static void
ComputeImmediateDominators(MIRGraph &graph)
{
// The default start block is a root and therefore only self-dominates.
MBasicBlock *startBlock = *graph.begin();
startBlock->setImmediateDominator(startBlock);
// Any OSR block is a root and therefore only self-dominates.
MBasicBlock *osrBlock = graph.osrBlock();
if (osrBlock)
osrBlock->setImmediateDominator(osrBlock);
bool changed = true;
while (changed) {
changed = false;
ReversePostorderIterator block = graph.rpoBegin();
// For each block in RPO, intersect all dominators.
for (; block != graph.rpoEnd(); block++) {
// If a node has once been found to have no exclusive dominator,
// it will never have an exclusive dominator, so it may be skipped.
if (block->immediateDominator() == *block)
continue;
MBasicBlock *newIdom = block->getPredecessor(0);
// Find the first common dominator.
for (size_t i = 1; i < block->numPredecessors(); i++) {
MBasicBlock *pred = block->getPredecessor(i);
if (pred->immediateDominator() != NULL)
newIdom = IntersectDominators(pred, newIdom);
// If there is no common dominator, the block self-dominates.
if (newIdom == NULL) {
block->setImmediateDominator(*block);
changed = true;
break;
}
}
if (newIdom && block->immediateDominator() != newIdom) {
block->setImmediateDominator(newIdom);
changed = true;
}
}
}
#ifdef DEBUG
// Assert that all blocks have dominator information.
for (MBasicBlockIterator block(graph.begin()); block != graph.end(); block++) {
JS_ASSERT(block->immediateDominator() != NULL);
}
#endif
}
void
ion::AssertGraphCoherency(MIRGraph &graph)
{
#ifdef DEBUG
// Assert successor and predecessor list coherency.
uint32_t count = 0;
for (MBasicBlockIterator block(graph.begin()); block != graph.end(); block++) {
count++;
for (size_t i = 0; i < block->numSuccessors(); i++)
JS_ASSERT(CheckSuccessorImpliesPredecessor(*block, block->getSuccessor(i)));
for (size_t i = 0; i < block->numPredecessors(); i++)
JS_ASSERT(CheckPredecessorImpliesSuccessor(*block, block->getPredecessor(i)));
for (MInstructionIterator ins = block->begin(); ins != block->end(); ins++) {
for (uint32_t i = 0; i < ins->numOperands(); i++)
JS_ASSERT(CheckMarkedAsUse(*ins, ins->getOperand(i)));
}
}
JS_ASSERT(graph.numBlocks() == count);
AssertReversePostOrder(graph);
#endif
}
void
ion::AssertExtendedGraphCoherency(MIRGraph &graph)
{
// Checks the basic GraphCoherency but also other conditions that
// do not hold immediately (such as the fact that critical edges
// are split)
#ifdef DEBUG
AssertGraphCoherency(graph);
uint32_t idx = 0;
for (MBasicBlockIterator block(graph.begin()); block != graph.end(); block++) {
JS_ASSERT(block->id() == idx++);
// No critical edges:
if (block->numSuccessors() > 1)
for (size_t i = 0; i < block->numSuccessors(); i++)
JS_ASSERT(block->getSuccessor(i)->numPredecessors() == 1);
if (block->isLoopHeader()) {
JS_ASSERT(block->numPredecessors() == 2);
MBasicBlock *backedge = block->getPredecessor(1);
JS_ASSERT(backedge->id() >= block->id());
JS_ASSERT(backedge->numSuccessors() == 1);
JS_ASSERT(backedge->getSuccessor(0) == *block);
}
if (!block->phisEmpty()) {
for (size_t i = 0; i < block->numPredecessors(); i++) {
MBasicBlock *pred = block->getPredecessor(i);
JS_ASSERT(pred->successorWithPhis() == *block);
JS_ASSERT(pred->positionInPhiSuccessor() == i);
}
}
uint32_t successorWithPhis = 0;
for (size_t i = 0; i < block->numSuccessors(); i++)
if (!block->getSuccessor(i)->phisEmpty())
successorWithPhis++;
JS_ASSERT(successorWithPhis <= 1);
JS_ASSERT_IF(successorWithPhis, block->successorWithPhis() != NULL);
// I'd like to assert this, but it's not necc. true. Sometimes we set this
// flag to non-NULL just because a successor has multiple preds, even if it
// does not actually have any phis.
