C++ MControlInstruction类代码示例

// Visit the control instruction at the end of |block|.
bool
ValueNumberer::visitControlInstruction(MBasicBlock* block, const MBasicBlock* dominatorRoot)
{
    // Look for a simplified form of the control instruction.
    MControlInstruction* control = block->lastIns();
    MDefinition* rep = simplified(control);
    if (rep == control)
        return true;

    if (rep == nullptr)
        return false;

    MControlInstruction* newControl = rep->toControlInstruction();
    MOZ_ASSERT(!newControl->block(),
               "Control instruction replacement shouldn't already be in a block");
#ifdef DEBUG
    JitSpew(JitSpew_GVN, "      Folded control instruction %s%u to %s%u",
            control->opName(), control->id(), newControl->opName(), graph_.getNumInstructionIds());
#endif

    // If the simplification removes any CFG edges, update the CFG and remove
    // any blocks that become dead.
    size_t oldNumSuccs = control->numSuccessors();
    size_t newNumSuccs = newControl->numSuccessors();
    if (newNumSuccs != oldNumSuccs) {
        MOZ_ASSERT(newNumSuccs < oldNumSuccs, "New control instruction has too many successors");
        for (size_t i = 0; i != oldNumSuccs; ++i) {
            MBasicBlock* succ = control->getSuccessor(i);
            if (HasSuccessor(newControl, succ))
                continue;
            if (succ->isMarked())
                continue;
            if (!removePredecessorAndCleanUp(succ, block))
                return false;
            if (succ->isMarked())
                continue;
            if (!rerun_) {
                if (!remainingBlocks_.append(succ))
                    return false;
            }
        }
    }

    if (!releaseOperands(control))
        return false;
    block->discardIgnoreOperands(control);
    block->end(newControl);
    if (block->entryResumePoint() && newNumSuccs != oldNumSuccs)
        block->flagOperandsOfPrunedBranches(newControl);
    return processDeadDefs();
}

MBasicBlock *
UnreachableCodeElimination::optimizableSuccessor(MBasicBlock *block)
{
    // If the last instruction in `block` is a test instruction of a
    // constant value, returns the successor that the branch will
    // always branch to at runtime. Otherwise, returns nullptr.

    MControlInstruction *ins = block->lastIns();
    if (!ins->isTest())
        return nullptr;

    MTest *testIns = ins->toTest();
    MDefinition *v = testIns->getOperand(0);
    if (!v->isConstant())
        return nullptr;

    BranchDirection bdir = v->toConstant()->valueToBoolean() ? TRUE_BRANCH : FALSE_BRANCH;
    return testIns->branchSuccessor(bdir);
}

// Given a block which has had predecessors removed but is still reachable, test
// whether the block's new dominator will be closer than its old one and whether
// it will expose potential optimization opportunities.
static bool
IsDominatorRefined(MBasicBlock* block)
{
    MBasicBlock* old = block->immediateDominator();
    MBasicBlock* now = ComputeNewDominator(block, old);

    // If this block is just a goto and it doesn't dominate its destination,
    // removing its predecessors won't refine the dominators of anything
    // interesting.
    MControlInstruction* control = block->lastIns();
    if (*block->begin() == control && block->phisEmpty() && control->isGoto() &&
        !block->dominates(control->toGoto()->target()))
    {
        return false;
    }

    // We've computed block's new dominator. Test whether there are any
    // newly-dominating definitions which look interesting.
    if (block == old)
        return block != now && ScanDominatorsForDefs(now);
    MOZ_ASSERT(block != now, "Non-self-dominating block became self-dominating");
    return ScanDominatorsForDefs(now, old);
}

bool
ValueNumberer::computeValueNumbers()
{
    // At the end of this function, we will have the value numbering stored in
    // each instruction.
    //
    // We also need an "optimistic" value number, for temporary use, which is
    // stored in a hashtable.
    //
    // For the instruction x := y op z, we map (op, VN[y], VN[z]) to a value
    // number, say v. If it is not in the map, we use the instruction id.
    //
    // If the instruction in question's value number is not already
    // v, we break the congruence and set it to v. We repeat until saturation.
    // This will take at worst O(d) time, where d is the loop connectedness
    // of the SSA def/use graph.
    //
    // The algorithm is the simple RPO-based algorithm from
    // "SCC-Based Value Numbering" by Cooper and Simpson.
    //
    // If we are performing a pessimistic pass, then we assume that every
    // definition is in its own congruence class, since we know nothing about
    // values that enter Phi nodes through back edges. We then make one pass
    // through the graph, ignoring back edges. This yields less congruences on
    // any graph with back-edges, but is much faster to perform.

    IonSpew(IonSpew_GVN, "Numbering instructions");

    if (!values.init())
        return false;
    // Stick a VN object onto every mdefinition
    for (ReversePostorderIterator block(graph_.rpoBegin()); block != graph_.rpoEnd(); block++) {
        for (MDefinitionIterator iter(*block); iter; iter++)
            iter->setValueNumberData(new ValueNumberData);
        MControlInstruction *jump = block->lastIns();
        jump->setValueNumberData(new ValueNumberData);
    }

    // Assign unique value numbers if pessimistic.
    // It might be productive to do this in the MDefinition constructor or
    // possibly in a previous pass, if it seems reasonable.
    if (pessimisticPass_) {
        for (ReversePostorderIterator block(graph_.rpoBegin()); block != graph_.rpoEnd(); block++) {
            for (MDefinitionIterator iter(*block); iter; iter++)
                iter->setValueNumber(iter->id());
        }
    } else {
        // For each root block, add all of its instructions to the worklist.
        markBlock(*(graph_.begin()));
        if (graph_.osrBlock())
            markBlock(graph_.osrBlock());
    }

    while (count_ > 0) {
#ifdef DEBUG
        if (!pessimisticPass_) {
            size_t debugCount = 0;
            IonSpew(IonSpew_GVN, "The following instructions require processing:");
            for (ReversePostorderIterator block(graph_.rpoBegin()); block != graph_.rpoEnd(); block++) {
                for (MDefinitionIterator iter(*block); iter; iter++) {
                    if (iter->isInWorklist()) {
                        IonSpew(IonSpew_GVN, "\t%d", iter->id());
                        debugCount++;
                    }
                }
                if (block->lastIns()->isInWorklist()) {
                    IonSpew(IonSpew_GVN, "\t%d", block->lastIns()->id());
                    debugCount++;
                }
            }
            if (!debugCount)
                IonSpew(IonSpew_GVN, "\tNone");
            JS_ASSERT(debugCount == count_);
        }
#endif
        for (ReversePostorderIterator block(graph_.rpoBegin()); block != graph_.rpoEnd(); block++) {
            for (MDefinitionIterator iter(*block); iter; ) {

                if (!isMarked(*iter)) {
                    iter++;
                    continue;
                }

                JS_ASSERT_IF(!pessimisticPass_, count_ > 0);
                unmarkDefinition(*iter);

                MDefinition *ins = simplify(*iter, false);

                if (ins != *iter) {
                    iter = block->discardDefAt(iter);
                    continue;
                }

                uint32 value = lookupValue(ins);

                if (!value)
                    return false; // Hashtable insertion failed

                if (ins->valueNumber() != value) {
                    IonSpew(IonSpew_GVN,
//.........这里部分代码省略.........