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loop exit edge -- new PHIs may be needed not only for the additional splits that are made to preserve LoopSimplify form, but also for the original split. Factor out the code that inserts new PHIs so that it can be used for both. Remove LoopRotation.cpp's code for manually updating LCSSA form, as it is now redundant. This fixes PR4934. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@81363 91177308-0d34-0410-b5e6-96231b3b80d8
377 lines
15 KiB
C++
377 lines
15 KiB
C++
//===- 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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// forward dominator (set, immediate dominators, tree, and frontier)
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// information.
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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/Compiler.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 VISIBILITY_HIDDEN 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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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<DominanceFrontier>();
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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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static RegisterPass<BreakCriticalEdges>
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X("break-crit-edges", "Break critical edges in CFG");
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// Publically exposed interface to pass...
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const PassInfo *const llvm::BreakCriticalEdgesID = &X;
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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)
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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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pred_const_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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if (*I != 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(*I);
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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 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 = PHINode::Create(PN->getType(), "split",
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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 and
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/// DominatorFrontier information if it is available, thus calling this pass
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/// will not invalidate any of them. This returns true if the edge was split,
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/// false otherwise. This ensures that all edges to that dest go to one block
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/// instead of each going to a different block.
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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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if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return 0;
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BasicBlock *TIBB = TI->getParent();
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BasicBlock *DestBB = TI->getSuccessor(SuccNum);
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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::Create(DestBB, NewBB);
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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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for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
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PHINode *PN = cast<PHINode>(I);
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// We no longer enter through TIBB, now we come in through NewBB. Revector
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// exactly one entry in the PHI node that used to come from TIBB to come
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// from NewBB.
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int BBIdx = PN->getBasicBlockIndex(TIBB);
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PN->setIncomingBlock(BBIdx, NewBB);
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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);
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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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// 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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for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
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if (*I != NewBB)
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OtherPreds.push_back(*I);
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bool NewBBDominatesDestBB = true;
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// Should we update DominatorTree information?
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if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
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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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// Should we update DominanceFrontier information?
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if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) {
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// If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
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if (!OtherPreds.empty()) {
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// FIXME: IMPLEMENT THIS!
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llvm_unreachable("Requiring domfrontiers but not idom/domtree/domset."
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" not implemented yet!");
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}
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// Since the new block is dominated by its only predecessor TIBB,
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// it cannot be in any block's dominance frontier. If NewBB dominates
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// DestBB, its dominance frontier is the same as DestBB's, otherwise it is
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// just {DestBB}.
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DominanceFrontier::DomSetType NewDFSet;
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if (NewBBDominatesDestBB) {
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DominanceFrontier::iterator I = DF->find(DestBB);
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if (I != DF->end()) {
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DF->addBasicBlock(NewBB, I->second);
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if (I->second.count(DestBB)) {
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// However NewBB's frontier does not include DestBB.
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DominanceFrontier::iterator NF = DF->find(NewBB);
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DF->removeFromFrontier(NF, DestBB);
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}
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}
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else
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DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
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} else {
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DominanceFrontier::DomSetType NewDFSet;
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NewDFSet.insert(DestBB);
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DF->addBasicBlock(NewBB, NewDFSet);
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}
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}
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// Update LoopInfo if it is around.
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if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>()) {
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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->getHeader())) {
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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->getHeader())) {
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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.
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if (P->mustPreserveAnalysisID(LCSSAID)) {
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SmallVector<BasicBlock *, 1> OrigPred;
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OrigPred.push_back(TIBB);
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CreatePHIsForSplitLoopExit(OrigPred, NewBB, DestBB);
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}
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// For each unique exit block...
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SmallVector<BasicBlock *, 4> ExitBlocks;
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TIL->getExitBlocks(ExitBlocks);
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
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// Collect all the preds that are inside the loop, and note
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// whether there are any preds outside the loop.
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SmallVector<BasicBlock *, 4> Preds;
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bool HasPredOutsideOfLoop = false;
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BasicBlock *Exit = ExitBlocks[i];
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for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit);
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I != E; ++I)
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if (TIL->contains(*I))
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Preds.push_back(*I);
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else
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HasPredOutsideOfLoop = true;
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// If there are any preds not in the loop, we'll need to split
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// the edges. The Preds.empty() check is needed because a block
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// may appear multiple times in the list. We can't use
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// getUniqueExitBlocks above because that depends on LoopSimplify
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// form, which we're in the process of restoring!
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if (!Preds.empty() && HasPredOutsideOfLoop) {
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BasicBlock *NewExitBB =
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SplitBlockPredecessors(Exit, Preds.data(), Preds.size(),
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"split", P);
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if (P->mustPreserveAnalysisID(LCSSAID))
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CreatePHIsForSplitLoopExit(Preds, NewExitBB, Exit);
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}
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}
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}
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// LCSSA form was updated above for the case where LoopSimplify is
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// available, which means that all predecessors of loop exit blocks
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// are within the loop. Without LoopSimplify form, it would be
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// necessary to insert a new phi.
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assert((!P->mustPreserveAnalysisID(LCSSAID) ||
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P->mustPreserveAnalysisID(LoopSimplifyID)) &&
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"SplitCriticalEdge doesn't know how to update LCCSA form "
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"without LoopSimplify!");
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}
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}
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// Update ProfileInfo if it is around.
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if (ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>()) {
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PI->splitEdge(TIBB,DestBB,NewBB,MergeIdenticalEdges);
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}
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return NewBB;
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}
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