-
- // Move the original header to the bottom of the loop, where it now more
- // naturally belongs. This isn't necessary for correctness, and CodeGen can
- // usually reorder blocks on its own to fix things like this up, but it's
- // still nice to keep the IR readable.
- //
- // The original header should have only one predecessor at this point, since
- // we checked that the loop had a proper preheader and unique backedge before
- // we started.
- assert(OrigHeader->getSinglePredecessor() &&
- "Original loop header has too many predecessors after loop rotation!");
- OrigHeader->moveAfter(OrigHeader->getSinglePredecessor());
-
- // Also, since this original header only has one predecessor, zap its
- // PHI nodes, which are now trivial.
- FoldSingleEntryPHINodes(OrigHeader);
-
- // TODO: We could just go ahead and merge OrigHeader into its predecessor
- // at this point, if we don't mind updating dominator info.
-
- // Establish a new preheader, update dominators, etc.
- preserveCanonicalLoopForm(L, OrigHeader, OrigPreHeader, OrigLatch,
- NewHeader, Exit);
-
- ++NumRotated;
- return true;
-}
-
-
-/// After loop rotation, loop pre-header has multiple sucessors.
-/// Insert one forwarding basic block to ensure that loop pre-header
-/// has only one successor.
-void LoopRotate::preserveCanonicalLoopForm(Loop *L, BasicBlock *OrigHeader,
- BasicBlock *OrigPreHeader,
- BasicBlock *OrigLatch,
- BasicBlock *NewHeader,
- BasicBlock *Exit) {
-
- // Right now original pre-header has two successors, new header and
- // exit block. Insert new block between original pre-header and
- // new header such that loop's new pre-header has only one successor.
- BasicBlock *NewPreHeader =
- BasicBlock::Create(OrigHeader->getContext(), "bb.nph",
- OrigHeader->getParent(), NewHeader);
- if (Loop *PL = LI->getLoopFor(OrigPreHeader))
- PL->addBasicBlockToLoop(NewPreHeader, LI->getBase());
- BranchInst::Create(NewHeader, NewPreHeader);
-
- BranchInst *OrigPH_BI = cast<BranchInst>(OrigPreHeader->getTerminator());
- if (OrigPH_BI->getSuccessor(0) == NewHeader)
- OrigPH_BI->setSuccessor(0, NewPreHeader);
- else {
- assert(OrigPH_BI->getSuccessor(1) == NewHeader &&
- "Unexpected original pre-header terminator");
- OrigPH_BI->setSuccessor(1, NewPreHeader);
- }
-
- PHINode *PN;
- for (BasicBlock::iterator I = NewHeader->begin();
- (PN = dyn_cast<PHINode>(I)); ++I) {
- int index = PN->getBasicBlockIndex(OrigPreHeader);
- assert(index != -1 && "Expected incoming value from Original PreHeader");
- PN->setIncomingBlock(index, NewPreHeader);
- assert(PN->getBasicBlockIndex(OrigPreHeader) == -1 &&
- "Expected only one incoming value from Original PreHeader");
- }
-
- if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) {
- DT->addNewBlock(NewPreHeader, OrigPreHeader);
- DT->changeImmediateDominator(L->getHeader(), NewPreHeader);
- DT->changeImmediateDominator(Exit, OrigPreHeader);
- for (Loop::block_iterator BI = L->block_begin(), BE = L->block_end();
- BI != BE; ++BI) {
- BasicBlock *B = *BI;
- if (L->getHeader() != B) {
- DomTreeNode *Node = DT->getNode(B);
- if (Node && Node->getBlock() == OrigHeader)
- DT->changeImmediateDominator(*BI, L->getHeader());
- }
- }
- DT->changeImmediateDominator(OrigHeader, OrigLatch);
- }
-
- if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>()) {
- // New Preheader's dominance frontier is Exit block.
- DominanceFrontier::DomSetType NewPHSet;
- NewPHSet.insert(Exit);
- DF->addBasicBlock(NewPreHeader, NewPHSet);
-
- // New Header's dominance frontier now includes itself and Exit block
- DominanceFrontier::iterator HeadI = DF->find(L->getHeader());
- if (HeadI != DF->end()) {
- DominanceFrontier::DomSetType & HeaderSet = HeadI->second;
- HeaderSet.clear();
- HeaderSet.insert(L->getHeader());
- HeaderSet.insert(Exit);
- } else {
- DominanceFrontier::DomSetType HeaderSet;
- HeaderSet.insert(L->getHeader());
- HeaderSet.insert(Exit);
- DF->addBasicBlock(L->getHeader(), HeaderSet);
+ assert(L->getHeader() == NewHeader && "Latch block is our new header");
+
+
+ // At this point, we've finished our major CFG changes. As part of cloning
+ // the loop into the preheader we've simplified instructions and the
+ // duplicated conditional branch may now be branching on a constant. If it is
+ // branching on a constant and if that constant means that we enter the loop,
+ // then we fold away the cond branch to an uncond branch. This simplifies the
+ // loop in cases important for nested loops, and it also means we don't have
+ // to split as many edges.
+ BranchInst *PHBI = cast<BranchInst>(OrigPreheader->getTerminator());
+ assert(PHBI->isConditional() && "Should be clone of BI condbr!");
+ if (!isa<ConstantInt>(PHBI->getCondition()) ||
+ PHBI->getSuccessor(cast<ConstantInt>(PHBI->getCondition())->isZero())
+ != NewHeader) {
+ // The conditional branch can't be folded, handle the general case.
+ // Update DominatorTree to reflect the CFG change we just made. Then split
+ // edges as necessary to preserve LoopSimplify form.
+ if (DT) {
+ // Everything that was dominated by the old loop header is now dominated
+ // by the original loop preheader. Conceptually the header was merged
+ // into the preheader, even though we reuse the actual block as a new
+ // loop latch.
+ DomTreeNode *OrigHeaderNode = DT->getNode(OrigHeader);
+ SmallVector<DomTreeNode *, 8> HeaderChildren(OrigHeaderNode->begin(),
+ OrigHeaderNode->end());
+ DomTreeNode *OrigPreheaderNode = DT->getNode(OrigPreheader);
+ for (unsigned I = 0, E = HeaderChildren.size(); I != E; ++I)
+ DT->changeImmediateDominator(HeaderChildren[I], OrigPreheaderNode);
+
+ assert(DT->getNode(Exit)->getIDom() == OrigPreheaderNode);
+ assert(DT->getNode(NewHeader)->getIDom() == OrigPreheaderNode);
+
+ // Update OrigHeader to be dominated by the new header block.
+ DT->changeImmediateDominator(OrigHeader, OrigLatch);