//===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // BreakCriticalEdges pass - Break all of the critical edges in the CFG by // inserting a dummy basic block. This pass may be "required" by passes that // cannot deal with critical edges. For this usage, the structure type is // forward declared. This pass obviously invalidates the CFG, but can update // dominator trees. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar.h" #include "llvm/ADT/SmallVector.h" #include "llvm/ADT/Statistic.h" #include "llvm/Analysis/AliasAnalysis.h" #include "llvm/Analysis/CFG.h" #include "llvm/Analysis/LoopInfo.h" #include "llvm/IR/CFG.h" #include "llvm/IR/Dominators.h" #include "llvm/IR/Function.h" #include "llvm/IR/Instructions.h" #include "llvm/IR/Type.h" #include "llvm/Support/ErrorHandling.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" using namespace llvm; #define DEBUG_TYPE "break-crit-edges" STATISTIC(NumBroken, "Number of blocks inserted"); namespace { struct BreakCriticalEdges : public FunctionPass { static char ID; // Pass identification, replacement for typeid BreakCriticalEdges() : FunctionPass(ID) { initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry()); } bool runOnFunction(Function &F) override { auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>(); auto *DT = DTWP ? &DTWP->getDomTree() : nullptr; auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>(); auto *LI = LIWP ? &LIWP->getLoopInfo() : nullptr; unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI)); NumBroken += N; return N > 0; } void getAnalysisUsage(AnalysisUsage &AU) const override { AU.addPreserved<DominatorTreeWrapperPass>(); AU.addPreserved<LoopInfoWrapperPass>(); // No loop canonicalization guarantees are broken by this pass. AU.addPreservedID(LoopSimplifyID); } }; } char BreakCriticalEdges::ID = 0; INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges", "Break critical edges in CFG", false, false) // Publicly exposed interface to pass... char &llvm::BreakCriticalEdgesID = BreakCriticalEdges::ID; FunctionPass *llvm::createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); } //===----------------------------------------------------------------------===// // Implementation of the external critical edge manipulation functions //===----------------------------------------------------------------------===// /// createPHIsForSplitLoopExit - When a loop exit edge is split, LCSSA form /// may require new PHIs in the new exit block. This function inserts the /// new PHIs, as needed. Preds is a list of preds inside the loop, SplitBB /// is the new loop exit block, and DestBB is the old loop exit, now the /// successor of SplitBB. static void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds, BasicBlock *SplitBB, BasicBlock *DestBB) { // SplitBB shouldn't have anything non-trivial in it yet. assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() || SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!"); // For each PHI in the destination block. for (BasicBlock::iterator I = DestBB->begin(); PHINode *PN = dyn_cast<PHINode>(I); ++I) { unsigned Idx = PN->getBasicBlockIndex(SplitBB); Value *V = PN->getIncomingValue(Idx); // If the input is a PHI which already satisfies LCSSA, don't create // a new one. if (const PHINode *VP = dyn_cast<PHINode>(V)) if (VP->getParent() == SplitBB) continue; // Otherwise a new PHI is needed. Create one and populate it. PHINode *NewPN = PHINode::Create(PN->getType(), Preds.size(), "split", SplitBB->isLandingPad() ? SplitBB->begin() : SplitBB->getTerminator()); for (unsigned i = 0, e = Preds.size(); i != e; ++i) NewPN->addIncoming(V, Preds[i]); // Update the original PHI. PN->setIncomingValue(Idx, NewPN); } } /// SplitCriticalEdge - If this edge is a critical edge, insert a new node to /// split the critical edge. This will update DominatorTree information if it /// is available, thus calling this pass will not invalidate either of them. /// This returns the new block if the edge was split, null otherwise. /// /// If MergeIdenticalEdges is true (not the default), *all* edges from TI to the /// specified successor will be merged into the same critical edge block. /// This is most commonly interesting with switch instructions, which may /// have many edges to any one destination. This ensures that all edges to that /// dest go to one block instead of each going to a different block, but isn't /// the standard definition of a "critical edge". /// /// It is invalid to call this function on a critical edge that starts at an /// IndirectBrInst. Splitting these edges will almost always create an invalid /// program because the address of the new block won't be the one that is jumped /// to. /// BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, const CriticalEdgeSplittingOptions &Options) { if (!isCriticalEdge(TI, SuccNum, Options.MergeIdenticalEdges)) return nullptr; assert(!isa<IndirectBrInst>(TI) && "Cannot split critical edge from IndirectBrInst"); BasicBlock *TIBB = TI->getParent(); BasicBlock *DestBB = TI->getSuccessor(SuccNum); // Splitting the critical edge to a landing pad block is non-trivial. Don't do // it in this generic function. if (DestBB->isLandingPad()) return nullptr; // Create a new basic block, linking it into the CFG. BasicBlock *NewBB = BasicBlock::Create(TI->getContext(), TIBB->getName() + "." + DestBB->getName() + "_crit_edge"); // Create our unconditional branch. BranchInst *NewBI = BranchInst::Create(DestBB, NewBB); NewBI->setDebugLoc(TI->getDebugLoc()); // Branch to the new block, breaking the edge. TI->setSuccessor(SuccNum, NewBB); // Insert the block into the function... right after the block TI lives in. Function &F = *TIBB->getParent(); Function::iterator FBBI = TIBB; F.getBasicBlockList().insert(++FBBI, NewBB); // If there are any PHI nodes in DestBB, we need to update them so that they // merge incoming