
+    this._active = undefined;
+    this._paused = false;
+
+    // Public API
+    this.sequencer = null;
+    this.testDurationMS = gDefaultTestDuration * 1000;
+  }
+
+  getByName(name) {
+    const mutator = this._loads.get(name);
+    if (!mutator) {
+      throw new Error(`invalid mutator '${name}'`);
+    }
+    return mutator;
+  }
+
+  activeLoad() {
+    return this._active;
+  }
+
+  setActiveLoad(mutator) {
+    if (this._active) {
+      this._active.stop();
+    }
+    this._active = mutator;
+    this._active.start();
+  }
+
+  deactivateLoad() {
+    this._active.stop();
+    this._active = undefined;
+  }
+
+  get paused() {
+    return this._paused;
+  }
+  set paused(pause) {
+    this._paused = pause;
+  }
+
+  load_running() {
+    return this._active;
+  }
+
+  change_garbagePiles(amount) {
+    if (this._active) {
+      this._active.garbagePiles = amount;
+      this._active.reload();
+    }
+  }
+
+  change_garbagePerFrame(amount) {
+    if (this._active) {
+      this._active.garbagePerFrame = amount;
+    }
+  }
+
+  tick(now = gHost.now()) {
+    this.lastActive = this._active;
+    let completed = false;
+
+    if (this.sequencer) {
+      if (this.sequencer.tick(now)) {
+        completed = true;
+        if (this.sequencer.current) {
+          this.setActiveLoad(this.sequencer.current);
+        } else {
+          this.deactivateLoad();
+        }
+        if (this.sequencer.done()) {
+          this.sequencer = null;
+        }
+      }
+    }
+
+    if (this._active && !this._paused) {
+      this._active.tick();
+    }
+
+    return completed;
+  }
+
+  startSequencer(sequencer, now = gHost.now()) {
+    this.sequencer = sequencer;
+    this.sequencer.start(now);
+    this.setActiveLoad(this.sequencer.current);
+  }
+
+  stopped() {
+    return !this.sequencer || this.sequencer.done();
+  }
+
+  currentLoadRemaining(now = gHost.now()) {
+    if (this.stopped()) {
+      return 0;
+    }
+
+    // TODO: The web UI displays a countdown to the end of the current mutator.
+    // This won't work for potential future things like "run until 3 major GCs
+    // have been seen", so the API will need to be modified to provide
+    // information in that case.
+    return this.testDurationMS - this.sequencer.currentLoadElapsed(now);
+  }
+};
+
+// Current test state.
+var gLoadMgr = undefined;
+
+function format_with_units(n, label, shortlabel, kbase) {
+  if (n < kbase * 4) {
+    return `${n} ${label}`;
+  } else if (n < kbase ** 2 * 4) {
+    return `${(n / kbase).toFixed(2)}K${shortlabel}`;
+  } else if (n < kbase ** 3 * 4) {
+    return `${(n / kbase ** 2).toFixed(2)}M${shortlabel}`;
+  }
+  return `${(n / kbase ** 3).toFixed(2)}G${shortlabel}`;
+}
+
+function format_bytes(bytes) {
+  return format_with_units(bytes, "bytes", "B", 1024);
+}
+
+function format_num(n) {
+  return format_with_units(n, "", "", 1000);
+}
+
+function update_histogram(histogram, delay) {
+  // Round to a whole number of 10us intervals to provide enough resolution to
+  // capture a 16ms target with adequate accuracy.
+  delay = Math.round(delay * 100) / 100;
+  var current = histogram.has(delay) ? histogram.get(delay) : 0;
+  histogram.set(delay, ++current);
+}
+
+// Compute a score based on the total ms we missed frames by per second.
+function compute_test_score(histogram) {
+  var score = 0;
+  for (let [delay, count] of histogram) {
+    score += Math.abs((delay - 1000 / 60) * count);
+  }
+  score = score / (gLoadMgr.testDurationMS / 1000);
+  return Math.round(score * 1000) / 1000;
+}
+
+// Build a spark-lines histogram for the test results to show with the aggregate score.
+function compute_spark_histogram_percents(histogram) {
+  var ranges = [
+    [-99999999, 16.6],
+    [16.6, 16.8],
+    [16.8, 25],
+    [25, 33.4],
+    [33.4, 60],
+    [60, 100],
+    [100, 300],
+    [300, 99999999],
+  ];
+  var rescaled = new Map();
+  for (let [delay] of histogram) {
+    for (var i = 0; i < ranges.length; ++i) {
+      var low = ranges[i][0];
+      var high = ranges[i][1];
+      if (low <= delay && delay < high) {
+        update_histogram(rescaled, i);
+        break;
+      }
+    }
+  }
+  var total = 0;
+  for (const [, count] of rescaled) {
+    total += count;
+  }
+
+  var spark = [];
+  for (let i = 0; i < ranges.length; ++i) {
+    const amt = rescaled.has(i) ? rescaled.get(i) : 0;
+    spark.push(amt / total);
+  }
+
+  return spark;
+}
diff --git a/js/src/devtools/gc-ubench/index.html b/js/src/devtools/gc-ubench/index.html
new file mode 100644
index 0000000000..4abce385f2
--- /dev/null
+++ b/js/src/devtools/gc-ubench/index.html
@@ -0,0 +1,90 @@
+<!-- This Source Code Form is subject to the terms of the Mozilla Public
+   - License, v. 2.0. If a copy of the MPL was not distributed with this file,
+   - You can obtain one at http://mozilla.org/MPL/2.0/. -->
+
+<html>
+<head>
+  <title>GC uBench</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+
+  <!-- Benchmark harness and UI -->
+  <script src="harness.js"></script>
+  <script src="perf.js"></script>
+  <script src="sequencer.js"></script>
+  <script src="ui.js"></script>
+
+  <!-- List of garbage-creating test loads -->
+  <script src="test_list.js"></script>
+
+  <!-- Collect all test loads into a `tests` Map -->
+  <script>
+    var tests = new Map();
+    foreach_test_file(path => import("./" + path));
+  </script>
+
+</head>
+
+<body onload="onload()" onunload="onunload()">
+
+<canvas id="graph" width="1080" height="400" style="padding-left:10px"></canvas>
+<canvas id="memgraph" width="1080" height="400" style="padding-left:10px"></canvas>
+<div id="memgraph-disabled" style="display: none"><i>No performance.mozMemory object available. If running Firefox, set dom.enable_memory_stats to True to see heap size info.</i></div>
+
+<hr>
+
+<div id='track-sizes-div'>
+  Show heap size graph: <input id='track-sizes' type='checkbox' onclick="trackHeapSizes(this.checked)">
+</div>
+
+<div>
+  Update display:
+  <input type="checkbox" id="do-graph" onchange="onUpdateDisplayChanged()" checked></input>
+</div>
+
+<div>
+  Run allocation load
+  <input type="checkbox" id="do-load" onchange="onDoLoadChange()" checked></input>
+</div>
+
+<div>
+    Allocation load:
+    <select id="test-selection" required onchange="onLoadChange()"></select>
+    <span id="load-running">(init)</span>
+</div>
+
+<div>
+    &nbsp;&nbsp;&nbsp;&nbsp;Garbage items per frame:
+    <input type="text" id="garbage-per-frame" size="5" value="8K"
+           onchange="garbage_per_frame_changed()"></input>
+</div>
+<div>
+    &nbsp;&nbsp;&nbsp;&nbsp;Garbage piles:
+    <input type="text" id="garbage-piles" size="5" value="8"
+           onchange="garbage_piles_changed()"></input>
+</div>
+
+<hr>
+
+<div>
+    Duration: <input type="text" id="test-duration" size="3" value="8" onchange="duration_changed()"></input>s
+    <input type="button" id="test-one" value="Run Test" onclick="run_one_test()"></input>
+    <input type="button" id="test-all" value="Run All Tests" onclick="run_all_tests()"></input>
+</div>
+
+<div>
+    &nbsp;&nbsp;&nbsp;&nbsp;Time remaining: <span id="test-progress">(not running)</span>
+</div
+
+<div>
+    &nbsp;&nbsp;&nbsp;&nbsp;60 fps: <span id="pct60">n/a</span>
+    &nbsp;&nbsp;&nbsp;&nbsp;45 fps: <span id="pct45">n/a</span>
+    &nbsp;&nbsp;&nbsp;&nbsp;30 fps: <span id="pct30">n/a</span>
+</div
+
+<div id="results-Area">
+    Test Results:
+    <div id="results-display" style="padding-left: 10px; border: 1px solid black;"></div>
+</div>
+
+</body>
+</html>
diff --git a/js/src/devtools/gc-ubench/perf.js b/js/src/devtools/gc-ubench/perf.js
new file mode 100644
index 0000000000..dc370ee0da
--- /dev/null
+++ b/js/src/devtools/gc-ubench/perf.js
@@ -0,0 +1,217 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Performance monitoring and calculation.