//
// JS_ASSERT_IF(!successorWithPhis, block->successorWithPhis() == NULL);
}
#endif
}
bool
ion::BuildPhiReverseMapping(MIRGraph &graph)
{
// Build a mapping such that given a basic block, whose successor has one or
// more phis, we can find our specific input to that phi. To make this fast
// mapping work we rely on a specific property of our structured control
// flow graph: For a block with phis, its predecessors each have only one
// successor with phis. Consider each case:
// * Blocks with less than two predecessors cannot have phis.
// * Breaks. A break always has exactly one successor, and the break
// catch block has exactly one predecessor for each break, as
// well as a final predecessor for the actual loop exit.
// * Continues. A continue always has exactly one successor, and the
// continue catch block has exactly one predecessor for each
// continue, as well as a final predecessor for the actual
// loop continuation. The continue itself has exactly one
// successor.
// * An if. Each branch as exactly one predecessor.
// * A switch. Each branch has exactly one predecessor.
// * Loop tail. A new block is always created for the exit, and if a
// break statement is present, the exit block will forward
// directly to the break block.
for (MBasicBlockIterator block(graph.begin()); block != graph.end(); block++) {
if (block->numPredecessors() < 2) {
JS_ASSERT(block->phisEmpty());
continue;
}
// Assert on the above.
for (size_t j = 0; j < block->numPredecessors(); j++) {
MBasicBlock *pred = block->getPredecessor(j);
#ifdef DEBUG
size_t numSuccessorsWithPhis = 0;
for (size_t k = 0; k < pred->numSuccessors(); k++) {
MBasicBlock *successor = pred->getSuccessor(k);
if (!successor->phisEmpty())
numSuccessorsWithPhis++;
}
JS_ASSERT(numSuccessorsWithPhis <= 1);
#endif
pred->setSuccessorWithPhis(*block, j);
}
}
return true;
}
// A critical edge is an edge which is neither its successor's only predecessor
// nor its predecessor's only successor. Critical edges must be split to
// prevent copy-insertion and code motion from affecting other edges.
bool
ion::SplitCriticalEdges(MIRGraph &graph)
{
for (MBasicBlockIterator block(graph.begin()); block != graph.end(); block++) {
if (block->numSuccessors() < 2)
continue;
for (size_t i = 0; i < block->numSuccessors(); i++) {
MBasicBlock *target = block->getSuccessor(i);
if (target->numPredecessors() < 2)
continue;
// Create a new block inheriting from the predecessor.
MBasicBlock *split = MBasicBlock::NewSplitEdge(graph, block->info(), *block);
split->setLoopDepth(block->loopDepth());
graph.insertBlockAfter(*block, split);
split->end(MGoto::New(target));
block->replaceSuccessor(i, split);
target->replacePredecessor(*block, split);
}
}
return true;
}
// A bounds check is considered redundant if it's dominated by another bounds
// check with the same length and the indexes differ by only a constant amount.
// In this case we eliminate the redundant bounds check and update the other one
// to cover the ranges of both checks.
//
// Bounds checks are added to a hash map and since the hash function ignores
// differences in constant offset, this offers a fast way to find redundant
// checks.
bool
ion::EliminateRedundantBoundsChecks(MIRGraph &graph)
{
BoundsCheckMap checks;
if (!checks.init())
return false;
// Stack for pre-order CFG traversal.
Vector<MBasicBlock *, 1, IonAllocPolicy> worklist;
// The index of the current block in the CFG traversal.
size_t index = 0;
// Add all self-dominating blocks to the worklist.
// This includes all roots. Order does not matter.
for (MBasicBlockIterator i(graph.begin()); i != graph.end(); i++) {
MBasicBlock *block = *i;
if (block->immediateDominator() == block) {
if (!worklist.append(block))
return false;
}
}
// Starting from each self-dominating block, traverse the CFG in pre-order.
while (!worklist.empty()) {
MBasicBlock *block = worklist.popCopy();
// Add all immediate dominators to the front of the worklist.
for (size_t i = 0; i < block->numImmediatelyDominatedBlocks(); i++) {
if (!worklist.append(block->getImmediatelyDominatedBlock(i)))
return false;
}
for (MDefinitionIterator iter(block); iter; ) {
if (!iter->isBoundsCheck()) {
iter++;
continue;
}
MBoundsCheck *check = iter->toBoundsCheck();
// Replace all uses of the bounds check with the actual index.
