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llvm-project/llvm/lib/Transforms/Utils/LoopSimplify.cpp

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//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs several transformations to transform natural loops into a
// simpler form, which makes subsequent analyses and transformations simpler and
// more effective.
//
// Loop pre-header insertion guarantees that there is a single, non-critical
// entry edge from outside of the loop to the loop header. This simplifies a
// number of analyses and transformations, such as LICM.
//
// Loop exit-block insertion guarantees that all exit blocks from the loop
// (blocks which are outside of the loop that have predecessors inside of the
// loop) only have predecessors from inside of the loop (and are thus dominated
// by the loop header). This simplifies transformations such as store-sinking
// that are built into LICM.
//
// This pass also guarantees that loops will have exactly one backedge.
//
// Note that the simplifycfg pass will clean up blocks which are split out but
// end up being unnecessary, so usage of this pass should not pessimize
// generated code.
//
// This pass obviously modifies the CFG, but updates loop information and
// dominator information.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "loopsimplify"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Function.h"
#include "llvm/Type.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Compiler.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/DepthFirstIterator.h"
using namespace llvm;
STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
STATISTIC(NumNested , "Number of nested loops split out");
namespace {
struct VISIBILITY_HIDDEN LoopSimplify : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
LoopSimplify() : FunctionPass((intptr_t)&ID) {}
// AA - If we have an alias analysis object to update, this is it, otherwise
// this is null.
AliasAnalysis *AA;
LoopInfo *LI;
DominatorTree *DT;
virtual bool runOnFunction(Function &F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
// We need loop information to identify the loops...
AU.addRequired<LoopInfo>();
AU.addRequired<DominatorTree>();
AU.addPreserved<LoopInfo>();
AU.addPreserved<DominatorTree>();
AU.addPreserved<DominanceFrontier>();
AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
}
/// verifyAnalysis() - Verify loop nest.
void verifyAnalysis() const {
#ifndef NDEBUG
LoopInfo *NLI = &getAnalysis<LoopInfo>();
for (LoopInfo::iterator I = NLI->begin(), E = NLI->end(); I != E; ++I)
(*I)->verifyLoop();
#endif
}
private:
bool ProcessLoop(Loop *L);
BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix,
const std::vector<BasicBlock*> &Preds);
BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
void InsertPreheaderForLoop(Loop *L);
Loop *SeparateNestedLoop(Loop *L);
void InsertUniqueBackedgeBlock(Loop *L);
void PlaceSplitBlockCarefully(BasicBlock *NewBB,
std::vector<BasicBlock*> &SplitPreds,
Loop *L);
};
char LoopSimplify::ID = 0;
RegisterPass<LoopSimplify>
X("loopsimplify", "Canonicalize natural loops", true);
}
// Publically exposed interface to pass...
const PassInfo *llvm::LoopSimplifyID = X.getPassInfo();
FunctionPass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
/// runOnFunction - Run down all loops in the CFG (recursively, but we could do
/// it in any convenient order) inserting preheaders...
///
bool LoopSimplify::runOnFunction(Function &F) {
bool Changed = false;
LI = &getAnalysis<LoopInfo>();
AA = getAnalysisToUpdate<AliasAnalysis>();
DT = &getAnalysis<DominatorTree>();
// Check to see that no blocks (other than the header) in loops have
// predecessors that are not in loops. This is not valid for natural loops,
// but can occur if the blocks are unreachable. Since they are unreachable we
// can just shamelessly destroy their terminators to make them not branch into
// the loop!
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
// This case can only occur for unreachable blocks. Blocks that are
// unreachable can't be in loops, so filter those blocks out.
if (LI->getLoopFor(BB)) continue;
bool BlockUnreachable = false;
TerminatorInst *TI = BB->getTerminator();
// Check to see if any successors of this block are non-loop-header loops
// that are not the header.
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
// If this successor is not in a loop, BB is clearly ok.
Loop *L = LI->getLoopFor(TI->getSuccessor(i));
if (!L) continue;
// If the succ is the loop header, and if L is a top-level loop, then this
// is an entrance into a loop through the header, which is also ok.
if (L->getHeader() == TI->getSuccessor(i) && L->getParentLoop() == 0)
continue;
// Otherwise, this is an entrance into a loop from some place invalid.
// Either the loop structure is invalid and this is not a natural loop (in
// which case the compiler is buggy somewhere else) or BB is unreachable.
