js/src/jit/Sink.cpp
author Jan de Mooij <jdemooij@mozilla.com>
Sat, 28 Mar 2015 12:08:37 +0100
changeset 236295 0c030f97a04f4e34c138b878c4352423f5e920f9
parent 231232 31d7b208abd151f02196eef85f45eb1b91e5c298
child 236301 5b892d8ef4538ea84378ebe4a352c49d8b9aa366
permissions -rw-r--r--
Bug 1144366 - Switch SpiderMonkey and XPConnect style from |T *t| to |T* t|. r=jorendorff

/* -*- 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/Sink.h"

#include "mozilla/Vector.h"

#include "jit/IonAnalysis.h"
#include "jit/JitSpewer.h"
#include "jit/MIR.h"
#include "jit/MIRGenerator.h"
#include "jit/MIRGraph.h"

namespace js {
namespace jit {

// Given the last found common dominator and a new definition to dominate, the
// CommonDominator function returns the basic block which dominate the last
// common dominator and the definition. If no such block exists, then this
// functions return null.
static MBasicBlock*
CommonDominator(MBasicBlock* commonDominator, MBasicBlock* defBlock)
{
    // This is the first instruction visited, record its basic block as being
    // the only interesting one.
    if (!commonDominator)
        return defBlock;

    // Iterate on immediate dominators of the known common dominator to find a
    // block which dominates all previous uses as well as this instruction.
    while (!commonDominator->dominates(defBlock)) {
        MBasicBlock* nextBlock = commonDominator->immediateDominator();
        // All uses are dominated, so, this cannot happen unless the graph
        // coherency is not respected.
        MOZ_ASSERT(commonDominator != nextBlock);
        commonDominator = nextBlock;
    }

    return commonDominator;
}

bool
Sink(MIRGenerator* mir, MIRGraph& graph)
{
    TempAllocator& alloc = graph.alloc();
    bool sinkEnabled = mir->optimizationInfo().sinkEnabled();

    for (PostorderIterator block = graph.poBegin(); block != graph.poEnd(); block++) {
        if (mir->shouldCancel("Sink"))
            return false;

        for (MInstructionReverseIterator iter = block->rbegin(); iter != block->rend(); ) {
            MInstruction* ins = *iter++;

            // Only instructions which can be recovered on bailout can be moved
            // into the bailout paths.
            if (ins->isGuard() || ins->isGuardRangeBailouts() ||
                ins->isRecoveredOnBailout() || !ins->canRecoverOnBailout())
            {
                continue;
            }

            // Compute a common dominator for all uses of the current
            // instruction.
            bool hasLiveUses = false;
            bool hasUses = false;
            MBasicBlock* usesDominator = nullptr;
            for (MUseIterator i(ins->usesBegin()), e(ins->usesEnd()); i != e; i++) {
                hasUses = true;
                MNode* consumerNode = (*i)->consumer();
                if (consumerNode->isResumePoint())
                    continue;

                MDefinition* consumer = consumerNode->toDefinition();
                if (consumer->isRecoveredOnBailout())
                    continue;

                hasLiveUses = true;

                // If the instruction is a Phi, then we should dominate the
                // predecessor from which the value is coming from.
                MBasicBlock* consumerBlock = consumer->block();
                if (consumer->isPhi())
                    consumerBlock = consumerBlock->getPredecessor(consumer->indexOf(*i));

                usesDominator = CommonDominator(usesDominator, consumerBlock);
                if (usesDominator == *block)
                    break;
            }

            // Leave this instruction for DCE.
            if (!hasUses)
                continue;

            // We have no uses, so sink this instruction in all the bailout
            // paths.
            if (!hasLiveUses) {
                MOZ_ASSERT(!usesDominator);
                ins->setRecoveredOnBailout();
                JitSpewDef(JitSpew_Sink, "  No live uses, recover the instruction on bailout\n", ins);
                continue;
            }

            // This guard is temporarly moved here as the above code deals with
            // Dead Code elimination, which got moved into this Sink phase, as
            // the Dead Code elimination used to move instructions with no-live
            // uses to the bailout path.
            if (!sinkEnabled)
                continue;

            // To move an effectful instruction, we would have to verify that the
            // side-effect is not observed. In the mean time, we just inhibit
            // this optimization on effectful instructions.
            if (ins->isEffectful())
                continue;

