void
RangeAnalysis::analyzeLoop(MBasicBlock *header)
{
// Try to compute an upper bound on the number of times the loop backedge
// will be taken. Look for tests that dominate the backedge and which have
// an edge leaving the loop body.
MBasicBlock *backedge = header->backedge();
// Ignore trivial infinite loops.
if (backedge == header)
return;
markBlocksInLoopBody(header, backedge);
LoopIterationBound *iterationBound = NULL;
MBasicBlock *block = backedge;
do {
BranchDirection direction;
MTest *branch = block->immediateDominatorBranch(&direction);
if (block == block->immediateDominator())
break;
block = block->immediateDominator();
if (branch) {
direction = NegateBranchDirection(direction);
MBasicBlock *otherBlock = branch->branchSuccessor(direction);
if (!otherBlock->isMarked()) {
iterationBound =
analyzeLoopIterationCount(header, branch, direction);
if (iterationBound)
break;
}
}
} while (block != header);
if (!iterationBound) {
graph_.unmarkBlocks();
return;
}
#ifdef DEBUG
if (IonSpewEnabled(IonSpew_Range)) {
Sprinter sp(GetIonContext()->cx);
sp.init();
iterationBound->sum.print(sp);
IonSpew(IonSpew_Range, "computed symbolic bound on backedges: %s",
sp.string());
}
#endif
// Try to compute symbolic bounds for the phi nodes at the head of this
// loop, expressed in terms of the iteration bound just computed.
for (MDefinitionIterator iter(header); iter; iter++) {
MDefinition *def = *iter;
if (def->isPhi())
analyzeLoopPhi(header, iterationBound, def->toPhi());
}
// Try to hoist any bounds checks from the loop using symbolic bounds.
Vector<MBoundsCheck *, 0, IonAllocPolicy> hoistedChecks;
for (ReversePostorderIterator iter(graph_.rpoBegin()); iter != graph_.rpoEnd(); iter++) {
MBasicBlock *block = *iter;
if (!block->isMarked())
continue;
for (MDefinitionIterator iter(block); iter; iter++) {
MDefinition *def = *iter;
if (def->isBoundsCheck() && def->isMovable()) {
if (tryHoistBoundsCheck(header, def->toBoundsCheck()))
hoistedChecks.append(def->toBoundsCheck());
}
}
}
// Note: replace all uses of the original bounds check with the
// actual index. This is usually done during bounds check elimination,
// but in this case it's safe to do it here since the load/store is
// definitely not loop-invariant, so we will never move it before
// one of the bounds checks we just added.
for (size_t i = 0; i < hoistedChecks.length(); i++) {
MBoundsCheck *ins = hoistedChecks[i];
ins->replaceAllUsesWith(ins->index());
ins->block()->discard(ins);
}
graph_.unmarkBlocks();
}