BlockUnreachable = true;
break;
}
// If this block is ok, check the next one.
if (!BlockUnreachable) continue;
// Otherwise, this block is dead. To clean up the CFG and to allow later
// loop transformations to ignore this case, we delete the edges into the
// loop by replacing the terminator.
// Remove PHI entries from the successors.
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
TI->getSuccessor(i)->removePredecessor(BB);
// Add a new unreachable instruction before the old terminator.
new UnreachableInst(TI);
// Delete the dead terminator.
if (AA) AA->deleteValue(TI);
if (!TI->use_empty())
TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
TI->eraseFromParent();
Changed |= true;
}
for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
Changed |= ProcessLoop(*I);
return Changed;
}
/// ProcessLoop - Walk the loop structure in depth first order, ensuring that
/// all loops have preheaders.
///
bool LoopSimplify::ProcessLoop(Loop *L) {
bool Changed = false;
ReprocessLoop:
// Canonicalize inner loops before outer loops. Inner loop canonicalization
// can provide work for the outer loop to canonicalize.
for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
Changed |= ProcessLoop(*I);
assert(L->getBlocks()[0] == L->getHeader() &&
"Header isn't first block in loop?");
// Does the loop already have a preheader? If so, don't insert one.
if (L->getLoopPreheader() == 0) {
InsertPreheaderForLoop(L);
NumInserted++;
Changed = true;
}
// Next, check to make sure that all exit nodes of the loop only have
// predecessors that are inside of the loop. This check guarantees that the
// loop preheader/header will dominate the exit blocks. If the exit block has
// predecessors from outside of the loop, split the edge now.
SmallVector<BasicBlock*, 8> ExitBlocks;
L->getExitBlocks(ExitBlocks);
SetVector<BasicBlock*> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end());
for (SetVector<BasicBlock*>::iterator I = ExitBlockSet.begin(),
E = ExitBlockSet.end(); I != E; ++I) {
BasicBlock *ExitBlock = *I;
for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
PI != PE; ++PI)
// Must be exactly this loop: no subloops, parent loops, or non-loop preds
// allowed.
if (!L->contains(*PI)) {
RewriteLoopExitBlock(L, ExitBlock);
NumInserted++;
Changed = true;
break;
}
}
// If the header has more than two predecessors at this point (from the
// preheader and from multiple backedges), we must adjust the loop.
unsigned NumBackedges = L->getNumBackEdges();
if (NumBackedges != 1) {
// If this is really a nested loop, rip it out into a child loop. Don't do
// this for loops with a giant number of backedges, just factor them into a
// common backedge instead.
if (NumBackedges < 8) {
if (Loop *NL = SeparateNestedLoop(L)) {
++NumNested;
// This is a big restructuring change, reprocess the whole loop.
ProcessLoop(NL);
Changed = true;
// GCC doesn't tail recursion eliminate this.
goto ReprocessLoop;
}
}
// If we either couldn't, or didn't want to, identify nesting of the loops,
// insert a new block that all backedges target, then make it jump to the
// loop header.
InsertUniqueBackedgeBlock(L);
NumInserted++;
Changed = true;
}
// Scan over the PHI nodes in the loop header. Since they now have only two
// incoming values (the loop is canonicalized), we may have simplified the PHI
// down to 'X = phi [X, Y]', which should be replaced with 'Y'.
PHINode *PN;
for (BasicBlock::iterator I = L->getHeader()->begin();
(PN = dyn_cast<PHINode>(I++)); )
if (Value *V = PN->hasConstantValue()) {
PN->replaceAllUsesWith(V);
PN->eraseFromParent();
}
return Changed;
}
/// SplitBlockPredecessors - Split the specified block into two blocks. We want
/// to move the predecessors specified in the Preds list to point to the new
/// block, leaving the remaining predecessors pointing to BB. This method
/// updates the SSA PHINode's, but no other analyses.
///
BasicBlock *LoopSimplify::SplitBlockPredecessors(BasicBlock *BB,
const char *Suffix,
const std::vector<BasicBlock*> &Preds) {
// Create new basic block, insert right before the original block...
BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB->getParent(), BB);
// The preheader first gets an unconditional branch to the loop header...
BranchInst *BI = new BranchInst(BB, NewBB);
// For every PHI node in the block, insert a PHI node into NewBB where the
// incoming values from the out of loop edges are moved to NewBB. We have two
// possible cases here. If the loop is dead, we just insert dummy entries
// into the PHI nodes for the new edge. If the loop is not dead, we move the
// incoming edges in BB into new PHI nodes in NewBB.
