474 lines
18 KiB
C++
474 lines
18 KiB
C++
//===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// BreakCriticalEdges pass - Break all of the critical edges in the CFG by
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// inserting a dummy basic block. This pass may be "required" by passes that
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// cannot deal with critical edges. For this usage, the structure type is
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// forward declared. This pass obviously invalidates the CFG, but can update
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// dominator trees.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Utils/BreakCriticalEdges.h"
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#include "llvm/ADT/SetVector.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/BlockFrequencyInfo.h"
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#include "llvm/Analysis/BranchProbabilityInfo.h"
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#include "llvm/Analysis/CFG.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/MemorySSAUpdater.h"
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#include "llvm/Analysis/PostDominators.h"
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#include "llvm/IR/CFG.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/Type.h"
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#include "llvm/Support/ErrorHandling.h"
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#include "llvm/Transforms/Utils.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/Cloning.h"
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#include "llvm/Transforms/Utils/ValueMapper.h"
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using namespace llvm;
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#define DEBUG_TYPE "break-crit-edges"
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STATISTIC(NumBroken, "Number of blocks inserted");
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namespace {
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struct BreakCriticalEdges : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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BreakCriticalEdges() : FunctionPass(ID) {
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initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &F) override {
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auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
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auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;
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auto *PDTWP = getAnalysisIfAvailable<PostDominatorTreeWrapperPass>();
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auto *PDT = PDTWP ? &PDTWP->getPostDomTree() : nullptr;
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auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>();
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auto *LI = LIWP ? &LIWP->getLoopInfo() : nullptr;
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unsigned N =
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SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI, nullptr, PDT));
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NumBroken += N;
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return N > 0;
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addPreserved<DominatorTreeWrapperPass>();
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AU.addPreserved<LoopInfoWrapperPass>();
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// No loop canonicalization guarantees are broken by this pass.
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AU.addPreservedID(LoopSimplifyID);
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}
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};
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}
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char BreakCriticalEdges::ID = 0;
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INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges",
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"Break critical edges in CFG", false, false)
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// Publicly exposed interface to pass...
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char &llvm::BreakCriticalEdgesID = BreakCriticalEdges::ID;
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FunctionPass *llvm::createBreakCriticalEdgesPass() {
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return new BreakCriticalEdges();
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}
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PreservedAnalyses BreakCriticalEdgesPass::run(Function &F,
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FunctionAnalysisManager &AM) {
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auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
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auto *LI = AM.getCachedResult<LoopAnalysis>(F);
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unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
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NumBroken += N;
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if (N == 0)
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return PreservedAnalyses::all();
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PreservedAnalyses PA;
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PA.preserve<DominatorTreeAnalysis>();
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PA.preserve<LoopAnalysis>();
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return PA;
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}
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//===----------------------------------------------------------------------===//
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// Implementation of the external critical edge manipulation functions
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//===----------------------------------------------------------------------===//
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/// When a loop exit edge is split, LCSSA form may require new PHIs in the new
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/// exit block. This function inserts the new PHIs, as needed. Preds is a list
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/// of preds inside the loop, SplitBB is the new loop exit block, and DestBB is
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/// the old loop exit, now the successor of SplitBB.
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static void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds,
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BasicBlock *SplitBB,
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BasicBlock *DestBB) {
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// SplitBB shouldn't have anything non-trivial in it yet.
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assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() ||
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SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!");
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// For each PHI in the destination block.
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for (PHINode &PN : DestBB->phis()) {
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unsigned Idx = PN.getBasicBlockIndex(SplitBB);
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Value *V = PN.getIncomingValue(Idx);
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// If the input is a PHI which already satisfies LCSSA, don't create
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// a new one.
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if (const PHINode *VP = dyn_cast<PHINode>(V))
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if (VP->getParent() == SplitBB)
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continue;
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// Otherwise a new PHI is needed. Create one and populate it.
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PHINode *NewPN = PHINode::Create(
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PN.getType(), Preds.size(), "split",
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SplitBB->isLandingPad() ? &SplitBB->front() : SplitBB->getTerminator());
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for (unsigned i = 0, e = Preds.size(); i != e; ++i)
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NewPN->addIncoming(V, Preds[i]);
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// Update the original PHI.
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PN.setIncomingValue(Idx, NewPN);
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}
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}
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BasicBlock *
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llvm::SplitCriticalEdge(Instruction *TI, unsigned SuccNum,
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const CriticalEdgeSplittingOptions &Options) {
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if (!isCriticalEdge(TI, SuccNum, Options.MergeIdenticalEdges))
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return nullptr;
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assert(!isa<IndirectBrInst>(TI) &&
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"Cannot split critical edge from IndirectBrInst");
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BasicBlock *TIBB = TI->getParent();
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BasicBlock *DestBB = TI->getSuccessor(SuccNum);
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// Splitting the critical edge to a pad block is non-trivial. Don't do
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// it in this generic function.
