C++ MDefinition::isDiscarded方法代码示例

// If an equivalent and dominating value already exists in the set, return it.
// Otherwise insert |def| into the set and return it.
MDefinition*
ValueNumberer::leader(MDefinition* def)
{
    // If the value isn't suitable for eliminating, don't bother hashing it. The
    // convention is that congruentTo returns false for node kinds that wish to
    // opt out of redundance elimination.
    // TODO: It'd be nice to clean up that convention (bug 1031406).
    if (!def->isEffectful() && def->congruentTo(def)) {
        // Look for a match.
        VisibleValues::AddPtr p = values_.findLeaderForAdd(def);
        if (p) {
            MDefinition* rep = *p;
            if (!rep->isDiscarded() && rep->block()->dominates(def->block())) {
                // We found a dominating congruent value.
                return rep;
            }

            // The congruent value doesn't dominate. It never will again in this
            // dominator tree, so overwrite it.
            values_.overwrite(p, def);
        } else {
            // No match. Add a new entry.
            if (!values_.add(p, def))
                return nullptr;
        }

#ifdef DEBUG
        JitSpew(JitSpew_GVN, "      Recording %s%u", def->opName(), def->id());
#endif
    }

    return def;
}

// Assuming |resume| is unreachable, release its operands.
// It might be nice to integrate this code with prepareForDiscard, however GVN
// needs it to call handleUseReleased so that it can observe when a definition
// becomes unused, so it isn't trivial to do.
bool
ValueNumberer::releaseResumePointOperands(MResumePoint *resume)
{
    for (size_t i = 0, e = resume->numOperands(); i < e; ++i) {
        if (!resume->hasOperand(i))
            continue;
        MDefinition *op = resume->getOperand(i);
        // TODO: We shouldn't leave discarded operands sitting around
        // (bug 1055690).
        if (op->isDiscarded())
            continue;
        resume->releaseOperand(i);

        // We set the UseRemoved flag when removing resume point operands,
        // because even though we may think we're certain that a particular
        // branch might not be taken, the type information might be incomplete.
        if (!handleUseReleased(op, SetUseRemoved))
            return false;
    }
    return true;
}

void
LoopUnroller::go(LoopIterationBound* bound)
{
    // For now we always unroll loops the same number of times.
    static const size_t UnrollCount = 10;

    JitSpew(JitSpew_Unrolling, "Attempting to unroll loop");

    header = bound->header;

    // UCE might have determined this isn't actually a loop.
    if (!header->isLoopHeader())
        return;

    backedge = header->backedge();
    oldPreheader = header->loopPredecessor();

    MOZ_ASSERT(oldPreheader->numSuccessors() == 1);

    // Only unroll loops with two blocks: an initial one ending with the
    // bound's test, and the body ending with the backedge.
    MTest* test = bound->test;
    if (header->lastIns() != test)
        return;
    if (test->ifTrue() == backedge) {
        if (test->ifFalse()->id() <= backedge->id())
            return;
    } else if (test->ifFalse() == backedge) {
        if (test->ifTrue()->id() <= backedge->id())
            return;
    } else {
        return;
    }
    if (backedge->numPredecessors() != 1 || backedge->numSuccessors() != 1)
        return;
    MOZ_ASSERT(backedge->phisEmpty());

    MBasicBlock* bodyBlocks[] = { header, backedge };

    // All instructions in the header and body must be clonable.
    for (size_t i = 0; i < ArrayLength(bodyBlocks); i++) {
        MBasicBlock* block = bodyBlocks[i];
        for (MInstructionIterator iter(block->begin()); iter != block->end(); iter++) {
            MInstruction* ins = *iter;
            if (ins->canClone())
                continue;
            if (ins->isTest() || ins->isGoto() || ins->isInterruptCheck())
                continue;
#ifdef DEBUG
            JitSpew(JitSpew_Unrolling, "Aborting: can't clone instruction %s", ins->opName());
#endif
            return;
        }
    }

    // Compute the linear inequality we will use for exiting the unrolled loop:
    //
    // iterationBound - iterationCount - UnrollCount >= 0
    //
    LinearSum remainingIterationsInequality(bound->boundSum);
    if (!remainingIterationsInequality.add(bound->currentSum, -1))
        return;
    if (!remainingIterationsInequality.add(-int32_t(UnrollCount)))
        return;

    // Terms in the inequality need to be either loop invariant or phis from
    // the original header.
    for (size_t i = 0; i < remainingIterationsInequality.numTerms(); i++) {
        MDefinition* def = remainingIterationsInequality.term(i).term;
        if (def->isDiscarded())
            return;
        if (def->block()->id() < header->id())
            continue;
        if (def->block() == header && def->isPhi())
            continue;
        return;
    }

    // OK, we've checked everything, now unroll the loop.