//
if (!Preds.empty()) { // Is the loop not obviously dead?
// Check to see if the values being merged into the new block need PHI
// nodes. If so, insert them.
for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) {
PHINode *PN = cast<PHINode>(I);
++I;
// Check to see if all of the values coming in are the same. If so, we
// don't need to create a new PHI node.
Value *InVal = PN->getIncomingValueForBlock(Preds[0]);
for (unsigned i = 1, e = Preds.size(); i != e; ++i)
if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
InVal = 0;
break;
}
// If the values coming into the block are not the same, we need a PHI.
if (InVal == 0) {
// Create the new PHI node, insert it into NewBB at the end of the block
PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
if (AA) AA->copyValue(PN, NewPHI);
// Move all of the edges from blocks outside the loop to the new PHI
for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
Value *V = PN->removeIncomingValue(Preds[i], false);
NewPHI->addIncoming(V, Preds[i]);
}
InVal = NewPHI;
} else {
// Remove all of the edges coming into the PHI nodes from outside of the
// block.
for (unsigned i = 0, e = Preds.size(); i != e; ++i)
PN->removeIncomingValue(Preds[i], false);
}
// Add an incoming value to the PHI node in the loop for the preheader
// edge.
PN->addIncoming(InVal, NewBB);
// Can we eliminate this phi node now?
if (Value *V = PN->hasConstantValue(true)) {
Instruction *I = dyn_cast<Instruction>(V);
// If I is in NewBB, the Dominator call will fail, because NewBB isn't
// registered in DominatorTree yet. Handle this case explicitly.
if (!I || (I->getParent() != NewBB &&
getAnalysis<DominatorTree>().dominates(I, PN))) {
PN->replaceAllUsesWith(V);
if (AA) AA->deleteValue(PN);
BB->getInstList().erase(PN);
}
}
}
// Now that the PHI nodes are updated, actually move the edges from
// Preds to point to NewBB instead of BB.
//
for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
TerminatorInst *TI = Preds[i]->getTerminator();
for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
if (TI->getSuccessor(s) == BB)
TI->setSuccessor(s, NewBB);
}
} else { // Otherwise the loop is dead...
for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
// Insert dummy values as the incoming value...
PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
}
}
return NewBB;
}
/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
/// preheader, this method is called to insert one. This method has two phases:
/// preheader insertion and analysis updating.
///
void LoopSimplify::InsertPreheaderForLoop(Loop *L) {
BasicBlock *Header = L->getHeader();
// Compute the set of predecessors of the loop that are not in the loop.
std::vector<BasicBlock*> OutsideBlocks;
for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
PI != PE; ++PI)
if (!L->contains(*PI)) // Coming in from outside the loop?
OutsideBlocks.push_back(*PI); // Keep track of it...
// Split out the loop pre-header.
BasicBlock *NewBB =
SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);
//===--------------------------------------------------------------------===//
// Update analysis results now that we have performed the transformation
//
// We know that we have loop information to update... update it now.
if (Loop *Parent = L->getParentLoop())
Parent->addBasicBlockToLoop(NewBB, LI->getBase());
DT->splitBlock(NewBB);
if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>())
DF->splitBlock(NewBB);
// Make sure that NewBB is put someplace intelligent, which doesn't mess up
// code layout too horribly.
PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
}
/// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
/// blocks. This method is used to split exit blocks that have predecessors
/// outside of the loop.
BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
std::vector<BasicBlock*> LoopBlocks;
for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
if (L->contains(*I))
LoopBlocks.push_back(*I);
assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks);
// Update Loop Information - we know that the new block will be in whichever
// loop the Exit block is in. Note that it may not be in that immediate loop,
// if the successor is some other loop header. In that case, we continue
// walking up the loop tree to find a loop that contains both the successor
// block and the predecessor block.