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if (DestBB->isEHPad()) return nullptr;
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// Don't split the non-fallthrough edge from a callbr.
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if (isa<CallBrInst>(TI) && SuccNum > 0)
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return nullptr;
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if (Options.IgnoreUnreachableDests &&
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isa<UnreachableInst>(DestBB->getFirstNonPHIOrDbgOrLifetime()))
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return nullptr;
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// Create a new basic block, linking it into the CFG.
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BasicBlock *NewBB = BasicBlock::Create(TI->getContext(),
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TIBB->getName() + "." + DestBB->getName() + "_crit_edge");
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// Create our unconditional branch.
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BranchInst *NewBI = BranchInst::Create(DestBB, NewBB);
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NewBI->setDebugLoc(TI->getDebugLoc());
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// Branch to the new block, breaking the edge.
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TI->setSuccessor(SuccNum, NewBB);
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// Insert the block into the function... right after the block TI lives in.
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Function &F = *TIBB->getParent();
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Function::iterator FBBI = TIBB->getIterator();
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F.getBasicBlockList().insert(++FBBI, NewBB);
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// If there are any PHI nodes in DestBB, we need to update them so that they
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// merge incoming values from NewBB instead of from TIBB.
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{
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unsigned BBIdx = 0;
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for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
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// We no longer enter through TIBB, now we come in through NewBB.
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// Revector exactly one entry in the PHI node that used to come from
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// TIBB to come from NewBB.
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PHINode *PN = cast<PHINode>(I);
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// Reuse the previous value of BBIdx if it lines up. In cases where we
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// have multiple phi nodes with *lots* of predecessors, this is a speed
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// win because we don't have to scan the PHI looking for TIBB. This
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// happens because the BB list of PHI nodes are usually in the same
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// order.
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if (PN->getIncomingBlock(BBIdx) != TIBB)
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BBIdx = PN->getBasicBlockIndex(TIBB);
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PN->setIncomingBlock(BBIdx, NewBB);
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}
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}
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// If there are any other edges from TIBB to DestBB, update those to go
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// through the split block, making those edges non-critical as well (and
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// reducing the number of phi entries in the DestBB if relevant).
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if (Options.MergeIdenticalEdges) {
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for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
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if (TI->getSuccessor(i) != DestBB) continue;
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// Remove an entry for TIBB from DestBB phi nodes.
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DestBB->removePredecessor(TIBB, Options.KeepOneInputPHIs);
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// We found another edge to DestBB, go to NewBB instead.
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TI->setSuccessor(i, NewBB);
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}
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}
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// If we have nothing to update, just return.
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auto *DT = Options.DT;
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auto *PDT = Options.PDT;
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auto *LI = Options.LI;
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auto *MSSAU = Options.MSSAU;
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if (MSSAU)
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MSSAU->wireOldPredecessorsToNewImmediatePredecessor(
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DestBB, NewBB, {TIBB}, Options.MergeIdenticalEdges);
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if (!DT && !PDT && !LI)
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return NewBB;
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if (DT || PDT) {
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// Update the DominatorTree.
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// ---> NewBB -----\
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// / V
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// TIBB -------\\------> DestBB
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//
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// First, inform the DT about the new path from TIBB to DestBB via NewBB,
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// then delete the old edge from TIBB to DestBB. By doing this in that order
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// DestBB stays reachable in the DT the whole time and its subtree doesn't
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// get disconnected.
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SmallVector<DominatorTree::UpdateType, 3> Updates;
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Updates.push_back({DominatorTree::Insert, TIBB, NewBB});
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Updates.push_back({DominatorTree::Insert, NewBB, DestBB});
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if (llvm::find(successors(TIBB), DestBB) == succ_end(TIBB))
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Updates.push_back({DominatorTree::Delete, TIBB, DestBB});
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if (DT)
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DT->applyUpdates(Updates);
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if (PDT)
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PDT->applyUpdates(Updates);
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}
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// Update LoopInfo if it is around.
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if (LI) {
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if (Loop *TIL = LI->getLoopFor(TIBB)) {
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// If one or the other blocks were not in a loop, the new block is not
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// either, and thus LI doesn't need to be updated.
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if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
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if (TIL == DestLoop) {
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// Both in the same loop, the NewBB joins loop.
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DestLoop->addBasicBlockToLoop(NewBB, *LI);
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} else if (TIL->contains(DestLoop)) {
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// Edge from an outer loop to an inner loop. Add to the outer loop.
