1 files changed, 139 insertions, 0 deletions
diff --git a/js/src/jit/WasmBCE.cpp b/js/src/jit/WasmBCE.cpp
new file mode 100644
index 0000000000..f855aab41d
--- /dev/null
+++ b/js/src/jit/WasmBCE.cpp
@@ -0,0 +1,139 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * 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/. */
+
+#include "jit/WasmBCE.h"
+
+#include "jit/JitSpewer.h"
+#include "jit/MIRGenerator.h"
+#include "jit/MIRGraph.h"
+
+using namespace js;
+using namespace js::jit;
+
+typedef js::HashMap<uint32_t, MDefinition*, DefaultHasher<uint32_t>,
+                    SystemAllocPolicy>
+    LastSeenMap;
+
+// The Wasm Bounds Check Elimination (BCE) pass looks for bounds checks
+// on SSA values that have already been checked. (in the same block or in a
+// dominating block). These bounds checks are redundant and thus eliminated.
+//
+// Note: This is safe in the presense of dynamic memory sizes as long as they
+// can ONLY GROW. If we allow SHRINKING the heap, this pass should be
+// RECONSIDERED.
+//
+// TODO (dbounov): Are there a lot of cases where there is no single dominating
+// check, but a set of checks that together dominate a redundant check?
+//
+// TODO (dbounov): Generalize to constant additions relative to one base
+bool jit::EliminateBoundsChecks(MIRGenerator* mir, MIRGraph& graph) {
+  JitSpew(JitSpew_WasmBCE, "Begin");
+  // Map for dominating block where a given definition was checked
+  LastSeenMap lastSeen;
+
+  for (ReversePostorderIterator bIter(graph.rpoBegin());
+       bIter != graph.rpoEnd(); bIter++) {
+    MBasicBlock* block = *bIter;
+    for (MDefinitionIterator dIter(block); dIter;) {
+      MDefinition* def = *dIter++;
+
+      switch (def->op()) {
+        case MDefinition::Opcode::WasmBoundsCheck: {
+          MWasmBoundsCheck* bc = def->toWasmBoundsCheck();
+          MDefinition* addr = bc->index();
+
+          // We only support bounds check elimination on wasm memory, not
+          // tables. See bug 1625891.
+          if (!bc->isMemory()) {
+            continue;
+          }
+
+          // Eliminate constant-address memory bounds checks to addresses below
+          // the heap minimum.
+          //
+          // The payload of the MConstant will be Double if the constant
+          // result is above 2^31-1, but we don't care about that for BCE.
+
+          if (addr->isConstant() &&
+              ((addr->toConstant()->type() == MIRType::Int32 &&
+                uint64_t(addr->toConstant()->toInt32()) <
+                    mir->minWasmHeapLength()) ||
+               (addr->toConstant()->type() == MIRType::Int64 &&
+                uint64_t(addr->toConstant()->toInt64()) <
+                    mir->minWasmHeapLength()))) {
+            bc->setRedundant();
+            if (JitOptions.spectreIndexMasking) {
+              bc->replaceAllUsesWith(addr);
+            } else {
+              MOZ_ASSERT(!bc->hasUses());
+            }
+          } else {
+            LastSeenMap::AddPtr ptr = lastSeen.lookupForAdd(addr->id());
+            if (ptr) {
+              MDefinition* prevCheckOrPhi = ptr->value();
+              if (prevCheckOrPhi->block()->dominates(block)) {
+                bc->setRedundant();
+                if (JitOptions.spectreIndexMasking) {
+                  bc->replaceAllUsesWith(prevCheckOrPhi);
+                } else {
+                  MOZ_ASSERT(!bc->hasUses());
+                }
+              }
+            } else {
+              if (!lastSeen.add(ptr, addr->id(), def)) {
+                return false;
+              }
+            }
+          }
+          break;
+        }
+        case MDefinition::Opcode::Phi: {
+          MPhi* phi = def->toPhi();
+          bool phiChecked = true;
+
+          MOZ_ASSERT(phi->numOperands() > 0);
+
+          // If all incoming values to a phi node are safe (i.e. have a
+          // check that dominates this block) then we can consider this
+          // phi node checked.
+          //
+          // Note that any phi that is part of a cycle
+          // will not be "safe" since the value coming on the backedge
+          // cannot be in lastSeen because its block hasn't been traversed yet.
+          for (int i = 0, nOps = phi->numOperands(); i < nOps; i++) {
+            MDefinition* src = phi->getOperand(i);
+
+            if (JitOptions.spectreIndexMasking) {
+              if (src->isWasmBoundsCheck()) {
+                src = src->toWasmBoundsCheck()->index();
+              }
+            } else {
+              MOZ_ASSERT(!src->isWasmBoundsCheck());
+            }
+
+            LastSeenMap::Ptr checkPtr = lastSeen.lookup(src->id());
+            if (!checkPtr || !checkPtr->value()->block()->dominates(block)) {
+              phiChecked = false;
+              break;
+            }
+          }
+
+          if (phiChecked) {
+            if (!lastSeen.put(def->id(), def)) {
+              return false;
+            }
+          }
+
+          break;
+        }
+        default:
+          break;
+      }
+    }
+  }
+
+  return true;
+}