C++ MInstructionIterator::isFolded方法代码示例

// Operands to a resume point which are dead at the point of the resume can be
// replaced with undefined values. This analysis supports limited detection of
// dead operands, pruning those which are defined in the resume point's basic
// block and have no uses outside the block or at points later than the resume
// point.
//
// This is intended to ensure that extra resume points within a basic block
// will not artificially extend the lifetimes of any SSA values. This could
// otherwise occur if the new resume point captured a value which is created
// between the old and new resume point and is dead at the new resume point.
bool
ion::EliminateDeadResumePointOperands(MIRGenerator *mir, MIRGraph &graph)
{
    for (PostorderIterator block = graph.poBegin(); block != graph.poEnd(); block++) {
        if (mir->shouldCancel("Eliminate Dead Resume Point Operands (main loop)"))
            return false;

        // The logic below can get confused on infinite loops.
        if (block->isLoopHeader() && block->backedge() == *block)
            continue;

        for (MInstructionIterator ins = block->begin(); ins != block->end(); ins++) {
            // No benefit to replacing constant operands with other constants.
            if (ins->isConstant())
                continue;

            // Scanning uses does not give us sufficient information to tell
            // where instructions that are involved in box/unbox operations or
            // parameter passing might be live. Rewriting uses of these terms
            // in resume points may affect the interpreter's behavior. Rather
            // than doing a more sophisticated analysis, just ignore these.
            if (ins->isUnbox() || ins->isParameter())
                continue;

            // If the instruction's behavior has been constant folded into a
            // separate instruction, we can't determine precisely where the
            // instruction becomes dead and can't eliminate its uses.
            if (ins->isFolded())
                continue;

            // Check if this instruction's result is only used within the
            // current block, and keep track of its last use in a definition
            // (not resume point). This requires the instructions in the block
            // to be numbered, ensured by running this immediately after alias
            // analysis.
            uint32_t maxDefinition = 0;
            for (MUseDefIterator uses(*ins); uses; uses++) {
                if (uses.def()->block() != *block ||
                    uses.def()->isBox() ||
                    uses.def()->isPassArg() ||
                    uses.def()->isPhi())
                {
                    maxDefinition = UINT32_MAX;
                    break;
                }
                maxDefinition = Max(maxDefinition, uses.def()->id());
            }
            if (maxDefinition == UINT32_MAX)
                continue;

            // Walk the uses a second time, removing any in resume points after
            // the last use in a definition.
            for (MUseIterator uses(ins->usesBegin()); uses != ins->usesEnd(); ) {
                if (uses->node()->isDefinition()) {
                    uses++;
                    continue;
                }
                MResumePoint *mrp = uses->node()->toResumePoint();
                if (mrp->block() != *block ||
                    !mrp->instruction() ||
                    mrp->instruction() == *ins ||
                    mrp->instruction()->id() <= maxDefinition)
                {
                    uses++;
                    continue;
                }

                // Store an undefined value in place of all dead resume point
                // operands. Making any such substitution can in general alter
                // the interpreter's behavior, even though the code is dead, as
                // the interpreter will still execute opcodes whose effects
                // cannot be observed. If the undefined value were to flow to,
                // say, a dead property access the interpreter could throw an
                // exception; we avoid this problem by removing dead operands
                // before removing dead code.
                MConstant *constant = MConstant::New(UndefinedValue());
                block->insertBefore(*(block->begin()), constant);
                uses = mrp->replaceOperand(uses, constant);
            }
        }
    }

    return true;
}