Loop *SuccLoop = LI->getLoopFor(Exit);
while (SuccLoop && !SuccLoop->contains(L->getHeader()))
SuccLoop = SuccLoop->getParentLoop();
if (SuccLoop)
SuccLoop->addBasicBlockToLoop(NewBB, LI->getBase());
// Update Dominator Information
DT->splitBlock(NewBB);
if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>())
DF->splitBlock(NewBB);
return NewBB;
}
/// AddBlockAndPredsToSet - Add the specified block, and all of its
/// predecessors, to the specified set, if it's not already in there. Stop
/// predecessor traversal when we reach StopBlock.
static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
std::set<BasicBlock*> &Blocks) {
std::vector<BasicBlock *> WorkList;
WorkList.push_back(InputBB);
do {
BasicBlock *BB = WorkList.back(); WorkList.pop_back();
if (Blocks.insert(BB).second && BB != StopBlock)
// If BB is not already processed and it is not a stop block then
// insert its predecessor in the work list
for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
BasicBlock *WBB = *I;
WorkList.push_back(WBB);
}
} while(!WorkList.empty());
}
/// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
/// PHI node that tells us how to partition the loops.
static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
AliasAnalysis *AA) {
for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
PHINode *PN = cast<PHINode>(I);
++I;
if (Value *V = PN->hasConstantValue())
if (!isa<Instruction>(V) || DT->dominates(cast<Instruction>(V), PN)) {
// This is a degenerate PHI already, don't modify it!
PN->replaceAllUsesWith(V);
if (AA) AA->deleteValue(PN);
PN->eraseFromParent();
continue;
}
// Scan this PHI node looking for a use of the PHI node by itself.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == PN &&
L->contains(PN->getIncomingBlock(i)))
// We found something tasty to remove.
return PN;
}
return 0;
}
// PlaceSplitBlockCarefully - If the block isn't already, move the new block to
// right after some 'outside block' block. This prevents the preheader from
// being placed inside the loop body, e.g. when the loop hasn't been rotated.
void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
std::vector<BasicBlock*>&SplitPreds,
Loop *L) {
// Check to see if NewBB is already well placed.
Function::iterator BBI = NewBB; --BBI;
for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
if (&*BBI == SplitPreds[i])
return;
}
// If it isn't already after an outside block, move it after one. This is
// always good as it makes the uncond branch from the outside block into a
// fall-through.
// Figure out *which* outside block to put this after. Prefer an outside
// block that neighbors a BB actually in the loop.
BasicBlock *FoundBB = 0;
for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
Function::iterator BBI = SplitPreds[i];
if (++BBI != NewBB->getParent()->end() &&
L->contains(BBI)) {
FoundBB = SplitPreds[i];
break;
}
}
// If our heuristic for a *good* bb to place this after doesn't find
// anything, just pick something. It's likely better than leaving it within
// the loop.
if (!FoundBB)
FoundBB = SplitPreds[0];
NewBB->moveAfter(FoundBB);
}
/// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
/// them out into a nested loop. This is important for code that looks like
/// this:
///
/// Loop:
/// ...
/// br cond, Loop, Next
/// ...
/// br cond2, Loop, Out
///
/// To identify this common case, we look at the PHI nodes in the header of the
/// loop. PHI nodes with unchanging values on one backedge correspond to values
/// that change in the "outer" loop, but not in the "inner" loop.
///
/// If we are able to separate out a loop, return the new outer loop that was
/// created.
///
Loop *LoopSimplify::SeparateNestedLoop(Loop *L) {
PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
if (PN == 0) return 0; // No known way to partition.
// Pull out all predecessors that have varying values in the loop. This
// handles the case when a PHI node has multiple instances of itself as
// arguments.
std::vector<BasicBlock*> OuterLoopPreds;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) != PN ||
!L->contains(PN->getIncomingBlock(i)))
OuterLoopPreds.push_back(PN->getIncomingBlock(i));
BasicBlock *Header = L->getHeader();
BasicBlock *NewBB = SplitBlockPredecessors(Header, ".outer", OuterLoopPreds);
// Update dominator information
DT->splitBlock(NewBB);
if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>())
DF->splitBlock(NewBB);
// Make sure that NewBB is put someplace intelligent, which doesn't mess up
// code layout too horribly.
PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
// Create the new outer loop.
Loop *NewOuter = new Loop();
// Change the parent loop to use the outer loop as its child now.
if (Loop *Parent = L->getParentLoop())
Parent->replaceChildLoopWith(L, NewOuter);
else
LI->changeTopLevelLoop(L, NewOuter);
// This block is going to be our new header block: add it to this loop and all
// parent loops.
NewOuter->addBasicBlockToLoop(NewBB, LI->getBase());
// L is now a subloop of our outer loop.
