1 files changed, 97 insertions, 0 deletions
diff --git a/js/src/jit/AlignmentMaskAnalysis.cpp b/js/src/jit/AlignmentMaskAnalysis.cpp
new file mode 100644
index 0000000000..5b19b0861c
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+++ b/js/src/jit/AlignmentMaskAnalysis.cpp
@@ -0,0 +1,97 @@
+/* -*- 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/AlignmentMaskAnalysis.h"
+#include "jit/MIR.h"
+#include "jit/MIRGraph.h"
+
+using namespace js;
+using namespace jit;
+
+static bool IsAlignmentMask(uint32_t m) {
+  // Test whether m is just leading ones and trailing zeros.
+  return (-m & ~m) == 0;
+}
+
+static void AnalyzeAsmHeapAddress(MDefinition* ptr, MIRGraph& graph) {
+  // Fold (a+i)&m to (a&m)+i, provided that this doesn't change the result,
+  // since the users of the BitAnd include heap accesses. This will expose
+  // the redundancy for GVN when expressions like this:
+  //   a&m
+  //   (a+1)&m,
+  //   (a+2)&m,
+  // are transformed into this:
+  //   a&m
+  //   (a&m)+1
+  //   (a&m)+2
+  // and it will allow the constants to be folded by the
+  // EffectiveAddressAnalysis pass.
+  //
+  // Putting the add on the outside might seem like it exposes other users of
+  // the expression to the possibility of i32 overflow, if we aren't in wasm
+  // and they aren't naturally truncating. However, since we use MAdd::New
+  // with MIRType::Int32, we make sure that the value is truncated, just as it
+  // would be by the MBitAnd.
+
+  MOZ_ASSERT(IsCompilingWasm());
+
+  if (!ptr->isBitAnd()) {
+    return;
+  }
+
+  MDefinition* lhs = ptr->toBitAnd()->getOperand(0);
+  MDefinition* rhs = ptr->toBitAnd()->getOperand(1);
+  if (lhs->isConstant()) {
+    std::swap(lhs, rhs);
+  }
+  if (!lhs->isAdd() || !rhs->isConstant()) {
+    return;
+  }
+
+  MDefinition* op0 = lhs->toAdd()->getOperand(0);
+  MDefinition* op1 = lhs->toAdd()->getOperand(1);
+  if (op0->isConstant()) {
+    std::swap(op0, op1);
+  }
+  if (!op1->isConstant()) {
+    return;
+  }
+
+  uint32_t i = op1->toConstant()->toInt32();
+  uint32_t m = rhs->toConstant()->toInt32();
+  if (!IsAlignmentMask(m) || (i & m) != i) {
+    return;
+  }
+
+  // The pattern was matched! Produce the replacement expression.
+  MInstruction* and_ = MBitAnd::New(graph.alloc(), op0, rhs, MIRType::Int32);
+  ptr->block()->insertBefore(ptr->toBitAnd(), and_);
+  auto* add = MAdd::New(graph.alloc(), and_, op1, TruncateKind::Truncate);
+  ptr->block()->insertBefore(ptr->toBitAnd(), add);
+  ptr->replaceAllUsesWith(add);
+  ptr->block()->discard(ptr->toBitAnd());
+}
+
+bool AlignmentMaskAnalysis::analyze() {
+  for (ReversePostorderIterator block(graph_.rpoBegin());
+       block != graph_.rpoEnd(); block++) {
+    for (MInstructionIterator i = block->begin(); i != block->end(); i++) {
+      if (!graph_.alloc().ensureBallast()) {
+        return false;
+      }
+
+      // Note that we don't check for MWasmCompareExchangeHeap
+      // or MWasmAtomicBinopHeap, because the backend and the OOB
+      // mechanism don't support non-zero offsets for them yet.
+      if (i->isAsmJSLoadHeap()) {
+        AnalyzeAsmHeapAddress(i->toAsmJSLoadHeap()->base(), graph_);
+      } else if (i->isAsmJSStoreHeap()) {
+        AnalyzeAsmHeapAddress(i->toAsmJSStoreHeap()->base(), graph_);
+      }
+    }
+  }
+  return true;
+}