+
+function round_up(val, interval) {
+  return val + (interval - (val % interval));
+}
+
+// Class for inter-frame timing, which handles being paused and resumed.
+var FrameTimer = class {
+  constructor() {
+    // Start time of the current active test, adjusted for any time spent
+    // stopped (so `now - this.start` is how long the current active test
+    // has run for.)
+    this.start = undefined;
+
+    // Timestamp of callback following the previous frame.
+    this.prev = undefined;
+
+    // Timestamp when drawing was paused, or zero if drawing is active.
+    this.stopped = 0;
+  }
+
+  is_stopped() {
+    return this.stopped != 0;
+  }
+
+  start_recording(now = gHost.now()) {
+    this.start = this.prev = now;
+  }
+
+  on_frame_finished(now = gHost.now()) {
+    const delay = now - this.prev;
+    this.prev = now;
+    return delay;
+  }
+
+  pause(now = gHost.now()) {
+    this.stopped = now;
+    // Abuse this.prev to store the time elapsed since the previous frame.
+    // This will be used to adjust this.prev when we resume.
+    this.prev = now - this.prev;
+  }
+
+  resume(now = gHost.now()) {
+    this.prev = now - this.prev;
+    const stop_duration = now - this.stopped;
+    this.start += stop_duration;
+    this.stopped = 0;
+  }
+};
+
+// Per-frame time sampling infra.
+var sampleTime = 16.666667; // ms
+var sampleIndex = 0;
+
+// Class for maintaining a rolling window of per-frame GC-related counters:
+// inter-frame delay, minor/major/slice GC counts, cumulative bytes, etc.
+var FrameHistory = class {
+  constructor(numSamples) {
+    // Private
+    this._frameTimer = new FrameTimer();
+    this._numSamples = numSamples;
+
+    // Public API
+    this.delays = new Array(numSamples);
+    this.gcBytes = new Array(numSamples);
+    this.mallocBytes = new Array(numSamples);
+    this.gcs = new Array(numSamples);
+    this.minorGCs = new Array(numSamples);
+    this.majorGCs = new Array(numSamples);
+    this.slices = new Array(numSamples);
+
+    sampleIndex = 0;
+    this.reset();
+  }
+
+  start(now = gHost.now()) {
+    this._frameTimer.start_recording(now);
+  }
+
+  reset() {
+    this.delays.fill(0);
+    this.gcBytes.fill(0);
+    this.mallocBytes.fill(0);
+    this.gcs.fill(this.gcs[sampleIndex]);
+    this.minorGCs.fill(this.minorGCs[sampleIndex]);
+    this.majorGCs.fill(this.majorGCs[sampleIndex]);
+    this.slices.fill(this.slices[sampleIndex]);
+
+    sampleIndex = 0;
+  }
+
+  get numSamples() {
+    return this._numSamples;
+  }
+
+  findMax(collection) {
+    // Depends on having at least one non-negative entry, and unfilled
+    // entries being <= max.
+    var maxIndex = 0;
+    for (let i = 0; i < this._numSamples; i++) {
+      if (collection[i] >= collection[maxIndex]) {
+        maxIndex = i;
+      }
+    }
+    return maxIndex;
+  }
+
+  findMaxDelay() {
+    return this.findMax(this.delays);
+  }
+
+  on_frame(now = gHost.now()) {
+    const delay = this._frameTimer.on_frame_finished(now);
+
+    // Total time elapsed while the active test has been running.
+    var t = now - this._frameTimer.start;
+    var newIndex = Math.round(t / sampleTime);
+    while (sampleIndex < newIndex) {
+      sampleIndex++;
+      var idx = sampleIndex % this._numSamples;
+      this.delays[idx] = delay;
+      if (gHost.features.haveMemorySizes) {
+        this.gcBytes[idx] = gHost.gcBytes;
+        this.mallocBytes[idx] = gHost.mallocBytes;
+      }
+      if (gHost.features.haveGCCounts) {
+        this.minorGCs[idx] = gHost.minorGCCount;
+        this.majorGCs[idx] = gHost.majorGCCount;
+        this.slices[idx] = gHost.GCSliceCount;
+      }
+    }
+
+    return delay;
+  }
+
+  pause() {
+    this._frameTimer.pause();
+  }
+
+  resume() {
+    this._frameTimer.resume();
+  }
+
+  is_stopped() {
+    return this._frameTimer.is_stopped();
+  }
+};
+
+var PerfTracker = class {
+  constructor() {
+    // Private
+    this._currentLoadStart = undefined;
+    this._frameCount = undefined;
+    this._mutating_ms = undefined;
+    this._suspend_sec = undefined;
+    this._minorGCs = undefined;
+    this._majorGCs = undefined;
+
+    // Public
+    this.results = [];
+  }
+
+  on_load_start(load, now = gHost.now()) {
+    this._currentLoadStart = now;
+    this._frameCount = 0;
+    this._mutating_ms = 0;
+    this._suspend_sec = 0;
+    this._majorGCs = gHost.majorGCCount;
+    this._minorGCs = gHost.minorGCCount;
+  }
+
+  on_load_end(load, now = gHost.now()) {
+    const elapsed_time = (now - this._currentLoadStart) / 1000;
+    const full_time = round_up(elapsed_time, 1 / 60);
+    const frame_60fps_limit = Math.round(full_time * 60);
+    const dropped_60fps_frames = frame_60fps_limit - this._frameCount;
+    const dropped_60fps_fraction = dropped_60fps_frames / frame_60fps_limit;
+
+    const mutating_and_gc_fraction = this._mutating_ms / (full_time * 1000);
+
+    const result = {
+      load,
+      elapsed_time,
+      mutating: this._mutating_ms / 1000,
+      mutating_and_gc_fraction,
+      suspended: this._suspend_sec,
+      full_time,
+      frames: this._frameCount,
+      dropped_60fps_frames,
+      dropped_60fps_fraction,
+      majorGCs: gHost.majorGCCount - this._majorGCs,
+      minorGCs: gHost.minorGCCount - this._minorGCs,
+    };
+    this.results.push(result);
+
+    this._currentLoadStart = undefined;
+    this._frameCount = 0;
+
+    return result;
+  }
+
+  after_suspend(wait_sec) {
+    this._suspend_sec += wait_sec;
+  }
+
+  before_mutator(now = gHost.now()) {
+    this._frameCount++;
+  }
+
+  after_mutator(start_time, end_time = gHost.now()) {
+    this._mutating_ms += end_time - start_time;
+  }
+};
diff --git a/js/src/devtools/gc-ubench/scheduler.js b/js/src/devtools/gc-ubench/scheduler.js
new file mode 100644
index 0000000000..8f4a483b33
--- /dev/null
+++ b/js/src/devtools/gc-ubench/scheduler.js
@@ -0,0 +1,64 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Frame schedulers: executing a frame's worth of mutation, and possibly
+// waiting for a later frame. (These schedulers will halt the main thread,
+// allowing background threads to continue working.)