    JitSpew(JitSpew_Unrolling, "Unrolling loop");

    // The old preheader will go before the unrolled loop, and the old loop
    // will need a new empty preheader.
    CompileInfo& info = oldPreheader->info();
    if (header->trackedPc()) {
        unrolledHeader =
            MBasicBlock::New(graph, nullptr, info,
                             oldPreheader, header->trackedSite(), MBasicBlock::LOOP_HEADER);
        unrolledBackedge =
            MBasicBlock::New(graph, nullptr, info,
                             unrolledHeader, backedge->trackedSite(), MBasicBlock::NORMAL);
        newPreheader =
            MBasicBlock::New(graph, nullptr, info,
                             unrolledHeader, oldPreheader->trackedSite(), MBasicBlock::NORMAL);
    } else {
        unrolledHeader = MBasicBlock::NewAsmJS(graph, info, oldPreheader, MBasicBlock::LOOP_HEADER);
        unrolledBackedge = MBasicBlock::NewAsmJS(graph, info, unrolledHeader, MBasicBlock::NORMAL);
        newPreheader = MBasicBlock::NewAsmJS(graph, info, unrolledHeader, MBasicBlock::NORMAL);
    }
//.........这里部分代码省略.........

// Visit |def|.
bool
ValueNumberer::visitDefinition(MDefinition *def)
{
    // If this instruction has a dependency() into an unreachable block, we'll
    // need to update AliasAnalysis.
    MDefinition *dep = def->dependency();
    if (dep != nullptr && (dep->isDiscarded() || dep->block()->isDead())) {
        JitSpew(JitSpew_GVN, "      AliasAnalysis invalidated");
        if (updateAliasAnalysis_ && !dependenciesBroken_) {
            // TODO: Recomputing alias-analysis could theoretically expose more
            // GVN opportunities.
            JitSpew(JitSpew_GVN, "        Will recompute!");
            dependenciesBroken_ = true;
        }
        // Temporarily clear its dependency, to protect foldsTo, which may
        // wish to use the dependency to do store-to-load forwarding.
        def->setDependency(def->toInstruction());
    } else {
        dep = nullptr;
    }

    // Look for a simplified form of |def|.
    MDefinition *sim = simplified(def);
    if (sim != def) {
        if (sim == nullptr)
            return false;

        // If |sim| doesn't belong to a block, insert it next to |def|.
        if (sim->block() == nullptr)
            def->block()->insertAfter(def->toInstruction(), sim->toInstruction());

#ifdef DEBUG
        JitSpew(JitSpew_GVN, "      Folded %s%u to %s%u",
                def->opName(), def->id(), sim->opName(), sim->id());
#endif
        ReplaceAllUsesWith(def, sim);

        // The node's foldsTo said |def| can be replaced by |rep|. If |def| is a
        // guard, then either |rep| is also a guard, or a guard isn't actually
        // needed, so we can clear |def|'s guard flag and let it be discarded.
        def->setNotGuardUnchecked();

        if (DeadIfUnused(def)) {
            if (!discardDefsRecursively(def))
                return false;
        }
        def = sim;
    }

    // Now that foldsTo is done, re-enable the original dependency. Even though
    // it may be pointing into a discarded block, it's still valid for the
    // purposes of detecting congruent loads.
    if (dep != nullptr)
        def->setDependency(dep);

    // Look for a dominating def which makes |def| redundant.
    MDefinition *rep = leader(def);
    if (rep != def) {
        if (rep == nullptr)
            return false;
        if (rep->updateForReplacement(def)) {
#ifdef DEBUG
            JitSpew(JitSpew_GVN,
                    "      Replacing %s%u with %s%u",
                    def->opName(), def->id(), rep->opName(), rep->id());
#endif
            ReplaceAllUsesWith(def, rep);