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TIL->addBasicBlockToLoop(NewBB, *LI);
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} else if (DestLoop->contains(TIL)) {
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// Edge from an inner loop to an outer loop. Add to the outer loop.
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DestLoop->addBasicBlockToLoop(NewBB, *LI);
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} else {
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// Edge from two loops with no containment relation. Because these
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// are natural loops, we know that the destination block must be the
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// header of its loop (adding a branch into a loop elsewhere would
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// create an irreducible loop).
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assert(DestLoop->getHeader() == DestBB &&
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"Should not create irreducible loops!");
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if (Loop *P = DestLoop->getParentLoop())
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P->addBasicBlockToLoop(NewBB, *LI);
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}
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}
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// If TIBB is in a loop and DestBB is outside of that loop, we may need
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// to update LoopSimplify form and LCSSA form.
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if (!TIL->contains(DestBB)) {
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assert(!TIL->contains(NewBB) &&
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"Split point for loop exit is contained in loop!");
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// Update LCSSA form in the newly created exit block.
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if (Options.PreserveLCSSA) {
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createPHIsForSplitLoopExit(TIBB, NewBB, DestBB);
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}
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// The only that we can break LoopSimplify form by splitting a critical
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// edge is if after the split there exists some edge from TIL to DestBB
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// *and* the only edge into DestBB from outside of TIL is that of
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// NewBB. If the first isn't true, then LoopSimplify still holds, NewBB
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// is the new exit block and it has no non-loop predecessors. If the
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// second isn't true, then DestBB was not in LoopSimplify form prior to
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// the split as it had a non-loop predecessor. In both of these cases,
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// the predecessor must be directly in TIL, not in a subloop, or again
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// LoopSimplify doesn't hold.
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SmallVector<BasicBlock *, 4> LoopPreds;
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for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E;
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++I) {
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BasicBlock *P = *I;
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if (P == NewBB)
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continue; // The new block is known.
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if (LI->getLoopFor(P) != TIL) {
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// No need to re-simplify, it wasn't to start with.
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LoopPreds.clear();
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break;
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}
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LoopPreds.push_back(P);
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}
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if (!LoopPreds.empty()) {
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assert(!DestBB->isEHPad() && "We don't split edges to EH pads!");
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BasicBlock *NewExitBB = SplitBlockPredecessors(
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DestBB, LoopPreds, "split", DT, LI, MSSAU, Options.PreserveLCSSA);
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if (Options.PreserveLCSSA)
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createPHIsForSplitLoopExit(LoopPreds, NewExitBB, DestBB);
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}
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}
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}
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}
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return NewBB;
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}
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// Return the unique indirectbr predecessor of a block. This may return null
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// even if such a predecessor exists, if it's not useful for splitting.
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// If a predecessor is found, OtherPreds will contain all other (non-indirectbr)
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// predecessors of BB.
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static BasicBlock *
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findIBRPredecessor(BasicBlock *BB, SmallVectorImpl<BasicBlock *> &OtherPreds) {
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// If the block doesn't have any PHIs, we don't care about it, since there's
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// no point in splitting it.
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PHINode *PN = dyn_cast<PHINode>(BB->begin());
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if (!PN)
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return nullptr;
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// Verify we have exactly one IBR predecessor.
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// Conservatively bail out if one of the other predecessors is not a "regular"
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// terminator (that is, not a switch or a br).
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BasicBlock *IBB = nullptr;
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for (unsigned Pred = 0, E = PN->getNumIncomingValues(); Pred != E; ++Pred) {
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BasicBlock *PredBB = PN->getIncomingBlock(Pred);
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Instruction *PredTerm = PredBB->getTerminator();
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switch (PredTerm->getOpcode()) {
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case Instruction::IndirectBr:
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if (IBB)
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return nullptr;
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IBB = PredBB;
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break;
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case Instruction::Br:
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case Instruction::Switch:
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OtherPreds.push_back(PredBB);
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continue;
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default:
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return nullptr;
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}
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}
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return IBB;
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}
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bool llvm::SplitIndirectBrCriticalEdges(Function &F,
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BranchProbabilityInfo *BPI,
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BlockFrequencyInfo *BFI) {
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// Check whether the function has any indirectbrs, and collect which blocks
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// they may jump to. Since most functions don't have indirect branches,
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// this lowers the common case's overhead to O(Blocks) instead of O(Edges).
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SmallSetVector<BasicBlock *, 16> Targets;
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for (auto &BB : F) {
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auto *IBI = dyn_cast<IndirectBrInst>(BB.getTerminator());
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if (!IBI)
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continue;
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for (unsigned Succ = 0, E = IBI->getNumSuccessors(); Succ != E; ++Succ)
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Targets.insert(IBI->getSuccessor(Succ));
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}
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if (Targets.empty())
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return false;
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bool ShouldUpdateAnalysis = BPI && BFI;
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bool Changed = false;
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for (BasicBlock *Target : Targets) {
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SmallVector<BasicBlock *, 16> OtherPreds;
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BasicBlock *IBRPred = findIBRPredecessor(Target, OtherPreds);
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// If we did not found an indirectbr, or the indirectbr is the only
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// incoming edge, this isn't the kind of edge we're looking for.
