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Diffstat (limited to 'contrib/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp')
| -rw-r--r-- | contrib/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp | 370 |
1 files changed, 370 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 000000000000..0e7f7f784401 --- /dev/null +++ b/contrib/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp @@ -0,0 +1,370 @@ +//===- 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. +// +//===----------------------------------------------------------------------===// + +#define DEBUG_TYPE "break-crit-edges" +#include "llvm/Transforms/Scalar.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/CFG.h" +#include "llvm/Analysis/Dominators.h" +#include "llvm/Analysis/LoopInfo.h" +#include "llvm/IR/Function.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/Type.h" +#include "llvm/Support/CFG.h" +#include "llvm/Support/ErrorHandling.h" +#include "llvm/Transforms/Utils/BasicBlockUtils.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) { + initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry()); + } + + virtual bool runOnFunction(Function &F); + + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.addPreserved<DominatorTree>(); + AU.addPreserved<LoopInfo>(); + + // 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(); +} + +// 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 +//===----------------------------------------------------------------------===// + +/// 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, + Pass *P, bool MergeIdenticalEdges, + bool DontDeleteUselessPhis, + bool SplitLandingPads) { + 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); + + // Splitting the critical edge to a landing pad block is non-trivial. Don't do + // it in this generic function. + if (DestBB->isLandingPad()) return 0; + + // 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 (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, DontDeleteUselessPhis); + + // 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>(); + LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>(); + + // If we have nothing to update, just return. + if (DT == 0 && LI == 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) { + 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 = 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); + } + } + } + + // 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)) + createPHIsForSplitLoopExit(TIBB, NewBB, DestBB); + + // For each unique exit block... + // FIXME: This code is functionally equivalent to the corresponding + // loop in LoopSimplify. + 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) { + BasicBlock *P = *I; + if (TIL->contains(P)) { + if (isa<IndirectBrInst>(P->getTerminator())) { + Preds.clear(); + break; + } + Preds.push_back(P); + } 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) { + if (!Exit->isLandingPad()) { + BasicBlock *NewExitBB = + SplitBlockPredecessors(Exit, Preds, "split", P); + if (P->mustPreserveAnalysisID(LCSSAID)) + createPHIsForSplitLoopExit(Preds, NewExitBB, Exit); + } else if (SplitLandingPads) { + SmallVector<BasicBlock*, 8> NewBBs; + SplitLandingPadPredecessors(Exit, Preds, + ".split1", ".split2", + P, NewBBs); + if (P->mustPreserveAnalysisID(LCSSAID)) + createPHIsForSplitLoopExit(Preds, NewBBs[0], 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!"); + } + } + + return NewBB; +} |