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Diffstat (limited to 'contrib/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp')
| -rw-r--r-- | contrib/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp | 438 |
1 files changed, 438 insertions, 0 deletions
diff --git a/contrib/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp b/contrib/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp new file mode 100644 index 0000000..8c25ad1 --- /dev/null +++ b/contrib/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp @@ -0,0 +1,438 @@ +//===- 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 +// forward dominator (set, immediate dominators, tree, and frontier) +// information. +// +//===----------------------------------------------------------------------===// + +#define DEBUG_TYPE "break-crit-edges" +#include "llvm/Transforms/Scalar.h" +#include "llvm/Transforms/Utils/BasicBlockUtils.h" +#include "llvm/Analysis/Dominators.h" +#include "llvm/Analysis/LoopInfo.h" +#include "llvm/Analysis/ProfileInfo.h" +#include "llvm/Function.h" +#include "llvm/Instructions.h" +#include "llvm/Type.h" +#include "llvm/Support/CFG.h" +#include "llvm/Support/ErrorHandling.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/ADT/Statistic.h" +using namespace llvm; + +STATISTIC(NumBroken, "Number of blocks inserted"); + +namespace { + struct BreakCriticalEdges : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + BreakCriticalEdges() : FunctionPass(&ID) {} + + virtual bool runOnFunction(Function &F); + + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.addPreserved<DominatorTree>(); + AU.addPreserved<DominanceFrontier>(); + AU.addPreserved<LoopInfo>(); + AU.addPreserved<ProfileInfo>(); + + // No loop canonicalization guarantees are broken by this pass. + AU.addPreservedID(LoopSimplifyID); + } + }; +} + +char BreakCriticalEdges::ID = 0; +static RegisterPass<BreakCriticalEdges> +X("break-crit-edges", "Break critical edges in CFG"); + +// Publically exposed interface to pass... +const PassInfo *const llvm::BreakCriticalEdgesID = &X; +FunctionPass *llvm::createBreakCriticalEdgesPass() { + return new BreakCriticalEdges(); +} + +// runOnFunction - Loop over all of the edges in the CFG, breaking critical +// edges as they are found. +// +bool BreakCriticalEdges::runOnFunction(Function &F) { + bool Changed = false; + for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) { + TerminatorInst *TI = I->getTerminator(); + if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI)) + for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) + if (SplitCriticalEdge(TI, i, this)) { + ++NumBroken; + Changed = true; + } + } + + return Changed; +} + +//===----------------------------------------------------------------------===// +// Implementation of the external critical edge manipulation functions +//===----------------------------------------------------------------------===// + +// isCriticalEdge - Return true if the specified edge is a critical edge. +// Critical edges are edges from a block with multiple successors to a block +// with multiple predecessors. +// +bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum, + bool AllowIdenticalEdges) { + assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!"); + if (TI->getNumSuccessors() == 1) return false; + + const BasicBlock *Dest = TI->getSuccessor(SuccNum); + const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest); + + // If there is more than one predecessor, this is a critical edge... + assert(I != E && "No preds, but we have an edge to the block?"); + const BasicBlock *FirstPred = *I; + ++I; // Skip one edge due to the incoming arc from TI. + if (!AllowIdenticalEdges) + return I != E; + + // If AllowIdenticalEdges is true, then we allow this edge to be considered + // non-critical iff all preds come from TI's block. + while (I != E) { + if (*I != FirstPred) + return true; + // Note: leave this as is until no one ever compiles with either gcc 4.0.1 + // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207 + E = pred_end(*I); + ++I; + } + return false; +} + +/// 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(SmallVectorImpl<BasicBlock *> &Preds, + BasicBlock *SplitBB, + BasicBlock *DestBB) { + // SplitBB shouldn't have anything non-trivial in it yet. + assert(SplitBB->getFirstNonPHI() == SplitBB->getTerminator() && + "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(), "split", + 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 and +/// DominatorFrontier 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, + Pass *P, bool MergeIdenticalEdges) { + if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return 0; + + assert(!isa<IndirectBrInst>(TI) && + "Cannot split critical edge from IndirectBrInst"); + + BasicBlock *TIBB = TI->getParent(); + BasicBlock *DestBB = TI->getSuccessor(SuccNum); + + // 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::Create(DestBB, NewBB); + + // 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. + if (PHINode *APHI = dyn_cast<PHINode>(DestBB->begin())) { + // This conceptually does: + // foreach (PHINode *PN in DestBB) + // PN->setIncomingBlock(PN->getIncomingBlock(TIBB), NewBB); + // but is optimized for two cases. + + if (APHI->getNumIncomingValues() <= 8) { // Small # preds case. + 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); + } + } else { + // However, the foreach loop is slow for blocks with lots of predecessors + // because PHINode::getIncomingBlock is O(n) in # preds. Instead, walk + // the user list of TIBB to find the PHI nodes. + SmallPtrSet<PHINode*, 16> UpdatedPHIs; + + for (Value::use_iterator UI = TIBB->use_begin(), E = TIBB->use_end(); + UI != E; ) { + Value::use_iterator Use = UI++; + if (PHINode *PN = dyn_cast<PHINode>(Use)) { + // Remove one entry from each PHI. + if (PN->getParent() == DestBB && UpdatedPHIs.insert(PN)) + PN->setOperand(Use.getOperandNo(), 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 (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); + + // We found another edge to DestBB, go to NewBB instead. + TI->setSuccessor(i, NewBB); + } + } + + + + // If we don't have a pass object, we can't update anything... + if (P == 0) return NewBB; + + DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>(); + DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>(); + LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>(); + ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>(); + + // If we have nothing to update, just return. + if (DT == 0 && DF == 0 && LI == 0 && PI == 0) + 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) + if (*I != NewBB) + OtherPreds.push_back(*I); + } + + 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 = 0; + + // 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); + } + } + } + + // Should we update DominanceFrontier information? + if (DF) { + // If NewBBDominatesDestBB hasn't been computed yet, do so with DF. + if (!OtherPreds.empty()) { + // FIXME: IMPLEMENT THIS! + llvm_unreachable("Requiring domfrontiers but not idom/domtree/domset." + " not implemented yet!"); + } + + // Since the new block is dominated by its only predecessor TIBB, + // it cannot be in any block's dominance frontier. If NewBB dominates + // DestBB, its dominance frontier is the same as DestBB's, otherwise it is + // just {DestBB}. + DominanceFrontier::DomSetType NewDFSet; + if (NewBBDominatesDestBB) { + DominanceFrontier::iterator I = DF->find(DestBB); + if (I != DF->end()) { + DF->addBasicBlock(NewBB, I->second); + + if (I->second.count(DestBB)) { + // However NewBB's frontier does not include DestBB. + DominanceFrontier::iterator NF = DF->find(NewBB); + DF->removeFromFrontier(NF, DestBB); + } + } + else + DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType()); + } else { + DominanceFrontier::DomSetType NewDFSet; + NewDFSet.insert(DestBB); + DF->addBasicBlock(NewBB, NewDFSet); + } + } + + // 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->getBase()); + } else if (TIL->contains(DestLoop)) { + // Edge from an outer loop to an inner loop. Add to the outer loop. + TIL->addBasicBlockToLoop(NewBB, LI->getBase()); + } else if (DestLoop->contains(TIL)) { + // Edge from an inner loop to an outer loop. Add to the outer loop. + DestLoop->addBasicBlockToLoop(NewBB, LI->getBase()); + } 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->getBase()); + } + } + // If TIBB is in a loop and DestBB is outside of that loop, split the + // other exit blocks of the loop that also have predecessors outside + // the loop, to maintain a LoopSimplify guarantee. + if (!TIL->contains(DestBB) && + P->mustPreserveAnalysisID(LoopSimplifyID)) { + assert(!TIL->contains(NewBB) && + "Split point for loop exit is contained in loop!"); + + // Update LCSSA form in the newly created exit block. + if (P->mustPreserveAnalysisID(LCSSAID)) { + SmallVector<BasicBlock *, 1> OrigPred; + OrigPred.push_back(TIBB); + CreatePHIsForSplitLoopExit(OrigPred, NewBB, DestBB); + } + + // For each unique exit block... + SmallVector<BasicBlock *, 4> ExitBlocks; + TIL->getExitBlocks(ExitBlocks); + for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { + // Collect all the preds that are inside the loop, and note + // whether there are any preds outside the loop. + SmallVector<BasicBlock *, 4> Preds; + bool HasPredOutsideOfLoop = false; + BasicBlock *Exit = ExitBlocks[i]; + for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); + I != E; ++I) + if (TIL->contains(*I)) + Preds.push_back(*I); + else + HasPredOutsideOfLoop = true; + // If there are any preds not in the loop, we'll need to split + // the edges. The Preds.empty() check is needed because a block + // may appear multiple times in the list. We can't use + // getUniqueExitBlocks above because that depends on LoopSimplify + // form, which we're in the process of restoring! + if (!Preds.empty() && HasPredOutsideOfLoop) { + BasicBlock *NewExitBB = + SplitBlockPredecessors(Exit, Preds.data(), Preds.size(), + "split", P); + if (P->mustPreserveAnalysisID(LCSSAID)) + CreatePHIsForSplitLoopExit(Preds, NewExitBB, Exit); + } + } + } + // LCSSA form was updated above for the case where LoopSimplify is + // available, which means that all predecessors of loop exit blocks + // are within the loop. Without LoopSimplify form, it would be + // necessary to insert a new phi. + assert((!P->mustPreserveAnalysisID(LCSSAID) || + P->mustPreserveAnalysisID(LoopSimplifyID)) && + "SplitCriticalEdge doesn't know how to update LCCSA form " + "without LoopSimplify!"); + } + } + + // Update ProfileInfo if it is around. + if (PI) + PI->splitEdge(TIBB, DestBB, NewBB, MergeIdenticalEdges); + + return NewBB; +} |
