413 lines
16 KiB
C++
413 lines
16 KiB
C++
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//===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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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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#define DEBUG_TYPE "break-crit-edges"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/ProfileInfo.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Type.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/ErrorHandling.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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using namespace llvm;
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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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virtual bool runOnFunction(Function &F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addPreserved<DominatorTree>();
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AU.addPreserved<LoopInfo>();
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AU.addPreserved<ProfileInfo>();
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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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// runOnFunction - Loop over all of the edges in the CFG, breaking critical
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// edges as they are found.
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//
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bool BreakCriticalEdges::runOnFunction(Function &F) {
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bool Changed = false;
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for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
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TerminatorInst *TI = I->getTerminator();
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if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI))
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for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
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if (SplitCriticalEdge(TI, i, this)) {
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++NumBroken;
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Changed = true;
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}
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}
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return Changed;
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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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// isCriticalEdge - Return true if the specified edge is a critical edge.
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// Critical edges are edges from a block with multiple successors to a block
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// with multiple predecessors.
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//
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bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
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bool AllowIdenticalEdges) {
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assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
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if (TI->getNumSuccessors() == 1) return false;
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const BasicBlock *Dest = TI->getSuccessor(SuccNum);
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const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
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// If there is more than one predecessor, this is a critical edge...
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assert(I != E && "No preds, but we have an edge to the block?");
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const BasicBlock *FirstPred = *I;
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++I; // Skip one edge due to the incoming arc from TI.
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if (!AllowIdenticalEdges)
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return I != E;
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// If AllowIdenticalEdges is true, then we allow this edge to be considered
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// non-critical iff all preds come from TI's block.
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while (I != E) {
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const BasicBlock *P = *I;
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if (P != FirstPred)
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return true;
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// Note: leave this as is until no one ever compiles with either gcc 4.0.1
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// or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
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E = pred_end(P);
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++I;
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}
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return false;
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}
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/// createPHIsForSplitLoopExit - When a loop exit edge is split, LCSSA form
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/// may require new PHIs in the new exit block. This function inserts the
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/// new PHIs, as needed. Preds is a list of preds inside the loop, SplitBB
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/// is the new loop exit block, and DestBB is the old loop exit, now the
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/// successor of SplitBB.
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static void createPHIsForSplitLoopExit(SmallVectorImpl<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 (BasicBlock::iterator I = DestBB->begin();
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PHINode *PN = dyn_cast<PHINode>(I); ++I) {
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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 =
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PHINode::Create(PN->getType(), Preds.size(), "split",
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SplitBB->isLandingPad() ?
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SplitBB->begin() : 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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/// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
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/// split the critical edge. This will update DominatorTree information if it
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/// is available, thus calling this pass will not invalidate either of them.
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/// This returns the new block if the edge was split, null otherwise.
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///
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/// If MergeIdenticalEdges is true (not the default), *all* edges from TI to the
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/// specified successor will be merged into the same critical edge block.
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/// This is most commonly interesting with switch instructions, which may
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/// have many edges to any one destination. This ensures that all edges to that
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/// dest go to one block instead of each going to a different block, but isn't
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/// the standard definition of a "critical edge".
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///
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/// It is invalid to call this function on a critical edge that starts at an
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/// IndirectBrInst. Splitting these edges will almost always create an invalid
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/// program because the address of the new block won't be the one that is jumped
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/// to.
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///
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BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
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Pass *P, bool MergeIdenticalEdges,
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bool DontDeleteUselessPhis,
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bool SplitLandingPads) {
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if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return 0;
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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 landing pad block is non-trivial. Don't do
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// it in this generic function.
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if (DestBB->isLandingPad()) return 0;
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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;
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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 (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, DontDeleteUselessPhis);
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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 don't have a pass object, we can't update anything...
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if (P == 0) return NewBB;
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DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>();
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LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>();
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ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>();
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// If we have nothing to update, just return.
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if (DT == 0 && LI == 0 && PI == 0)
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return NewBB;
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// Now update analysis information. Since the only predecessor of NewBB is
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// the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
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// anything, as there are other successors of DestBB. However, if all other
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// predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
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// loop header) then NewBB dominates DestBB.
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SmallVector<BasicBlock*, 8> OtherPreds;
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// If there is a PHI in the block, loop over predecessors with it, which is
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// faster than iterating pred_begin/end.
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if (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if (PN->getIncomingBlock(i) != NewBB)
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OtherPreds.push_back(PN->getIncomingBlock(i));
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} else {
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for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB);
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I != E; ++I) {
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BasicBlock *P = *I;
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if (P != NewBB)
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OtherPreds.push_back(P);
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}
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}
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bool NewBBDominatesDestBB = true;
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// Should we update DominatorTree information?
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if (DT) {
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DomTreeNode *TINode = DT->getNode(TIBB);
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// The new block is not the immediate dominator for any other nodes, but
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// TINode is the immediate dominator for the new node.
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//
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if (TINode) { // Don't break unreachable code!
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DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
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DomTreeNode *DestBBNode = 0;
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// If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
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if (!OtherPreds.empty()) {
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DestBBNode = DT->getNode(DestBB);
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while (!OtherPreds.empty() && NewBBDominatesDestBB) {
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if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back()))
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NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode);
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OtherPreds.pop_back();
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}
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OtherPreds.clear();
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}
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// If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
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// doesn't dominate anything.
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if (NewBBDominatesDestBB) {
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if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
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DT->changeImmediateDominator(DestBBNode, NewBBNode);
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}
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}
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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->getBase());
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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->getBase());
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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->getBase());
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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->getBase());
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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, split the
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// other exit blocks of the loop that also have predecessors outside
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// the loop, to maintain a LoopSimplify guarantee.
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if (!TIL->contains(DestBB) &&
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P->mustPreserveAnalysisID(LoopSimplifyID)) {
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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.
|
||
|
|
if (P->mustPreserveAnalysisID(LCSSAID)) {
|
||
|
|
SmallVector<BasicBlock *, 1> OrigPred;
|
||
|
|
OrigPred.push_back(TIBB);
|
||
|
|
createPHIsForSplitLoopExit(OrigPred, 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!");
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
// Update ProfileInfo if it is around.
|
||
|
|
if (PI)
|
||
|
|
PI->splitEdge(TIBB, DestBB, NewBB, MergeIdenticalEdges);
|
||
|
|
|
||
|
|
return NewBB;
|
||
|
|
}
|