| //===- 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; |
| } |