+
+var Scheduler = class {
+  constructor(perfMonitor) {
+    this._perfMonitor = perfMonitor;
+  }
+
+  start(loadMgr, timestamp) {
+    return loadMgr.start(timestamp);
+  }
+  tick(loadMgr, timestamp) {}
+  wait_for_next_frame(t0, tick_start, tick_end) {}
+};
+
+// "Sync to vsync" scheduler: after the mutator is done for a frame, wait until
+// the beginning of the next 60fps frame.
+var VsyncScheduler = class extends Scheduler {
+  tick(loadMgr, timestamp) {
+    this._perfMonitor.before_mutator(timestamp);
+    gHost.start_turn();
+    const completed = loadMgr.tick(timestamp);
+    gHost.end_turn();
+    this._perfMonitor.after_mutator(timestamp);
+    return completed;
+  }
+
+  wait_for_next_frame(t0, tick_start, tick_end) {
+    // Compute how long until the next 60fps vsync event, and wait that long.
+    const elapsed = (tick_end - t0) / 1000;
+    const period = 1 / FPS;
+    const used = elapsed % period;
+    const delay = period - used;
+    gHost.suspend(delay);
+    this._perfMonitor.after_suspend(delay);
+  }
+};
+
+// Try to maintain 60fps, but if we overrun a frame, do more processing
+// immediately to make the next frame come up as soon as possible.
+var OptimizeForFrameRate = class extends Scheduler {
+  tick(loadMgr, timestamp) {
+    this._perfMonitor.before_mutator(timestamp);
+    gHost.start_turn();
+    const completed = loadMgr.tick(timestamp);
+    gHost.end_turn();
+    this._perfMonitor.after_mutator(timestamp);
+    return completed;
+  }
+
+  wait_for_next_frame(t0, tick_start, tick_end) {
+    const next_frame_ms = round_up(tick_start, 1000 / FPS);
+    if (tick_end < next_frame_ms) {
+      const delay = (next_frame_ms - tick_end) / 1000;
+      gHost.suspend(delay);
+      this._perfMonitor.after_suspend(delay);
+    }
+  }
+};
diff --git a/js/src/devtools/gc-ubench/sequencer.js b/js/src/devtools/gc-ubench/sequencer.js
new file mode 100644
index 0000000000..0271140c04
--- /dev/null
+++ b/js/src/devtools/gc-ubench/sequencer.js
@@ -0,0 +1,298 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// A Sequencer handles transitioning between different mutators. Typically, it
+// will base the decision to transition on things like elapsed time, number of
+// GCs observed, or similar. However, they might also implement a search for
+// some result value by running for some time while measuring, tweaking
+// parameters, and re-running until an in-range result is found.
+
+var Sequencer = class {
+  // Return the current mutator (of class AllocationLoad).
+  get current() {
+    throw new Error("unimplemented");
+  }
+
+  start(now = gHost.now()) {
+    this.started = now;
+  }
+
+  // Called by user to handle advancing time. Subclasses will normally override
+  // do_tick() instead. Returns the results of a trial if complete (the mutator
+  // reached its allotted time or otherwise determined that its timing data
+  // should be valid), and falsy otherwise.
+  tick(now = gHost.now()) {
+    if (this.done()) {
+      throw new Error("tick() called on completed sequencer");
+    }
+
+    return this.do_tick(now);
+  }
+
+  // Implement in subclass to handle time advancing. Must return trial's result
+  // if complete. Called by tick(), above.
+  do_tick(now = gHost.now()) {
+    throw new Error("unimplemented");
+  }
+
+  // Returns whether this sequencer is done running trials.
+  done() {
+    throw new Error("unimplemented");
+  }
+
+  restart(now = gHost.now()) {
+    this.reset();
+    this.start(now);
+  }
+
+  // Returns how long the current load has been running.
+  currentLoadElapsed(now = gHost.now()) {
+    return now - this.started;
+  }
+};
+
+// Run a single trial of a mutator and be done.
+var SingleMutatorSequencer = class extends Sequencer {
+  constructor(mutator, perf, duration_sec) {
+    super();
+    this.mutator = mutator;
+    this.perf = perf;
+    if (!(duration_sec > 0)) {
+      throw new Error(`invalid duration '${duration_sec}'`);
+    }
+    this.duration = duration_sec * 1000;
+    this.state = 'init'; // init -> running -> done
+    this.lastResult = undefined;
+  }
+
+  get current() {
+    return this.state === 'done' ? undefined : this.mutator;
+  }
+
+  reset() {
+    this.state = 'init';
+  }
+
+  start(now = gHost.now()) {
+    if (this.state !== 'init') {
+      throw new Error("cannot restart a single-mutator sequencer");
+    }
+    super.start(now);
+    this.state = 'running';
+    this.perf.on_load_start(this.current, now);
+  }
+
+  do_tick(now) {
+    if (this.currentLoadElapsed(now) < this.duration) {
+      return false;
+    }
+
+    const load = this.current;
+    this.state = 'done';
+    return this.perf.on_load_end(load, now);
+  }
+
+  done() {
+    return this.state === 'done';
+  }
+};
+
+// For each of series of sequencers, run until done.
+var ChainSequencer = class extends Sequencer {
+  constructor(sequencers) {
+    super();
+    this.sequencers = sequencers;
+    this.idx = -1;
+    this.state = sequencers.length ? 'init' : 'done'; // init -> running -> done
+  }
+
+  get current() {
+    return this.idx >= 0 ? this.sequencers[this.idx].current : undefined;
+  }
+
+  reset() {
+    this.state = 'init';
+    this.idx = -1;
+  }
+
+  start(now = gHost.now()) {
+    super.start(now);
+    if (this.sequencers.length === 0) {
+      this.state = 'done';
+      return;
+    }
+
+    this.idx = 0;
+    this.sequencers[0].start(now);
+    this.state = 'running';
+  }
+
+  do_tick(now) {
+    const sequencer = this.sequencers[this.idx];
+    const trial_result = sequencer.do_tick(now);
+    if (!trial_result) {
+      return false; // Trial is still going.