// This is (a) necessary, because we can coalesce two different
// bounds checks and would otherwise use the wrong index and
// (b) helps register allocation. Note that this is safe since
// no other pass after bounds check elimination moves instructions.
check->replaceAllUsesWith(check->index());
if (!check->isMovable()) {
iter++;
continue;
}
MBoundsCheck *dominating = FindDominatingBoundsCheck(checks, check, index);
if (!dominating)
return false;
if (dominating == check) {
// We didn't find a dominating bounds check.
iter++;
continue;
}
bool eliminated = false;
if (!TryEliminateBoundsCheck(dominating, check, &eliminated))
return false;
if (eliminated)
iter = check->block()->discardDefAt(iter);
else
iter++;
}
index++;
}
JS_ASSERT(index == graph.numBlocks());
return true;
}
bool
ion::BuildDominatorTree(MIRGraph &graph)
{
ComputeImmediateDominators(graph);
// Traversing through the graph in post-order means that every use
// of a definition is visited before the def itself. Since a def
// dominates its uses, by the time we reach a particular
// block, we have processed all of its dominated children, so
// block->numDominated() is accurate.
for (PostorderIterator i(graph.poBegin()); i != graph.poEnd(); i++) {
MBasicBlock *child = *i;
MBasicBlock *parent = child->immediateDominator();
// If the block only self-dominates, it has no definite parent.
if (child == parent)
continue;
if (!parent->addImmediatelyDominatedBlock(child))
return false;
// An additional +1 for the child block.
parent->addNumDominated(child->numDominated() + 1);
}
#ifdef DEBUG
// If compiling with OSR, many blocks will self-dominate.
// Without OSR, there is only one root block which dominates all.
if (!graph.osrBlock())
JS_ASSERT(graph.begin()->numDominated() == graph.numBlocks() - 1);
#endif
// Now, iterate through the dominator tree and annotate every
// block with its index in the pre-order traversal of the
// dominator tree.
Vector<MBasicBlock *, 1, IonAllocPolicy> worklist;
// The index of the current block in the CFG traversal.
size_t index = 0;
// Add all self-dominating blocks to the worklist.
// This includes all roots. Order does not matter.
for (MBasicBlockIterator i(graph.begin()); i != graph.end(); i++) {
MBasicBlock *block = *i;
if (block->immediateDominator() == block) {
if (!worklist.append(block))
return false;
}
}
// Starting from each self-dominating block, traverse the CFG in pre-order.
while (!worklist.empty()) {
MBasicBlock *block = worklist.popCopy();
block->setDomIndex(index);
for (size_t i = 0; i < block->numImmediatelyDominatedBlocks(); i++) {
if (!worklist.append(block->getImmediatelyDominatedBlock(i)))
return false;
}
index++;
}
return true;
}
// Eliminate checks which are redundant given each other or other instructions.
//
// A type barrier is considered redundant if all missing types have been tested
// for by earlier control instructions.
//
// A bounds check is considered redundant if it's dominated by another bounds
// check with the same length and the indexes differ by only a constant amount.
// In this case we eliminate the redundant bounds check and update the other one
// to cover the ranges of both checks.
//
// Bounds checks are added to a hash map and since the hash function ignores
// differences in constant offset, this offers a fast way to find redundant
// checks.
bool
ion::EliminateRedundantChecks(MIRGraph &graph)
{
BoundsCheckMap checks;
if (!checks.init())
return false;
// Stack for pre-order CFG traversal.
Vector<MBasicBlock *, 1, IonAllocPolicy> worklist;
// The index of the current block in the CFG traversal.
size_t index = 0;
// Add all self-dominating blocks to the worklist.
// This includes all roots. Order does not matter.
for (MBasicBlockIterator i(graph.begin()); i != graph.end(); i++) {
MBasicBlock *block = *i;
if (block->immediateDominator() == block) {
if (!worklist.append(block))
return false;
}
}
// Starting from each self-dominating block, traverse the CFG in pre-order.
while (!worklist.empty()) {
MBasicBlock *block = worklist.popCopy();
// Add all immediate dominators to the front of the worklist.
for (size_t i = 0; i < block->numImmediatelyDominatedBlocks(); i++) {
if (!worklist.append(block->getImmediatelyDominatedBlock(i)))
return false;
}
for (MDefinitionIterator iter(block); iter; ) {
bool eliminated = false;
if (iter->isBoundsCheck()) {
if (!TryEliminateBoundsCheck(checks, index, iter->toBoundsCheck(), &eliminated))
return false;
} else if (iter->isTypeBarrier()) {
if (!TryEliminateTypeBarrier(iter->toTypeBarrier(), &eliminated))
return false;
} else if (iter->isConvertElementsToDoubles()) {
// Now that code motion passes have finished, replace any
// ConvertElementsToDoubles with the actual elements.
MConvertElementsToDoubles *ins = iter->toConvertElementsToDoubles();
ins->replaceAllUsesWith(ins->elements());
}
if (eliminated)
iter = block->discardDefAt(iter);
else
iter++;
}
index++;
}
JS_ASSERT(index == graph.numBlocks());
return true;
}