values from NewBB instead of from TIBB. { unsigned BBIdx = 0; for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) { // We no longer enter through TIBB, now we come in through NewBB. // Revector exactly one entry in the PHI node that used to come from // TIBB to come from NewBB. PHINode *PN = cast<PHINode>(I); // Reuse the previous value of BBIdx if it lines up. In cases where we // have multiple phi nodes with *lots* of predecessors, this is a speed // win because we don't have to scan the PHI looking for TIBB. This // happens because the BB list of PHI nodes are usually in the same // order. if (PN->getIncomingBlock(BBIdx) != TIBB) BBIdx = PN->getBasicBlockIndex(TIBB); PN->setIncomingBlock(BBIdx, NewBB); } } // If there are any other edges from TIBB to DestBB, update those to go // through the split block, making those edges non-critical as well (and // reducing the number of phi entries in the DestBB if relevant). if (Options.MergeIdenticalEdges) { for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) { if (TI->getSuccessor(i) != DestBB) continue; // Remove an entry for TIBB from DestBB phi nodes. DestBB->removePredecessor(TIBB, Options.DontDeleteUselessPHIs); // We found another edge to DestBB, go to NewBB instead. TI->setSuccessor(i, NewBB); } } // If we have nothing to update, just return. auto *AA = Options.AA; auto *DT = Options.DT; auto *LI = Options.LI; if (!DT && !LI) return NewBB; // Now update analysis information. Since the only predecessor of NewBB is // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate // anything, as there are other successors of DestBB. However, if all other // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a // loop header) then NewBB dominates DestBB. SmallVector<BasicBlock*, 8> OtherPreds; // If there is a PHI in the block, loop over predecessors with it, which is // faster than iterating pred_begin/end. if (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) { for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) if (PN->getIncomingBlock(i) != NewBB) OtherPreds.push_back(PN->getIncomingBlock(i)); } else { for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I) { BasicBlock *P = *I; if (P != NewBB) OtherPreds.push_back(P); } } bool NewBBDominatesDestBB = true; // Should we update DominatorTree information? if (DT) { DomTreeNode *TINode = DT->getNode(TIBB); // The new block is not the immediate dominator for any other nodes, but // TINode is the immediate dominator for the new node. // if (TINode) { // Don't break unreachable code! DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB); DomTreeNode *DestBBNode = nullptr; // If NewBBDominatesDestBB hasn't been computed yet, do so with DT. if (!OtherPreds.empty()) { DestBBNode = DT->getNode(DestBB); while (!OtherPreds.empty() && NewBBDominatesDestBB) { if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back())) NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode); OtherPreds.pop_back(); } OtherPreds.clear(); } // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it // doesn't dominate anything. if (NewBBDominatesDestBB) { if (!DestBBNode) DestBBNode = DT->getNode(DestBB); DT->changeImmediateDominator(DestBBNode, NewBBNode); } } } // Update LoopInfo if it is around. if (LI) { if (Loop *TIL = LI->getLoopFor(TIBB)) { // If one or the other blocks were not in a loop, the new block is not // either, and thus LI doesn't need to be updated. if (Loop *DestLoop = LI->getLoopFor(DestBB)) { if (TIL == DestLoop) { // Both in the same loop, the NewBB joins loop. DestLoop->addBasicBlockToLoop(NewBB, *LI); } else if (TIL->contains(DestLoop)) { // Edge from an outer loop to an inner loop. Add to the outer loop. TIL->addBasicBlockToLoop(NewBB, *LI); } else if (DestLoop->contains(TIL)) { // Edge from an inner loop to an outer loop. Add to the outer loop. DestLoop->addBasicBlockToLoop(NewBB, *LI); } else { // Edge from two loops with no containment relation. Because these // are natural loops, we know that the destination block must be the // header of its loop (adding a branch into a loop elsewhere would // create an irreducible loop). assert(DestLoop->getHeader() == DestBB && "Should not create irreducible loops!"); if (Loop *P = DestLoop->getParentLoop()) P->addBasicBlockToLoop(NewBB, *LI); } } // If TIBB is in a loop and DestBB is outside of that loop, we may need // to update LoopSimplify form and LCSSA form. if (!TIL->contains(DestBB)) { assert(!TIL->contains(NewBB) && "Split point for loop exit is contained in loop!"); // Update LCSSA form in the newly created exit block. if (Options.PreserveLCSSA) { createPHIsForSplitLoopExit(TIBB, NewBB, DestBB); } // The only that we can break LoopSimplify form by splitting a critical // edge is if after the split there exists some edge from TIL to DestBB // *and* the only edge into DestBB from outside of TIL is that of // NewBB. If the first isn't true, then LoopSimplify still holds, NewBB // is the new exit block and it has no non-loop predecessors. If the // second isn't true, then DestBB was not in LoopSimplify form prior to // the split as it had a non-loop predecessor. In both of these cases, // the predecessor must be directly in TIL, not in a subloop, or again // LoopSimplify doesn't hold. SmallVector<BasicBlock *, 4> LoopPreds; for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I) { BasicBlock *P = *I; if (P == NewBB) continue; // The new block is known. if (LI->getLoopFor(P) != TIL) { // No need to re-simplify, it wasn't to start with. LoopPreds.clear(); break; } LoopPreds.push_back(P); } if (!LoopPreds.empty()) { assert(!DestBB->isLandingPad() && "We don't split edges to landing pads!"); BasicBlock *NewExitBB = SplitBlockPredecessors( DestBB, LoopPreds, "split", AA, DT, LI, Options.PreserveLCSSA); if (Options.PreserveLCSSA) createPHIsForSplitLoopExit(LoopPreds, NewExitBB, DestBB); } } } } return NewBB; }