+    }
+
+    if (!sequencer.done()) {
+      // A single trial has completed, but the sequencer is not yet done.
+      return trial_result;
+    }
+
+    this.idx++;
+    if (this.idx < this.sequencers.length) {
+      this.sequencers[this.idx].start();
+    } else {
+      this.idx = -1;
+      this.state = 'done';
+    }
+
+    return trial_result;
+  }
+
+  done() {
+    return this.state === 'done';
+  }
+};
+
+var RunUntilSequencer = class extends Sequencer {
+  constructor(sequencer, loadMgr) {
+    super();
+    this.loadMgr = loadMgr;
+    this.sequencer = sequencer;
+
+    // init -> running -> done
+    this.state = sequencer.done() ? 'done' : 'init';
+  }
+
+  get current() {
+    return this.sequencer?.current;
+  }
+
+  reset() {
+    this.sequencer.reset();
+    this.state = 'init';
+  }
+
+  start(now) {
+    super.start(now);
+    this.sequencer.start(now);
+    this.initSearch(now);
+    this.state = 'running';
+  }
+
+  initSearch(now) {}
+
+  done() {
+    return this.state === 'done';
+  }
+
+  do_tick(now) {
+    const trial_result = this.sequencer.do_tick(now);
+    if (trial_result) {
+      if (this.searchComplete(trial_result)) {
+        this.state = 'done';
+      } else {
+        this.sequencer.restart(now);
+      }
+    }
+    return trial_result;
+  }
+
+  // Take the result of the last mutator run into account (only notified after
+  // a mutator is complete, so cannot be used to decide when to end the
+  // mutator.)
+  searchComplete(result) {
+    throw new Error("must implement in subclass");
+  }
+};
+
+// Run trials, adjusting garbagePerFrame, until 50% of the frames are dropped.
+var Find50Sequencer = class extends RunUntilSequencer {
+  constructor(sequencer, loadMgr, goal=0.5, low_range=0.45, high_range=0.55) {
+    super(sequencer, loadMgr);
+
+    // Run trials with varying garbagePerFrame, looking for a setting that
+    // drops 50% of the frames, until we have been searching in the range for
+    // `persistence` times.
+    this.low_range = low_range;
+    this.goal = goal;
+    this.high_range = high_range;
+    this.persistence = 3;
+
+    this.clear();
+  }
+
+  reset() {
+    super.reset();
+    this.clear();
+  }
+
+  clear() {
+    this.garbagePerFrame = undefined;
+
+    this.good = undefined;
+    this.goodAt = undefined;
+    this.bad = undefined;
+    this.badAt = undefined;
+
+    this.numInRange = 0;
+  }
+
+  start(now) {
+    super.start(now);
+    if (!this.done()) {
+      this.garbagePerFrame = this.sequencer.current.garbagePerFrame;
+    }
+  }
+
+  searchComplete(result) {
+    print(
+      `Saw ${percent(result.dropped_60fps_fraction)} with garbagePerFrame=${this.garbagePerFrame}`
+    );
+
+    // This is brittle with respect to noise. It might be better to do a linear
+    // regression and stop at an error threshold.
+    if (result.dropped_60fps_fraction < this.goal) {
+      if (this.goodAt === undefined || this.goodAt < this.garbagePerFrame) {
+        this.goodAt = this.garbagePerFrame;
+        this.good = result.dropped_60fps_fraction;
+      }
+      if (this.badAt !== undefined) {
+        this.garbagePerFrame = Math.trunc(
+          (this.garbagePerFrame + this.badAt) / 2
+        );
+      } else {
+        this.garbagePerFrame *= 2;
+      }
+    } else {
+      if (this.badAt === undefined || this.badAt > this.garbagePerFrame) {
+        this.badAt = this.garbagePerFrame;
+        this.bad = result.dropped_60fps_fraction;
+      }
+      if (this.goodAt !== undefined) {
+        this.garbagePerFrame = Math.trunc(
+          (this.garbagePerFrame + this.goodAt) / 2
+        );
+      } else {
+        this.garbagePerFrame = Math.trunc(this.garbagePerFrame / 2);
+      }
+    }
+
+    if (
+      this.low_range < result.dropped_60fps_fraction &&
+      result.dropped_60fps_fraction < this.high_range
+    ) {
+      this.numInRange++;
+      if (this.numInRange >= this.persistence) {
+        return true;
+      }
+    }
+
+    print(`next run with ${this.garbagePerFrame}`);
+    this.loadMgr.change_garbagePerFrame(this.garbagePerFrame);
+
+    return false;
+  }
+};
diff --git a/js/src/devtools/gc-ubench/shell-bench.js b/js/src/devtools/gc-ubench/shell-bench.js
new file mode 100644
index 0000000000..9640cddce9
--- /dev/null
+++ b/js/src/devtools/gc-ubench/shell-bench.js
@@ -0,0 +1,147 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+var FPS = 60;
+var gNumSamples = 500;
+
+// This requires a gHost to have been created that provides host-specific
+// facilities. See eg spidermonkey.js.
