1 files changed, 82 insertions, 0 deletions
diff --git a/js/src/jit-test/tests/ion/fromcharcode-charcodeat-zero.js b/js/src/jit-test/tests/ion/fromcharcode-charcodeat-zero.js
new file mode 100644
index 0000000000..b8a5cb6b56
--- /dev/null
+++ b/js/src/jit-test/tests/ion/fromcharcode-charcodeat-zero.js
@@ -0,0 +1,82 @@
+function toCharCodes(str) {
+  return [...str].map(s => s.length === 1 ? [s.charCodeAt(0)] : [s.charCodeAt(0), s.charCodeAt(1)])
+                 .flat();
+}
+
+function test() {
+  // [Ascii; Latin-1, non-Ascii; non-Latin-1; non-BMP]
+  const constant = "AaÁáĀā𝐀𝐚";
+  const charCodes = toCharCodes(constant);
+
+  // Create a linear, but non-constant string with the same contents.
+  const linear = String.fromCharCode(...charCodes);
+  assertEq(linear, constant);
+
+  // Optimisation applies for |MCharCodeAt(MFromCharCode(MCharCodeAt(str, idx)), 0|.
+  for (let i = 0; i < 500; ++i) {
+    let idx = i & 3;
+
+    // str[idx] is compiled to |MFromCharCode(MCharCodeAt(str, idx))|.
+    assertEq(constant[idx].charCodeAt(0), charCodes[idx]);
+    assertEq(linear[idx].charCodeAt(0), charCodes[idx]);
+
+    // str.charAt(idx) is compiled to |MFromCharCode(MCharCodeAt(str, idx))|.
+    assertEq(constant.charAt(idx).charCodeAt(0), charCodes[idx]);
+    assertEq(linear.charAt(idx).charCodeAt(0), charCodes[idx]);
+  }
+}
+for (let i = 0; i < 4; ++i) {
+  test();
+}
+
+function testNonConstantIndex() {
+  // [Ascii; Latin-1, non-Ascii; non-Latin-1; non-BMP]
+  const constant = "AaÁáĀā𝐀𝐚";
+  const charCodes = toCharCodes(constant);
+
+  // Create a linear, but non-constant string with the same contents.
+  const linear = String.fromCharCode(...charCodes);
+  assertEq(linear, constant);
+
+  // No optimisation when the index isn't a constant zero.
+  let indices = [0, 1, 2, 3];
+  for (let i = 0; i < 500; ++i) {
+    let idx = i & 3;
+
+    // Always zero, but too complicated to infer at compile time.
+    let zero = indices[idx] - idx;
+
+    assertEq(constant[idx].charCodeAt(zero), charCodes[idx]);
+    assertEq(linear[idx].charCodeAt(zero), charCodes[idx]);
+
+    assertEq(constant.charAt(idx).charCodeAt(zero), charCodes[idx]);
+    assertEq(linear.charAt(idx).charCodeAt(zero), charCodes[idx]);
+  }
+}
+for (let i = 0; i < 4; ++i) {
+  testNonConstantIndex();
+}
+
+function testOOB() {
+  // [Ascii; Latin-1, non-Ascii; non-Latin-1; non-BMP]
+  const constant = "AaÁáĀā𝐀𝐚";
+  const charCodes = toCharCodes(constant);
+
+  // Create a linear, but non-constant string with the same contents.
+  const linear = String.fromCharCode(...charCodes);
+  assertEq(linear, constant);
+
+  // No optimisation when the index is out-of-bounds.
+  for (let i = 0; i < 500; ++i) {
+    let idx = i & 3;
+
+    assertEq(constant[idx].charCodeAt(1), NaN);
+    assertEq(linear[idx].charCodeAt(1), NaN);
+
+    assertEq(constant.charAt(idx).charCodeAt(1), NaN);
+    assertEq(linear.charAt(idx).charCodeAt(1), NaN);
+  }
+}
+for (let i = 0; i < 4; ++i) {
+  testOOB();
+}