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if (!IBRPred || OtherPreds.empty())
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continue;
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// Don't even think about ehpads/landingpads.
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Instruction *FirstNonPHI = Target->getFirstNonPHI();
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if (FirstNonPHI->isEHPad() || Target->isLandingPad())
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continue;
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BasicBlock *BodyBlock = Target->splitBasicBlock(FirstNonPHI, ".split");
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if (ShouldUpdateAnalysis) {
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// Copy the BFI/BPI from Target to BodyBlock.
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for (unsigned I = 0, E = BodyBlock->getTerminator()->getNumSuccessors();
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I < E; ++I)
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BPI->setEdgeProbability(BodyBlock, I,
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BPI->getEdgeProbability(Target, I));
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BFI->setBlockFreq(BodyBlock, BFI->getBlockFreq(Target).getFrequency());
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}
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// It's possible Target was its own successor through an indirectbr.
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// In this case, the indirectbr now comes from BodyBlock.
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if (IBRPred == Target)
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IBRPred = BodyBlock;
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// At this point Target only has PHIs, and BodyBlock has the rest of the
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// block's body. Create a copy of Target that will be used by the "direct"
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// preds.
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ValueToValueMapTy VMap;
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BasicBlock *DirectSucc = CloneBasicBlock(Target, VMap, ".clone", &F);
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BlockFrequency BlockFreqForDirectSucc;
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for (BasicBlock *Pred : OtherPreds) {
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// If the target is a loop to itself, then the terminator of the split
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// block (BodyBlock) needs to be updated.
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BasicBlock *Src = Pred != Target ? Pred : BodyBlock;
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Src->getTerminator()->replaceUsesOfWith(Target, DirectSucc);
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if (ShouldUpdateAnalysis)
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BlockFreqForDirectSucc += BFI->getBlockFreq(Src) *
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BPI->getEdgeProbability(Src, DirectSucc);
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}
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if (ShouldUpdateAnalysis) {
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BFI->setBlockFreq(DirectSucc, BlockFreqForDirectSucc.getFrequency());
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BlockFrequency NewBlockFreqForTarget =
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BFI->getBlockFreq(Target) - BlockFreqForDirectSucc;
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BFI->setBlockFreq(Target, NewBlockFreqForTarget.getFrequency());
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BPI->eraseBlock(Target);
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}
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// Ok, now fix up the PHIs. We know the two blocks only have PHIs, and that
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// they are clones, so the number of PHIs are the same.
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// (a) Remove the edge coming from IBRPred from the "Direct" PHI
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// (b) Leave that as the only edge in the "Indirect" PHI.
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// (c) Merge the two in the body block.
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BasicBlock::iterator Indirect = Target->begin(),
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End = Target->getFirstNonPHI()->getIterator();
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BasicBlock::iterator Direct = DirectSucc->begin();
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BasicBlock::iterator MergeInsert = BodyBlock->getFirstInsertionPt();
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assert(&*End == Target->getTerminator() &&
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"Block was expected to only contain PHIs");
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while (Indirect != End) {
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PHINode *DirPHI = cast<PHINode>(Direct);
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PHINode *IndPHI = cast<PHINode>(Indirect);
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// Now, clean up - the direct block shouldn't get the indirect value,
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// and vice versa.
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DirPHI->removeIncomingValue(IBRPred);
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Direct++;
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// Advance the pointer here, to avoid invalidation issues when the old
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// PHI is erased.
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Indirect++;
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PHINode *NewIndPHI = PHINode::Create(IndPHI->getType(), 1, "ind", IndPHI);
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NewIndPHI->addIncoming(IndPHI->getIncomingValueForBlock(IBRPred),
|
|
IBRPred);
|
|
|
|
// Create a PHI in the body block, to merge the direct and indirect
|
|
// predecessors.
|
|
PHINode *MergePHI =
|
|
PHINode::Create(IndPHI->getType(), 2, "merge", &*MergeInsert);
|
|
MergePHI->addIncoming(NewIndPHI, Target);
|
|
MergePHI->addIncoming(DirPHI, DirectSucc);
|
|
|
|
IndPHI->replaceAllUsesWith(MergePHI);
|
|
IndPHI->eraseFromParent();
|
|
}
|
|
|
|
Changed = true;
|
|
}
|
|
|
|
return Changed;
|
|
}
|