+
+loadRelativeToScript("argparse.js");
+loadRelativeToScript("harness.js");
+loadRelativeToScript("sequencer.js");
+loadRelativeToScript("scheduler.js");
+loadRelativeToScript("perf.js");
+loadRelativeToScript("test_list.js");
+
+var gPerf = new PerfTracker();
+
+var tests = new Map();
+foreach_test_file(f => loadRelativeToScript(f));
+for (const [name, info] of tests.entries()) {
+  if ("enabled" in info && !info.enabled) {
+    tests.delete(name);
+  }
+}
+
+function tick(loadMgr, timestamp) {
+  gPerf.before_mutator(timestamp);
+  gHost.start_turn();
+  const events = loadMgr.tick(timestamp);
+  gHost.end_turn();
+  gPerf.after_mutator(timestamp);
+  return events;
+}
+
+function run(opts, loads) {
+  const sequence = [];
+  for (const mut of loads) {
+    if (tests.has(mut)) {
+      sequence.push(mut);
+    } else if (mut === "all") {
+      sequence.push(...tests.keys());
+    } else {
+      sequence.push(...[...tests.keys()].filter(t => t.includes(mut)));
+    }
+  }
+  if (loads.length === 0) {
+    sequence.push(...tests.keys());
+  }
+
+  const loadMgr = new AllocationLoadManager(tests);
+  const perf = new FrameHistory(gNumSamples);
+
+  const mutators = sequence.map(name => new SingleMutatorSequencer(loadMgr.getByName(name), gPerf, opts.duration));
+  let sequencer;
+  if (opts.sequencer == 'cycle') {
+    sequencer = new ChainSequencer(mutators);
+  } else if (opts.sequencer == 'find50') {
+    const seekers = mutators.map(s => new Find50Sequencer(s, loadMgr));
+    sequencer = new ChainSequencer(seekers);
+  }
+
+  const schedulerCtors = {
+    keepup: OptimizeForFrameRate,
+    vsync: VsyncScheduler,
+  };
+  const scheduler = new schedulerCtors[opts.sched](gPerf);
+
+  perf.start();
+
+  const t0 = gHost.now();
+
+  let possible = 0;
+  let frames = 0;
+  loadMgr.startSequencer(sequencer);
+  print(`${loadMgr.activeLoad().name} starting`);
+  while (loadMgr.load_running()) {
+    const timestamp = gHost.now();
+    const completed = scheduler.tick(loadMgr, timestamp);
+    const after_tick = gHost.now();
+
+    perf.on_frame(timestamp);
+
+    if (completed) {
+      print(`${loadMgr.lastActive.name} ended`);
+      if (loadMgr.load_running()) {
+        print(`${loadMgr.activeLoad().name} starting`);
+      }
+    }
+
+    frames++;
+    if (completed) {
+      possible += (loadMgr.testDurationMS / 1000) * FPS;
+      const elapsed = ((after_tick - t0) / 1000).toFixed(2);
+      print(`  observed ${frames} / ${possible} frames in ${elapsed} seconds`);
+    }
+
+    scheduler.wait_for_next_frame(t0, timestamp, after_tick);
+  }
+}
+
+function report_results() {
+  for (const result of gPerf.results) {
+    const {
+      load,
+      elapsed_time,
+      mutating,
+      mutating_and_gc_fraction,
+      suspended,
+      full_time,
+      frames,
+      dropped_60fps_frames,
+      dropped_60fps_fraction,
+      minorGCs,
+      majorGCs,
+    } = result;
+
+    const drop_pct = percent(dropped_60fps_fraction);
+    const mut_pct = percent(mutating_and_gc_fraction);
+    const mut_sec = mutating.toFixed(2);
+    const full_sec = full_time.toFixed(2);
+    const susp_sec = suspended.toFixed(2);
+    print(`${load.name}:
+  ${frames} (60fps) frames seen out of expected ${Math.floor(full_time * 60)}
+  ${dropped_60fps_frames} = ${drop_pct} 60fps frames dropped
+  ${mut_pct} of run spent mutating and GCing (${mut_sec}sec out of ${full_sec}sec vs ${susp_sec} sec waiting)
+  ${minorGCs} minor GCs, ${majorGCs} major GCs
+`);
+  }
+}
+
+var argparse = new ArgParser("JS shell microbenchmark runner");
+argparse.add_argument(["--duration", "-d"], {
+  default: gDefaultTestDuration,
+  help: "how long to run mutators for (in seconds)"
+});
+argparse.add_argument("--sched", {
+  default: "keepup",
+  options: ["keepup", "vsync"],
+  help: "frame scheduler"
+});
+argparse.add_argument("--sequencer", {
+  default: "cycle",
+  options: ["cycle", "find50"],
+  help: "mutator sequencer"
+});
diff --git a/js/src/devtools/gc-ubench/spidermonkey.js b/js/src/devtools/gc-ubench/spidermonkey.js
new file mode 100644
index 0000000000..0ad0aa9771
--- /dev/null
+++ b/js/src/devtools/gc-ubench/spidermonkey.js
@@ -0,0 +1,57 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// SpiderMonkey JS shell benchmark script
+//
+// Usage: run $JS spidermonkey.js --help
+
+loadRelativeToScript("shell-bench.js");
+
+var SpiderMonkey = class extends Host {
+  start_turn() {}
+
+  end_turn() {
+    clearKeptObjects();
+    maybegc();
+    drainJobQueue();
+  }
+
+  suspend(duration) {
+    sleep(duration);
+  }
+
+  get minorGCCount() {
+    return performance.mozMemory.gc.minorGCCount;
+  }
+  get majorGCCount() {
+    return performance.mozMemory.gc.majorGCCount;
+  }
+  get GCSliceCount() {
+    return performance.mozMemory.gc.sliceCount;
+  }
+  get gcBytes() {
+    return performance.mozMemory.gc.zone.gcBytes;
+  }
+  get gcAllocTrigger() {
+    return performance.mozMemory.gc.zone.gcAllocTrigger;
+  }
+
+  features = {
+    haveMemorySizes: true,
+    haveGCCounts: true,
+  };
+};
+
+var gHost = new SpiderMonkey();
+var { opts, rest: mutators } = argparse.parse_args(scriptArgs);
+run(opts, mutators);
+
+print("\nTest results:\n");
+report_results();
+
+var outfile = "spidermonkey-results.json";
+var origOut = redirect(outfile);
+print(JSON.stringify(gPerf.results));
+redirect(origOut);
+print(`Wrote detailed results to ${outfile}`);
diff --git a/js/src/devtools/gc-ubench/test_list.js b/js/src/devtools/gc-ubench/test_list.js
new file mode 100644
index 0000000000..03ed30cf9e
--- /dev/null
+++ b/js/src/devtools/gc-ubench/test_list.js
@@ -0,0 +1,20 @@
+function foreach_test_file(callback) {
+  callback("benchmarks/noAllocation.js");
+  callback("benchmarks/globalArrayNewObject.js");
+  callback("benchmarks/globalArrayArrayLiteral.js");
+  callback("benchmarks/globalArrayLargeArray.js");
+  callback("benchmarks/globalArrayLargeObject.js");
+  callback("benchmarks/globalArrayObjectLiteral.js");
+  callback("benchmarks/globalArrayReallocArray.js");
+  callback("benchmarks/globalArrayBuffer.js");
+  callback("benchmarks/globalArrayFgFinalized.js");
+  callback("benchmarks/largeArrayPropertyAndElements.js");
+  callback("benchmarks/selfCyclicWeakMap.js");
+  callback("benchmarks/pairCyclicWeakMap.js");
+  callback("benchmarks/deepWeakMap.js");
+  callback("benchmarks/textNodes.js");
+  callback("benchmarks/bigTextNodes.js");
+  callback("benchmarks/events.js");
+  callback("benchmarks/expandoEvents.js");
+  callback("benchmarks/propertyTreeSplitting.js");
+}
diff --git a/js/src/devtools/gc-ubench/ui.js b/js/src/devtools/gc-ubench/ui.js
new file mode 100644
index 0000000000..f4a8357951
--- /dev/null
+++ b/js/src/devtools/gc-ubench/ui.js
@@ -0,0 +1,700 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+var stroke = {
+  gcslice: "rgb(255,100,0)",
+  minor: "rgb(0,255,100)",
+  initialMajor: "rgb(180,60,255)",
+};
+
+var numSamples = 500;
+
+var gHistogram = new Map(); // {ms: count}
+var gHistory = new FrameHistory(numSamples);
+var gPerf = new PerfTracker();
+
+var latencyGraph;
+var memoryGraph;
+var ctx;
+var memoryCtx;
+
+var loadState = "(init)"; // One of '(active)', '(inactive)', '(N/A)'
+var testState = "idle"; // One of 'idle' or 'running'.
+var enabled = { trackingSizes: false };
+
+var gMemory = performance.mozMemory?.gc || performance.mozMemory || {};
+
+var Firefox = class extends Host {
+  start_turn() {
+    // Handled by Gecko.
+  }
+
+  end_turn() {
+    // Handled by Gecko.
+  }
+
+  suspend(duration) {
+    // Not used; requestAnimationFrame takes its place.