NewOuter->addChildLoop(L);
for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i)
NewOuter->addBlockEntry(L->getBlocks()[i]);
// Determine which blocks should stay in L and which should be moved out to
// the Outer loop now.
std::set<BasicBlock*> BlocksInL;
for (pred_iterator PI = pred_begin(Header), E = pred_end(Header); PI!=E; ++PI)
if (DT->dominates(Header, *PI))
AddBlockAndPredsToSet(*PI, Header, BlocksInL);
// Scan all of the loop children of L, moving them to OuterLoop if they are
// not part of the inner loop.
const std::vector<Loop*> &SubLoops = L->getSubLoops();
for (size_t I = 0; I != SubLoops.size(); )
if (BlocksInL.count(SubLoops[I]->getHeader()))
++I; // Loop remains in L
else
NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
// Now that we know which blocks are in L and which need to be moved to
// OuterLoop, move any blocks that need it.
for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
BasicBlock *BB = L->getBlocks()[i];
if (!BlocksInL.count(BB)) {
// Move this block to the parent, updating the exit blocks sets
L->removeBlockFromLoop(BB);
if ((*LI)[BB] == L)
LI->changeLoopFor(BB, NewOuter);
--i;
}
}
return NewOuter;
}
/// InsertUniqueBackedgeBlock - This method is called when the specified loop
/// has more than one backedge in it. If this occurs, revector all of these
/// backedges to target a new basic block and have that block branch to the loop
/// header. This ensures that loops have exactly one backedge.
///
void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) {
assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
// Get information about the loop
BasicBlock *Preheader = L->getLoopPreheader();
BasicBlock *Header = L->getHeader();
Function *F = Header->getParent();
// Figure out which basic blocks contain back-edges to the loop header.
std::vector<BasicBlock*> BackedgeBlocks;
for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I)
if (*I != Preheader) BackedgeBlocks.push_back(*I);
// Create and insert the new backedge block...
BasicBlock *BEBlock = new BasicBlock(Header->getName()+".backedge", F);
BranchInst *BETerminator = new BranchInst(Header, BEBlock);
// Move the new backedge block to right after the last backedge block.
Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
// Now that the block has been inserted into the function, create PHI nodes in
// the backedge block which correspond to any PHI nodes in the header block.
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
PHINode *NewPN = new PHINode(PN->getType(), PN->getName()+".be",
BETerminator);
NewPN->reserveOperandSpace(BackedgeBlocks.size());
if (AA) AA->copyValue(PN, NewPN);
// Loop over the PHI node, moving all entries except the one for the
// preheader over to the new PHI node.
unsigned PreheaderIdx = ~0U;
bool HasUniqueIncomingValue = true;
Value *UniqueValue = 0;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *IBB = PN->getIncomingBlock(i);
Value *IV = PN->getIncomingValue(i);
if (IBB == Preheader) {
PreheaderIdx = i;
} else {
NewPN->addIncoming(IV, IBB);
if (HasUniqueIncomingValue) {
if (UniqueValue == 0)
UniqueValue = IV;
else if (UniqueValue != IV)
HasUniqueIncomingValue = false;
}
}
}
// Delete all of the incoming values from the old PN except the preheader's
assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
if (PreheaderIdx != 0) {
PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
}
// Nuke all entries except the zero'th.
for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
PN->removeIncomingValue(e-i, false);
// Finally, add the newly constructed PHI node as the entry for the BEBlock.
PN->addIncoming(NewPN, BEBlock);
// As an optimization, if all incoming values in the new PhiNode (which is a
// subset of the incoming values of the old PHI node) have the same value,
// eliminate the PHI Node.
if (HasUniqueIncomingValue) {
NewPN->replaceAllUsesWith(UniqueValue);
if (AA) AA->deleteValue(NewPN);
BEBlock->getInstList().erase(NewPN);
}
}
// Now that all of the PHI nodes have been inserted and adjusted, modify the
// backedge blocks to just to the BEBlock instead of the header.
for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
if (TI->getSuccessor(Op) == Header)
TI->setSuccessor(Op, BEBlock);
}
//===--- Update all analyses which we must preserve now -----------------===//
// Update Loop Information - we know that this block is now in the current
// loop and all parent loops.
L->addBasicBlockToLoop(BEBlock, LI->getBase());
// Update dominator information
DT->splitBlock(BEBlock);
if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>())
DF->splitBlock(BEBlock);
}
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