Example #1
0
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++) {
        if (mir->shouldCancel("Value Numbering (preparation loop"))
            return false;
        for (MDefinitionIterator iter(*block); iter; iter++)
            iter->setValueNumberData(new(alloc()) ValueNumberData);
        MControlInstruction *jump = block->lastIns();
        jump->setValueNumberData(new(alloc()) 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++) {
            if (mir->shouldCancel("Value Numbering (main loop)"))
                return false;
            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;
                }

                // Don't bother storing this instruction in the HashMap if
                // (a) eliminateRedundancies will never eliminate it (because
                // it's non-movable or effectful) and (b) no other instruction's
                // value number depends on it.
                if (!ins->hasDefUses() && (!ins->isMovable() || ins->isEffectful())) {
                    iter++;
                    continue;
                }

                uint32_t value = lookupValue(ins);

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

                if (ins->valueNumber() != value) {
                    IonSpew(IonSpew_GVN,
                            "Broke congruence for instruction %d (%p) with VN %d (now using %d)",
                            ins->id(), (void *) ins, ins->valueNumber(), value);
                    ins->setValueNumber(value);
                    markConsumers(ins);
                }

                iter++;
            }
            // Process control flow instruction:
            MControlInstruction *jump = block->lastIns();
            jump = simplifyControlInstruction(jump);

            // If we are pessimistic, then this will never get set.
            if (!jump->isInWorklist())
                continue;
            unmarkDefinition(jump);
            if (jump->valueNumber() == 0) {
                jump->setValueNumber(jump->id());
                for (size_t i = 0; i < jump->numSuccessors(); i++)
                    markBlock(jump->getSuccessor(i));
            }

        }

        // If we are doing a pessimistic pass, we only go once through the
        // instruction list.
        if (pessimisticPass_)
            break;
    }
#ifdef DEBUG
    for (ReversePostorderIterator block(graph_.rpoBegin()); block != graph_.rpoEnd(); block++) {
        for (MDefinitionIterator iter(*block); iter; iter++) {
            JS_ASSERT(!iter->isInWorklist());
            JS_ASSERT_IF(iter->valueNumber() == 0,
                         !iter->hasDefUses() && (!iter->isMovable() || iter->isEffectful()));
        }
    }
#endif
    return true;
}