+    throw new Error("unimplemented");
+  }
+
+  get minorGCCount() {
+    return gMemory.minorGCCount;
+  }
+  get majorGCCount() {
+    return gMemory.majorGCCount;
+  }
+  get GCSliceCount() {
+    return gMemory.sliceCount;
+  }
+  get gcBytes() {
+    return gMemory.zone.gcBytes;
+  }
+  get gcAllocTrigger() {
+    return gMemory.zone.gcAllocTrigger;
+  }
+
+  features = {
+    haveMemorySizes: 'gcBytes' in gMemory,
+    haveGCCounts: 'majorGCCount' in gMemory,
+  };
+};
+
+var gHost = new Firefox();
+
+function parse_units(v) {
+  if (!v.length) {
+    return NaN;
+  }
+  var lastChar = v[v.length - 1].toLowerCase();
+  if (!isNaN(parseFloat(lastChar))) {
+    return parseFloat(v);
+  }
+  var units = parseFloat(v.substr(0, v.length - 1));
+  if (lastChar == "k") {
+    return units * 1e3;
+  }
+  if (lastChar == "m") {
+    return units * 1e6;
+  }
+  if (lastChar == "g") {
+    return units * 1e9;
+  }
+  return NaN;
+}
+
+var Graph = class {
+  constructor(ctx) {
+    this.ctx = ctx;
+
+    var { height } = ctx.canvas;
+    this.layout = {
+      xAxisLabel_Y: height - 20,
+    };
+  }
+
+  xpos(index) {
+    return index * 2;
+  }
+
+  clear() {
+    const { width, height } = this.ctx.canvas;
+    this.ctx.clearRect(0, 0, width, height);
+  }
+
+  drawScale(delay) {
+    this.drawHBar(delay, `${delay}ms`, "rgb(150,150,150)");
+  }
+
+  draw60fps() {
+    this.drawHBar(1000 / 60, "60fps", "#00cf61", 25);
+  }
+
+  draw30fps() {
+    this.drawHBar(1000 / 30, "30fps", "#cf0061", 25);
+  }
+
+  drawAxisLabels(x_label, y_label) {
+    const ctx = this.ctx;
+    const { width, height } = ctx.canvas;
+
+    ctx.fillText(x_label, width / 2, this.layout.xAxisLabel_Y);
+
+    ctx.save();
+    ctx.rotate(Math.PI / 2);
+    var start = height / 2 - ctx.measureText(y_label).width / 2;
+    ctx.fillText(y_label, start, -width + 20);
+    ctx.restore();
+  }
+
+  drawFrame() {
+    const ctx = this.ctx;
+    const { width, height } = ctx.canvas;
+
+    // Draw frame to show size
+    ctx.strokeStyle = "rgb(0,0,0)";
+    ctx.fillStyle = "rgb(0,0,0)";
+    ctx.beginPath();
+    ctx.moveTo(0, 0);
+    ctx.lineTo(width, 0);
+    ctx.lineTo(width, height);
+    ctx.lineTo(0, height);
+    ctx.closePath();
+    ctx.stroke();
+  }
+};
+
+var LatencyGraph = class extends Graph {
+  constructor(ctx) {
+    super(ctx);
+    console.log(this.ctx);
+  }
+
+  ypos(delay) {
+    const { height } = this.ctx.canvas;
+
+    const r = height + 100 - Math.log(delay) * 64;
+    if (r < 5) {
+      return 5;
+    }
+    return r;
+  }
+
+  drawHBar(delay, label, color = "rgb(0,0,0)", label_offset = 0) {
+    const ctx = this.ctx;
+
+    ctx.fillStyle = color;
+    ctx.strokeStyle = color;
+    ctx.fillText(
+      label,
+      this.xpos(numSamples) + 4 + label_offset,
+      this.ypos(delay) + 3
+    );
+
+    ctx.beginPath();
+    ctx.moveTo(this.xpos(0), this.ypos(delay));
+    ctx.lineTo(this.xpos(numSamples) + label_offset, this.ypos(delay));
+    ctx.stroke();
+    ctx.strokeStyle = "rgb(0,0,0)";
+    ctx.fillStyle = "rgb(0,0,0)";
+  }
+
+  draw() {
+    const ctx = this.ctx;
+
+    this.clear();
+    this.drawFrame();
+
+    for (var delay of [10, 20, 30, 50, 100, 200, 400, 800]) {
+      this.drawScale(delay);
+    }
+    this.draw60fps();
+    this.draw30fps();
+
+    var worst = 0,
+      worstpos = 0;
+    ctx.beginPath();
+    for (let i = 0; i < numSamples; i++) {
+      ctx.lineTo(this.xpos(i), this.ypos(gHistory.delays[i]));
+      if (gHistory.delays[i] >= worst) {
+        worst = gHistory.delays[i];
+        worstpos = i;
+      }
+    }
+    ctx.stroke();
+
+    // Draw vertical lines marking minor and major GCs
+    if (gHost.features.haveGCCounts) {
+      ctx.strokeStyle = stroke.gcslice;
+      let idx = sampleIndex % numSamples;
+      const count = {
+        major: gHistory.majorGCs[idx],
+        minor: 0,
+        slice: gHistory.slices[idx],
+      };
+      for (let i = 0; i < numSamples; i++) {
+        idx = (sampleIndex + i) % numSamples;
+        const isMajorStart = count.major < gHistory.majorGCs[idx];
+        if (count.slice < gHistory.slices[idx]) {
+          if (isMajorStart) {
+            ctx.strokeStyle = stroke.initialMajor;
+          }
+          ctx.beginPath();
+          ctx.moveTo(this.xpos(idx), 0);
+          ctx.lineTo(this.xpos(idx), this.layout.xAxisLabel_Y);
+          ctx.stroke();
+          if (isMajorStart) {
+            ctx.strokeStyle = stroke.gcslice;
+          }
+        }
+        count.major = gHistory.majorGCs[idx];
+        count.slice = gHistory.slices[idx];
+      }
+
+      ctx.strokeStyle = stroke.minor;
+      idx = sampleIndex % numSamples;
+      count.minor = gHistory.minorGCs[idx];
+      for (let i = 0; i < numSamples; i++) {
+        idx = (sampleIndex + i) % numSamples;
+        if (count.minor < gHistory.minorGCs[idx]) {
+          ctx.beginPath();
+          ctx.moveTo(this.xpos(idx), 0);
+          ctx.lineTo(this.xpos(idx), 20);
+          ctx.stroke();
+        }
+        count.minor = gHistory.minorGCs[idx];
+      }
+    }
+
+    ctx.fillStyle = "rgb(255,0,0)";
+    if (worst) {
+      ctx.fillText(
+        `${worst.toFixed(2)}ms`,
+        this.xpos(worstpos) - 10,
+        this.ypos(worst) - 14
+      );
+    }
+
+    // Mark and label the slowest frame
+    ctx.beginPath();
+    var where = sampleIndex % numSamples;
+    ctx.arc(
+      this.xpos(where),
+      this.ypos(gHistory.delays[where]),
+      5,
+      0,
+      Math.PI * 2,
+      true
+    );
+    ctx.fill();
+    ctx.fillStyle = "rgb(0,0,0)";
+
+    this.drawAxisLabels("Time", "Pause between frames (log scale)");
+  }
+};
+
+var MemoryGraph = class extends Graph {
+  constructor(ctx) {
+    super(ctx);
+    this.worstEver = this.bestEver = gHost.gcBytes();
