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/* -*- 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 */
#include "jit/AlignmentMaskAnalysis.h"
#include "jit/MIR-wasm.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.
if (!ptr->isBitAnd()) {
MDefinition* lhs = ptr->toBitAnd()->getOperand(0);
MDefinition* rhs = ptr->toBitAnd()->getOperand(1);
if (lhs->isConstant()) {
std::swap(lhs, rhs);
if (!lhs->isAdd() || !rhs->isConstant()) {
MDefinition* op0 = lhs->toAdd()->getOperand(0);
MDefinition* op1 = lhs->toAdd()->getOperand(1);
if (op0->isConstant()) {
std::swap(op0, op1);
if (!op1->isConstant()) {
uint32_t i = op1->toConstant()->toInt32();
uint32_t m = rhs->toConstant()->toInt32();
if (!IsAlignmentMask(m) || (i & m) != i) {
// 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);
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;