C++ FrameEntry::setNotCopied方法代码示例

void
FrameState::storeLocal(uint32 n, bool popGuaranteed, bool typeChange)
{
    FrameEntry *localFe = getLocal(n);
    bool cacheable = !eval && !escaping[n];

    if (!popGuaranteed && !cacheable) {
        JS_ASSERT_IF(base[localIndex(n)] && (!eval || n < script->nfixed),
                     entries[localIndex(n)].type.inMemory() &&
                     entries[localIndex(n)].data.inMemory());
        Address local(JSFrameReg, sizeof(JSStackFrame) + n * sizeof(Value));
        storeTo(peek(-1), local, false);
        forgetAllRegs(getLocal(n));
        localFe->resetSynced();
        return;
    }

    bool wasSynced = localFe->type.synced();

    /* Detect something like (x = x) which is a no-op. */
    FrameEntry *top = peek(-1);
    if (top->isCopy() && top->copyOf() == localFe) {
        JS_ASSERT(localFe->isCopied());
        return;
    }

    /* Completely invalidate the local variable. */
    if (localFe->isCopied()) {
        uncopy(localFe);
        if (!localFe->isCopied())
            forgetAllRegs(localFe);
    } else {
        forgetAllRegs(localFe);
    }

    localFe->resetUnsynced();

    /* Constants are easy to propagate. */
    if (top->isConstant()) {
        localFe->setCopyOf(NULL);
        localFe->setNotCopied();
        localFe->setConstant(Jsvalify(top->getValue()));
        return;
    }

    /*
     * When dealing with copies, there are two important invariants:
     *
     * 1) The backing store precedes all copies in the tracker.
     * 2) The backing store of a local is never a stack slot, UNLESS the local
     *    variable itself is a stack slot (blocks) that precedes the stack
     *    slot.
     *
     * If the top is a copy, and the second condition holds true, the local
     * can be rewritten as a copy of the original backing slot. If the first
     * condition does not hold, force it to hold by swapping in-place.
     */
    FrameEntry *backing = top;
    if (top->isCopy()) {
        backing = top->copyOf();
        JS_ASSERT(backing->trackerIndex() < top->trackerIndex());

        uint32 backingIndex = indexOfFe(backing);
        uint32 tol = uint32(spBase - base);
        if (backingIndex < tol || backingIndex < localIndex(n)) {
            /* local.idx < backing.idx means local cannot be a copy yet */
            if (localFe->trackerIndex() < backing->trackerIndex())
                swapInTracker(backing, localFe);
            localFe->setNotCopied();
            localFe->setCopyOf(backing);
            if (backing->isTypeKnown())
                localFe->setType(backing->getKnownType());
            else
                localFe->type.invalidate();
            localFe->data.invalidate();
            localFe->isNumber = backing->isNumber;
            return;
        }

        /*
         * If control flow lands here, then there was a bytecode sequence like
         *
         *  ENTERBLOCK 2
         *  GETLOCAL 1
         *  SETLOCAL 0
         *
         * The problem is slot N can't be backed by M if M could be popped
         * before N. We want a guarantee that when we pop M, even if it was
         * copied, it has no outstanding copies.
         * 
         * Because of |let| expressions, it's kind of hard to really know
         * whether a region on the stack will be popped all at once. Bleh!
         *
         * This should be rare except in browser code (and maybe even then),
         * but even so there's a quick workaround. We take all copies of the
         * backing fe, and redirect them to be copies of the destination.
         */
        FrameEntry *tos = tosFe();
        for (uint32 i = backing->trackerIndex() + 1; i < tracker.nentries; i++) {
            FrameEntry *fe = tracker[i];
//.........这里部分代码省略.........

FrameEntry *
FrameState::uncopy(FrameEntry *original)
{
    JS_ASSERT(original->isCopied());

    /*
     * Copies have two critical invariants:
     *  1) The backing store precedes all copies in the tracker.
     *  2) The backing store of a copy cannot be popped from the stack
     *     while the copy is still live.
     *
     * Maintaining this invariant iteratively is kind of hard, so we choose
     * the "lowest" copy in the frame up-front.
     *
     * For example, if the stack is:
     *    [A, B, C, D]
     * And the tracker has:
     *    [A, D, C, B]
     *
     * If B, C, and D are copies of A - we will walk the tracker to the end
     * and select D, not B (see bug 583684).
     */
    uint32 firstCopy = InvalidIndex;
    FrameEntry *tos = tosFe();
    FrameEntry *bestFe = NULL;
    uint32 ncopies = 0;
    for (uint32 i = 0; i < tracker.nentries; i++) {
        FrameEntry *fe = tracker[i];
        if (fe >= tos)
            continue;
        if (fe->isCopy() && fe->copyOf() == original) {
            if (firstCopy == InvalidIndex) {
                firstCopy = i;
                bestFe = fe;
            } else if (fe < bestFe) {
                bestFe = fe;
            }
            ncopies++;
        }
    }

    if (!ncopies) {
        JS_ASSERT(firstCopy == InvalidIndex);
        JS_ASSERT(!bestFe);
        original->copied = false;
        return NULL;
    }

    JS_ASSERT(firstCopy != InvalidIndex);
    JS_ASSERT(bestFe);

    /* Mark all extra copies as copies of the new backing index. */
    bestFe->setCopyOf(NULL);
    if (ncopies > 1) {
        bestFe->setCopied();
        for (uint32 i = firstCopy; i < tracker.nentries; i++) {
            FrameEntry *other = tracker[i];
            if (other >= tos || other == bestFe)
                continue;

            /* The original must be tracked before copies. */
            JS_ASSERT(other != original);

            if (!other->isCopy() || other->copyOf() != original)
                continue;

            other->setCopyOf(bestFe);

            /*
             * This is safe even though we're mutating during iteration. There
             * are two cases. The first is that both indexes are <= i, and :.
             * will never be observed. The other case is we're placing the
             * other FE such that it will be observed later. Luckily, copyOf()
             * will return != original, so nothing will happen.
             */
            if (other->trackerIndex() < bestFe->trackerIndex())
                swapInTracker(bestFe, other);
        }
    } else {
        bestFe->setNotCopied();
    }

    FrameEntry *fe = bestFe;

    /*
     * Switch the new backing store to the old backing store. During
     * this process we also necessarily make sure the copy can be
     * synced.
     */
    if (!original->isTypeKnown()) {
        /*
         * If the copy is unsynced, and the original is in memory,
         * give the original a register. We do this below too; it's
         * okay if it's spilled.
         */
        if (original->type.inMemory() && !fe->type.synced())
            tempRegForType(original);
        fe->type.inherit(original->type);
        if (fe->type.inRegister())
            moveOwnership(fe->type.reg(), fe);
//.........这里部分代码省略.........