+    this.limit = Math.max(this.worstEver, gHost.gcAllocTrigger);
+  }
+
+  ypos(size) {
+    const { height } = this.ctx.canvas;
+
+    const range = this.limit - this.bestEver;
+    const percent = (size - this.bestEver) / range;
+
+    return (1 - percent) * height * 0.9 + 20;
+  }
+
+  drawHBar(size, label, color = "rgb(150,150,150)") {
+    const ctx = this.ctx;
+
+    const y = this.ypos(size);
+
+    ctx.fillStyle = color;
+    ctx.strokeStyle = color;
+    ctx.fillText(label, this.xpos(numSamples) + 4, y + 3);
+
+    ctx.beginPath();
+    ctx.moveTo(this.xpos(0), y);
+    ctx.lineTo(this.xpos(numSamples), y);
+    ctx.stroke();
+    ctx.strokeStyle = "rgb(0,0,0)";
+    ctx.fillStyle = "rgb(0,0,0)";
+  }
+
+  draw() {
+    const ctx = this.ctx;
+
+    this.clear();
+    this.drawFrame();
+
+    var worst = 0,
+      worstpos = 0;
+    for (let i = 0; i < numSamples; i++) {
+      if (gHistory.gcBytes[i] >= worst) {
+        worst = gHistory.gcBytes[i];
+        worstpos = i;
+      }
+      if (gHistory.gcBytes[i] < this.bestEver) {
+        this.bestEver = gHistory.gcBytes[i];
+      }
+    }
+
+    if (this.worstEver < worst) {
+      this.worstEver = worst;
+      this.limit = Math.max(this.worstEver, gHost.gcAllocTrigger);
+    }
+
+    this.drawHBar(
+      this.bestEver,
+      `${format_bytes(this.bestEver)} min`,
+      "#00cf61"
+    );
+    this.drawHBar(
+      this.worstEver,
+      `${format_bytes(this.worstEver)} max`,
+      "#cc1111"
+    );
+    this.drawHBar(
+      gHost.gcAllocTrigger,
+      `${format_bytes(gHost.gcAllocTrigger)} trigger`,
+      "#cc11cc"
+    );
+
+    ctx.fillStyle = "rgb(255,0,0)";
+    if (worst) {
+      ctx.fillText(
+        format_bytes(worst),
+        this.xpos(worstpos) - 10,
+        this.ypos(worst) - 14
+      );
+    }
+
+    ctx.beginPath();
+    var where = sampleIndex % numSamples;
+    ctx.arc(
+      this.xpos(where),
+      this.ypos(gHistory.gcBytes[where]),
+      5,
+      0,
+      Math.PI * 2,
+      true
+    );
+    ctx.fill();
+
+    ctx.beginPath();
+    for (let i = 0; i < numSamples; i++) {
+      if (i == (sampleIndex + 1) % numSamples) {
+        ctx.moveTo(this.xpos(i), this.ypos(gHistory.gcBytes[i]));
+      } else {
+        ctx.lineTo(this.xpos(i), this.ypos(gHistory.gcBytes[i]));
+      }
+      if (i == where) {
+        ctx.stroke();
+      }
+    }
+    ctx.stroke();
+
+    this.drawAxisLabels("Time", "Heap Memory Usage");
+  }
+};
+
+function onUpdateDisplayChanged() {
+  const do_graph = document.getElementById("do-graph");
+  if (do_graph.checked) {
+    window.requestAnimationFrame(handler);
+    gHistory.resume();
+  } else {
+    gHistory.pause();
+  }
+  update_load_state_indicator();
+}
+
+function onDoLoadChange() {
+  const do_load = document.getElementById("do-load");
+  gLoadMgr.paused = !do_load.checked;
+  console.log(`load paused: ${gLoadMgr.paused}`);
+  update_load_state_indicator();
+}
+
+var previous = 0;
+function handler(timestamp) {
+  if (gHistory.is_stopped()) {
+    return;
+  }
+
+  const completed = gLoadMgr.tick(timestamp);
+  if (completed) {
+    end_test(timestamp, gLoadMgr.lastActive);
+    if (!gLoadMgr.stopped()) {
+      start_test();
+    }
+    update_load_display();
+  }
+
+  if (testState == "running") {
+    document.getElementById("test-progress").textContent =
+      (gLoadMgr.currentLoadRemaining(timestamp) / 1000).toFixed(1) + " sec";
+  }
+
+  const delay = gHistory.on_frame(timestamp);
+
+  update_histogram(gHistogram, delay);
+
+  latencyGraph.draw();
+  if (memoryGraph) {
+    memoryGraph.draw();
+  }
+  window.requestAnimationFrame(handler);
+}
+
+// For interactive debugging.
+//
+// ['a', 'b', 'b', 'b', 'c', 'c'] => ['a', 'b x 3', 'c x 2']
+function summarize(arr) {
+  if (!arr.length) {
+    return [];
+  }
+
+  var result = [];
+  var run_start = 0;
+  var prev = arr[0];
+  for (let i = 1; i <= arr.length; i++) {
+    if (i == arr.length || arr[i] != prev) {
+      if (i == run_start + 1) {
+        result.push(arr[i]);
+      } else {
+        result.push(prev + " x " + (i - run_start));
+      }
+      run_start = i;
+    }
+    if (i != arr.length) {
+      prev = arr[i];
+    }
+  }
+
+  return result;
+}
+
+function reset_draw_state() {
+  gHistory.reset();
+}
+
+function onunload() {
+  gLoadMgr.deactivateLoad();
+}
+
+function onload() {
+  // The order of `tests` is currently based on their asynchronous load
+  // order, rather than the listed order. Rearrange by extracting the test
+  // names from their filenames, which is kind of gross.
+  _tests = tests;
+  tests = new Map();
+  foreach_test_file(fn => {
+    // "benchmarks/foo.js" => "foo"
+    const name = fn.split(/\//)[1].split(/\./)[0];
+    tests.set(name, _tests.get(name));
+  });
+  _tests = undefined;
+
+  gLoadMgr = new AllocationLoadManager(tests);
+
+  // Load initial test duration.
+  duration_changed();
+
+  // Load initial garbage size.
+  garbage_piles_changed();
+  garbage_per_frame_changed();
+
+  // Populate the test selection dropdown.
+  var select = document.getElementById("test-selection");
+  for (var [name, test] of tests) {
+    test.name = name;
+    var option = document.createElement("option");
+    option.id = name;
+    option.text = name;
+    option.title = test.description;
+    select.add(option);
+  }
+
+  // Load the initial test.
+  gLoadMgr.setActiveLoad(gLoadMgr.getByName("noAllocation"));
+  update_load_display();
+  document.getElementById("test-selection").value = "noAllocation";
+
+  // Polyfill rAF.
+  var requestAnimationFrame =
+    window.requestAnimationFrame ||
+    window.mozRequestAnimationFrame ||
+    window.webkitRequestAnimationFrame ||
+    window.msRequestAnimationFrame;
+  window.requestAnimationFrame = requestAnimationFrame;
+
+  // Acquire our canvas.