            // If all the uses are under a loop, we might not want to work
            // against LICM by moving everything back into the loop, but if the
            // loop is it-self inside an if, then we still want to move the
            // computation under this if statement.
            while (block->loopDepth() < usesDominator->loopDepth()) {
                MOZ_ASSERT(usesDominator != usesDominator->immediateDominator());
                usesDominator = usesDominator->immediateDominator();
            }

            // Only move instructions if there is a branch between the dominator
            // of the uses and the original instruction. This prevent moving the
            // computation of the arguments into an inline function if there is
            // no major win.
            MBasicBlock* lastJoin = usesDominator;
            while (*block != lastJoin && lastJoin->numPredecessors() == 1) {
                MOZ_ASSERT(lastJoin != lastJoin->immediateDominator());
                MBasicBlock* next = lastJoin->immediateDominator();
                if (next->numSuccessors() > 1)
                    break;
                lastJoin = next;
            }
            if (*block == lastJoin)
                continue;

            // Skip to the next instruction if we cannot find a common dominator
            // for all the uses of this instruction, or if the common dominator
            // correspond to the block of the current instruction.
            if (!usesDominator || usesDominator == *block)
                continue;

            // Only instruction which can be recovered on bailout and which are
            // sinkable can be moved into blocks which are below while filling
            // the resume points with a clone which is recovered on bailout.

            // If the instruction has live uses and if it is clonable, then we
            // can clone the instruction for all non-dominated uses and move the
            // instruction into the block which is dominating all live uses.
            if (!ins->canClone())
                continue;

            // If the block is a split-edge block, which is created for folding
            // test conditions, then the block has no resume point and has
            // multiple predecessors.  In such case, we cannot safely move
            // bailing instruction to these blocks as we have no way to bailout.
            if (!usesDominator->entryResumePoint() && usesDominator->numPredecessors() != 1)
                continue;

            JitSpewDef(JitSpew_Sink, "  Can Clone & Recover, sink instruction\n", ins);
            JitSpew(JitSpew_Sink, "  into Block %u", usesDominator->id());

            // Copy the arguments and clone the instruction.
            MDefinitionVector operands(alloc);
            for (size_t i = 0, end = ins->numOperands(); i < end; i++) {
                if (!operands.append(ins->getOperand(i)))
                    return false;
            }

            MInstruction* clone = ins->clone(alloc, operands);
            ins->block()->insertBefore(ins, clone);
            clone->setRecoveredOnBailout();

            // We should not update the producer of the entry resume point, as
            // it cannot refer to any instruction within the basic block excepts
            // for Phi nodes.
            MResumePoint* entry = usesDominator->entryResumePoint();

            // Replace the instruction by its clone in all the resume points /
            // recovered-on-bailout instructions which are not in blocks which
            // are dominated by the usesDominator block.
            for (MUseIterator i(ins->usesBegin()), e(ins->usesEnd()); i != e; ) {
                MUse* use = *i++;
                MNode* consumer = use->consumer();

                // If the consumer is a Phi, then we look for the index of the
                // use to find the corresponding predecessor block, which is
                // then used as the consumer block.
                MBasicBlock* consumerBlock = consumer->block();
                if (consumer->isDefinition() && consumer->toDefinition()->isPhi()) {
                    consumerBlock = consumerBlock->getPredecessor(
                        consumer->toDefinition()->toPhi()->indexOf(use));
                }

                // Keep the current instruction for all dominated uses, except
                // for the entry resume point of the block in which the
                // instruction would be moved into.
                if (usesDominator->dominates(consumerBlock) &&
                    (!consumer->isResumePoint() || consumer->toResumePoint() != entry))
                {
                    continue;
                }

                use->replaceProducer(clone);
            }

            // As we move this instruction in a different block, we should
            // verify that we do not carry over a resume point which would refer
            // to an outdated state of the control flow.
            if (ins->resumePoint())
                ins->clearResumePoint();

            // Now, that all uses which are not dominated by usesDominator are
            // using the cloned instruction, we can safely move the instruction
            // into the usesDominator block.
            MInstruction* at = usesDominator->safeInsertTop(nullptr, MBasicBlock::IgnoreRecover);
            block->moveBefore(at, ins);
        }
    }

    return true;
}

}
}