            // The node's congruentTo said |def| is congruent to |rep|, and it's
            // dominated by |rep|. If |def| is a guard, it's covered by |rep|,
            // so we can clear |def|'s guard flag and let it be discarded.
            def->setNotGuardUnchecked();

            if (DeadIfUnused(def)) {
                // discardDef should not add anything to the deadDefs, as the
                // redundant operation should have the same input operands.
                mozilla::DebugOnly<bool> r = discardDef(def);
                MOZ_ASSERT(r, "discardDef shouldn't have tried to add anything to the worklist, "
                              "so it shouldn't have failed");
                MOZ_ASSERT(deadDefs_.empty(),
                           "discardDef shouldn't have added anything to the worklist");
            }
            def = rep;
        }
    }

    return true;
}

// Visit |def|.
bool
ValueNumberer::visitDefinition(MDefinition* def)
{
    // Nop does not fit in any of the previous optimization, as its only purpose
    // is to reduce the register pressure by keeping additional resume
    // point. Still, there is no need consecutive list of MNop instructions, and
    // this will slow down every other iteration on the Graph.
    if (def->isNop()) {
        MNop* nop = def->toNop();
        MBasicBlock* block = nop->block();

        // We look backward to know if we can remove the previous Nop, we do not
        // look forward as we would not benefit from the folding made by GVN.
        MInstructionReverseIterator iter = ++block->rbegin(nop);

        // This nop is at the beginning of the basic block, just replace the
        // resume point of the basic block by the one from the resume point.
        if (iter == block->rend()) {
            JitSpew(JitSpew_GVN, "      Removing Nop%u", nop->id());
            nop->moveResumePointAsEntry();
            block->discard(nop);
            return true;
        }

        // The previous instruction is also a Nop, no need to keep it anymore.
        MInstruction* prev = *iter;
        if (prev->isNop()) {
            JitSpew(JitSpew_GVN, "      Removing Nop%u", prev->id());
            block->discard(prev);
            return true;
        }

        // The Nop is introduced to capture the result and make sure the operands
        // are not live anymore when there are no further uses. Though when
        // all operands are still needed the Nop doesn't decrease the liveness
        // and can get removed.
        MResumePoint* rp = nop->resumePoint();
        if (rp && rp->numOperands() > 0 &&
            rp->getOperand(rp->numOperands() - 1) == prev &&
            !nop->block()->lastIns()->isThrow() &&
            !prev->isAssertRecoveredOnBailout())
        {
            size_t numOperandsLive = 0;
            for (size_t j = 0; j < prev->numOperands(); j++) {
                for (size_t i = 0; i < rp->numOperands(); i++) {
                    if (prev->getOperand(j) == rp->getOperand(i)) {
                        numOperandsLive++;
                        break;
                    }
                }
            }

            if (numOperandsLive == prev->numOperands()) {
                JitSpew(JitSpew_GVN, "      Removing Nop%u", nop->id());
                block->discard(nop);
            }
        }

        return true;
    }

    // Skip optimizations on instructions which are recovered on bailout, to
    // avoid mixing instructions which are recovered on bailouts with
    // instructions which are not.
    if (def->isRecoveredOnBailout())
        return true;

    // If this instruction has a dependency() into an unreachable block, we'll
    // need to update AliasAnalysis.
    MDefinition* dep = def->dependency();
    if (dep != nullptr && (dep->isDiscarded() || dep->block()->isDead())) {
        JitSpew(JitSpew_GVN, "      AliasAnalysis invalidated");
        if (updateAliasAnalysis_ && !dependenciesBroken_) {
            // TODO: Recomputing alias-analysis could theoretically expose more
            // GVN opportunities.
            JitSpew(JitSpew_GVN, "        Will recompute!");
            dependenciesBroken_ = true;
        }
        // Temporarily clear its dependency, to protect foldsTo, which may
        // wish to use the dependency to do store-to-load forwarding.
        def->setDependency(def->toInstruction());
    } else {
        dep = nullptr;
    }

    // Look for a simplified form of |def|.
    MDefinition* sim = simplified(def);
    if (sim != def) {
        if (sim == nullptr)
            return false;

        bool isNewInstruction = sim->block() == nullptr;

        // If |sim| doesn't belong to a block, insert it next to |def|.
        if (isNewInstruction)
            def->block()->insertAfter(def->toInstruction(), sim->toInstruction());

#ifdef JS_JITSPEW
        JitSpew(JitSpew_GVN, "      Folded %s%u to %s%u",
//.........这里部分代码省略.........