Example #1
0
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();

    JS_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;
    JS_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->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->trackedSite().pc()) {
        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);
    }

    unrolledHeader->discardAllResumePoints();
    unrolledBackedge->discardAllResumePoints();
    newPreheader->discardAllResumePoints();

    // Insert new blocks at their RPO position, and update block ids.
    graph.insertBlockAfter(oldPreheader, unrolledHeader);
    graph.insertBlockAfter(unrolledHeader, unrolledBackedge);
    graph.insertBlockAfter(unrolledBackedge, newPreheader);
    graph.renumberBlocksAfter(oldPreheader);

    if (!unrolledDefinitions.init())
        CrashAtUnhandlableOOM("LoopUnroller::go");

    // Add phis to the unrolled loop header which correspond to the phis in the
    // original loop header.
    JS_ASSERT(header->getPredecessor(0) == oldPreheader);
    for (MPhiIterator iter(header->phisBegin()); iter != header->phisEnd(); iter++) {
        MPhi *old = *iter;
        JS_ASSERT(old->numOperands() == 2);
        MPhi *phi = MPhi::New(alloc);
        phi->setResultType(old->type());
        phi->setResultTypeSet(old->resultTypeSet());
        phi->setRange(old->range());

        unrolledHeader->addPhi(phi);

        if (!phi->reserveLength(2))
            CrashAtUnhandlableOOM("LoopUnroller::go");

        // Set the first input for the phi for now. We'll set the second after
        // finishing the unroll.
        phi->addInput(old->getOperand(0));

        // The old phi will now take the value produced by the unrolled loop.
        old->replaceOperand(0, phi);

        if (!unrolledDefinitions.putNew(old, phi))
            CrashAtUnhandlableOOM("LoopUnroller::go");
    }

    // The loop condition can bail out on e.g. integer overflow, so make a
    // resume point based on the initial resume point of the original header.
    MResumePoint *headerResumePoint = header->entryResumePoint();
    if (headerResumePoint) {
        MResumePoint *rp = makeReplacementResumePoint(unrolledHeader, headerResumePoint);
        unrolledHeader->setEntryResumePoint(rp);

        // Perform an interrupt check at the start of the unrolled loop.
        unrolledHeader->add(MInterruptCheck::New(alloc));
    }

    // Generate code for the test in the unrolled loop.
    for (size_t i = 0; i < remainingIterationsInequality.numTerms(); i++) {
        MDefinition *def = remainingIterationsInequality.term(i).term;
        MDefinition *replacement = getReplacementDefinition(def);
        remainingIterationsInequality.replaceTerm(i, replacement);
    }
    MCompare *compare = ConvertLinearInequality(alloc, unrolledHeader, remainingIterationsInequality);
    MTest *unrolledTest = MTest::New(alloc, compare, unrolledBackedge, newPreheader);
    unrolledHeader->end(unrolledTest);

    // Make an entry resume point for the unrolled body. The unrolled header
    // does not have side effects on stack values, even if the original loop
    // header does, so use the same resume point as for the unrolled header.
    if (headerResumePoint) {
        MResumePoint *rp = makeReplacementResumePoint(unrolledBackedge, headerResumePoint);
        unrolledBackedge->setEntryResumePoint(rp);
    }

    // Make an entry resume point for the new preheader. There are no
    // instructions which use this but some other stuff wants one to be here.
    if (headerResumePoint) {
        MResumePoint *rp = makeReplacementResumePoint(newPreheader, headerResumePoint);
        newPreheader->setEntryResumePoint(rp);
    }

    // Generate the unrolled code.
    JS_ASSERT(UnrollCount > 1);
    size_t unrollIndex = 0;
    while (true) {
        // Clone the contents of the original loop into the unrolled loop body.
        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()) {
                    makeReplacementInstruction(*iter);
                } else {
                    // Control instructions are handled separately.
                    JS_ASSERT(ins->isTest() || ins->isGoto() || ins->isInterruptCheck());
                }
            }
        }

        // Compute the value of each loop header phi after the execution of
        // this unrolled iteration.
        MDefinitionVector phiValues(alloc);
        JS_ASSERT(header->getPredecessor(1) == backedge);
        for (MPhiIterator iter(header->phisBegin()); iter != header->phisEnd(); iter++) {
            MPhi *old = *iter;
            MDefinition *oldInput = old->getOperand(1);
            if (!phiValues.append(getReplacementDefinition(oldInput)))
                CrashAtUnhandlableOOM("LoopUnroller::go");
        }

        unrolledDefinitions.clear();

        if (unrollIndex == UnrollCount - 1) {
            // We're at the end of the last unrolled iteration, set the
            // backedge input for the unrolled loop phis.
            size_t phiIndex = 0;
            for (MPhiIterator iter(unrolledHeader->phisBegin()); iter != unrolledHeader->phisEnd(); iter++) {
                MPhi *phi = *iter;
                phi->addInput(phiValues[phiIndex++]);
            }
            JS_ASSERT(phiIndex == phiValues.length());
            break;
        }

        // Update the map for the phis in the next iteration.
        size_t phiIndex = 0;
        for (MPhiIterator iter(header->phisBegin()); iter != header->phisEnd(); iter++) {
            MPhi *old = *iter;
            if (!unrolledDefinitions.putNew(old, phiValues[phiIndex++]))
                CrashAtUnhandlableOOM("LoopUnroller::go");
        }
        JS_ASSERT(phiIndex == phiValues.length());

        unrollIndex++;
    }

    MGoto *backedgeJump = MGoto::New(alloc, unrolledHeader);
    unrolledBackedge->end(backedgeJump);

    // Place the old preheader before the unrolled loop.
    JS_ASSERT(oldPreheader->lastIns()->isGoto());
    oldPreheader->discardLastIns();
    oldPreheader->end(MGoto::New(alloc, unrolledHeader));

    // Place the new preheader before the original loop.
    newPreheader->end(MGoto::New(alloc, header));

    // Cleanup the MIR graph.
    if (!unrolledHeader->addPredecessorWithoutPhis(unrolledBackedge))
        CrashAtUnhandlableOOM("LoopUnroller::go");
    header->replacePredecessor(oldPreheader, newPreheader);
    oldPreheader->setSuccessorWithPhis(unrolledHeader, 0);
    newPreheader->setSuccessorWithPhis(header, 0);
    unrolledBackedge->setSuccessorWithPhis(unrolledHeader, 1);
}