src/third_party/mozjs-45/js/src/jit/AlignmentMaskAnalysis.cpp

/* -*- 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 asm.js
    // and they aren't naturally truncating. However, since we use MAdd::NewAsmJS
    // with MIRType_Int32, we make sure that the value is truncated, just as it
    // would be by the MBitAnd.

    if (!ptr->isBitAnd())
        return;

    MDefinition* lhs = ptr->toBitAnd()->getOperand(0);
    MDefinition* rhs = ptr->toBitAnd()->getOperand(1);
    if (lhs->isConstantValue())
        mozilla::Swap(lhs, rhs);
    if (!lhs->isAdd() || !rhs->isConstantValue())
        return;

    MDefinition* op0 = lhs->toAdd()->getOperand(0);
    MDefinition* op1 = lhs->toAdd()->getOperand(1);
    if (op0->isConstantValue())
        mozilla::Swap(op0, op1);
    if (!op1->isConstantValue())
        return;

    uint32_t i = op1->constantValue().toInt32();
    uint32_t m = rhs->constantValue().toInt32();
    if (!IsAlignmentMask(m) || (i & m) != i)
        return;

    // The pattern was matched! Produce the replacement expression.
    MInstruction* and_ = MBitAnd::NewAsmJS(graph.alloc(), op0, rhs);
    ptr->block()->insertBefore(ptr->toBitAnd(), and_);
    MInstruction* add = MAdd::NewAsmJS(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++) {
            // 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()->ptr(), graph_);
            else if (i->isAsmJSStoreHeap())
                AnalyzeAsmHeapAddress(i->toAsmJSStoreHeap()->ptr(), graph_);
        }
    }
    return true;
}