1 files changed, 94 insertions, 0 deletions
diff --git a/js/src/jit/AlignmentMaskAnalysis.cpp b/js/src/jit/AlignmentMaskAnalysis.cpp
new file mode 100644
index 000000000..d4fefec07
--- /dev/null
+++ b/js/src/jit/AlignmentMaskAnalysis.cpp
@@ -0,0 +1,94 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * 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())
+        mozilla::Swap(lhs, rhs);
+    if (!lhs->isAdd() || !rhs->isConstant())
+        return;
+
+    MDefinition* op0 = lhs->toAdd()->getOperand(0);
+    MDefinition* op1 = lhs->toAdd()->getOperand(1);
+    if (op0->isConstant())
+        mozilla::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_);
+    MInstruction* add = MAdd::New(graph.alloc(), and_, op1, MIRType::Int32);
+    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 MAsmJSCompareExchangeHeap
+            // or MAsmJSAtomicBinopHeap, 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;
+}