+  var canvas = document.getElementById("graph");
+  latencyGraph = new LatencyGraph(canvas.getContext("2d"));
+
+  if (!gHost.features.haveMemorySizes) {
+    document.getElementById("memgraph-disabled").style.display = "block";
+    document.getElementById("track-sizes-div").style.display = "none";
+  }
+
+  trackHeapSizes(document.getElementById("track-sizes").checked);
+
+  update_load_state_indicator();
+  gHistory.start();
+
+  // Start drawing.
+  reset_draw_state();
+  window.requestAnimationFrame(handler);
+}
+
+function run_one_test() {
+  start_test_cycle([gLoadMgr.activeLoad().name]);
+}
+
+function run_all_tests() {
+  start_test_cycle([...tests.keys()]);
+}
+
+function start_test_cycle(tests_to_run) {
+  // Convert from an iterable to an array for pop.
+  const duration = gLoadMgr.testDurationMS / 1000;
+  const mutators = tests_to_run.map(name => new SingleMutatorSequencer(gLoadMgr.getByName(name), gPerf, duration));
+  const sequencer = new ChainSequencer(mutators);
+  gLoadMgr.startSequencer(sequencer);
+  testState = "running";
+  gHistogram.clear();
+  reset_draw_state();
+}
+
+function update_load_state_indicator() {
+  if (
+    !gLoadMgr.load_running() ||
+    gLoadMgr.activeLoad().name == "noAllocation"
+  ) {
+    loadState = "(none)";
+  } else if (gHistory.is_stopped() || gLoadMgr.paused) {
+    loadState = "(inactive)";
+  } else {
+    loadState = "(active)";
+  }
+  document.getElementById("load-running").textContent = loadState;
+}
+
+function start_test() {
+  console.log(`Running test: ${gLoadMgr.activeLoad().name}`);
+  document.getElementById("test-selection").value = gLoadMgr.activeLoad().name;
+  update_load_state_indicator();
+}
+
+function end_test(timestamp, load) {
+  document.getElementById("test-progress").textContent = "(not running)";
+  report_test_result(load, gHistogram);
+  gHistogram.clear();
+  console.log(`Ending test ${load.name}`);
+  if (gLoadMgr.stopped()) {
+    testState = "idle";
+  }
+  update_load_state_indicator();
+  reset_draw_state();
+}
+
+function compute_test_spark_histogram(histogram) {
+  const percents = compute_spark_histogram_percents(histogram);
+
+  var sparks = "▁▂▃▄▅▆▇█";
+  var colors = [
+    "#aaaa00",
+    "#007700",
+    "#dd0000",
+    "#ff0000",
+    "#ff0000",
+    "#ff0000",
+    "#ff0000",
+    "#ff0000",
+  ];
+  var line = "";
+  for (let i = 0; i < percents.length; ++i) {
+    var spark = sparks.charAt(parseInt(percents[i] * sparks.length));
+    line += `<span style="color:${colors[i]}">${spark}</span>`;
+  }
+  return line;
+}
+
+function report_test_result(load, histogram) {
+  var resultList = document.getElementById("results-display");
+  var resultElem = document.createElement("div");
+  var score = compute_test_score(histogram);
+  var sparks = compute_test_spark_histogram(histogram);
+  var params = `(${format_num(load.garbagePerFrame)},${format_num(
+    load.garbagePiles
+  )})`;
+  resultElem.innerHTML = `${score.toFixed(3)} ms/s : ${sparks} : ${
+    load.name
+  }${params} - ${load.description}`;
+  resultList.appendChild(resultElem);
+}
+
+function update_load_display() {
+  const garbage = gLoadMgr.activeLoad()
+    ? gLoadMgr.activeLoad().garbagePerFrame
+    : parse_units(gDefaultGarbagePerFrame);
+  document.getElementById("garbage-per-frame").value = format_num(garbage);
+  const piles = gLoadMgr.activeLoad()
+    ? gLoadMgr.activeLoad().garbagePiles
+    : parse_units(gDefaultGarbagePiles);
+  document.getElementById("garbage-piles").value = format_num(piles);
+  update_load_state_indicator();
+}
+
+function duration_changed() {
+  var durationInput = document.getElementById("test-duration");
+  gLoadMgr.testDurationMS = parseInt(durationInput.value) * 1000;
+  console.log(
+    `Updated test duration to: ${gLoadMgr.testDurationMS / 1000} seconds`
+  );
+}
+
+function onLoadChange() {
+  var select = document.getElementById("test-selection");
+  console.log(`Switching to test: ${select.value}`);
+  gLoadMgr.setActiveLoad(gLoadMgr.getByName(select.value));
+  update_load_display();
+  gHistogram.clear();
+  reset_draw_state();
+}
+
+function garbage_piles_changed() {
+  const input = document.getElementById("garbage-piles");
+  const value = parse_units(input.value);
+  if (isNaN(value)) {
+    update_load_display();
+    return;
+  }
+
+  if (gLoadMgr.load_running()) {
+    gLoadMgr.change_garbagePiles(value);
+    console.log(
+      `Updated garbage-piles to ${gLoadMgr.activeLoad().garbagePiles} items`
+    );
+  }
+  gHistogram.clear();
+  reset_draw_state();
+}
+
+function garbage_per_frame_changed() {
+  const input = document.getElementById("garbage-per-frame");
+  var value = parse_units(input.value);
+  if (isNaN(value)) {
+    update_load_display();
+    return;
+  }
+  if (gLoadMgr.load_running()) {
+    gLoadMgr.change_garbagePerFrame = value;
+    console.log(
+      `Updated garbage-per-frame to ${
+        gLoadMgr.activeLoad().garbagePerFrame
+      } items`
+    );
+  }
+}
+
+function trackHeapSizes(track) {
+  enabled.trackingSizes = track && gHost.features.haveMemorySizes;
+
+  var canvas = document.getElementById("memgraph");
+
+  if (enabled.trackingSizes) {
+    canvas.style.display = "block";
+    memoryGraph = new MemoryGraph(canvas.getContext("2d"));
+  } else {
+    canvas.style.display = "none";
+    memoryGraph = null;
+  }
+}
diff --git a/js/src/devtools/gc-ubench/v8.js b/js/src/devtools/gc-ubench/v8.js
new file mode 100644
index 0000000000..0c46c2b4c4
--- /dev/null
+++ b/js/src/devtools/gc-ubench/v8.js
@@ -0,0 +1,42 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// V8 JS shell benchmark script
+//
+// Usage: run d8 v8.js -- --help
+
+globalThis.loadRelativeToScript = load;
+
+load("shell-bench.js");
+
+var V8 = class extends Host {
+  constructor() {
+    super();
+    this.waitTA = new Int32Array(new SharedArrayBuffer(4));
+  }
+
+  start_turn() {}
+
+  end_turn() {}
+
+  suspend(duration) {
+    const response = Atomics.wait(this.waitTA, 0, 0, duration * 1000);
+    if (response !== 'timed-out') {
+      throw new Exception(`unexpected response from Atomics.wait: ${response}`);
+    }
+  }
+
+  features = {
+    haveMemorySizes: false,
+    haveGCCounts: false
+  };
+};
+
+var gHost = new V8();
+
+var { opts, rest: mutators } = argparse.parse_args(arguments);
+run(opts, mutators);
+
+print("\nTest results:\n");
+report_results();