// A loop is about to be made reachable only through an OSR entry into one of
// its nested loops. Fix everything up.
bool
ValueNumberer::fixupOSROnlyLoop(MBasicBlock* block, MBasicBlock* backedge)
{
// Create an empty and unreachable(!) block which jumps to |block|. This
// allows |block| to remain marked as a loop header, so we don't have to
// worry about moving a different block into place as the new loop header,
// which is hard, especially if the OSR is into a nested loop. Doing all
// that would produce slightly more optimal code, but this is so
// extraordinarily rare that it isn't worth the complexity.
MBasicBlock* fake = MBasicBlock::New(graph_, block->info(), nullptr, MBasicBlock::NORMAL);
if (fake == nullptr)
return false;
graph_.insertBlockBefore(block, fake);
fake->setImmediateDominator(fake);
fake->addNumDominated(1);
fake->setDomIndex(fake->id());
fake->setUnreachable();
// Create zero-input phis to use as inputs for any phis in |block|.
// Again, this is a little odd, but it's the least-odd thing we can do
// without significant complexity.
for (MPhiIterator iter(block->phisBegin()), end(block->phisEnd()); iter != end; ++iter) {
MPhi* phi = *iter;
MPhi* fakePhi = MPhi::New(graph_.alloc(), phi->type());
fake->addPhi(fakePhi);
if (!phi->addInputSlow(fakePhi))
return false;
}
fake->end(MGoto::New(graph_.alloc(), block));
if (!block->addPredecessorWithoutPhis(fake))
return false;
// Restore |backedge| as |block|'s loop backedge.
block->clearLoopHeader();
block->setLoopHeader(backedge);
JitSpew(JitSpew_GVN, " Created fake block%u", fake->id());
hasOSRFixups_ = true;
return true;
}
