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refactor TryToSimplifyUncondBranchFromEmptyBlock out of SimplifyCFG.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86666 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2009-11-10 05:59:26 +00:00
parent c5e1ae1313
commit dce94d92df
3 changed files with 181 additions and 165 deletions
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9
include/llvm/Transforms/Utils/Local.h
View File

@ -85,7 +85,14 @@ void RecursivelyDeleteDeadPHINode(PHINode *PN);
///
void MergeBasicBlockIntoOnlyPred(BasicBlock *BB, Pass *P = 0);
/// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an
/// unconditional branch, and contains no instructions other than PHI nodes,
/// potential debug intrinsics and the branch. If possible, eliminate BB by
/// rewriting all the predecessors to branch to the successor block and return
/// true. If we can't transform, return false.
bool TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB);
/// SimplifyCFG - This function is used to do simplification of a CFG. For
/// example, it adjusts branches to branches to eliminate the extra hop, it
/// eliminates unreachable basic blocks, and does other "peephole" optimization

171
lib/Transforms/Utils/Local.cpp
View File

@ -26,8 +26,11 @@
#include "llvm/Analysis/DebugInfo.h"
#include "llvm/Analysis/ProfileInfo.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
//===----------------------------------------------------------------------===//
@ -362,6 +365,174 @@ void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) {
PredBB->eraseFromParent();
}
/// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
/// almost-empty BB ending in an unconditional branch to Succ, into succ.
///
/// Assumption: Succ is the single successor for BB.
///
static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
DEBUG(errs() << "Looking to fold " << BB->getName() << " into "
<< Succ->getName() << "\n");
// Shortcut, if there is only a single predecessor it must be BB and merging
// is always safe
if (Succ->getSinglePredecessor()) return true;
// Make a list of the predecessors of BB
typedef SmallPtrSet<BasicBlock*, 16> BlockSet;
BlockSet BBPreds(pred_begin(BB), pred_end(BB));
// Use that list to make another list of common predecessors of BB and Succ
BlockSet CommonPreds;
for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);
PI != PE; ++PI)
if (BBPreds.count(*PI))
CommonPreds.insert(*PI);
// Shortcut, if there are no common predecessors, merging is always safe
if (CommonPreds.empty())
return true;
// Look at all the phi nodes in Succ, to see if they present a conflict when
// merging these blocks
for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
// If the incoming value from BB is again a PHINode in
// BB which has the same incoming value for *PI as PN does, we can
// merge the phi nodes and then the blocks can still be merged
PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB));
if (BBPN && BBPN->getParent() == BB) {
for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end();
PI != PE; PI++) {
if (BBPN->getIncomingValueForBlock(*PI)
!= PN->getIncomingValueForBlock(*PI)) {
DEBUG(errs() << "Can't fold, phi node " << PN->getName() << " in "
<< Succ->getName() << " is conflicting with "
<< BBPN->getName() << " with regard to common predecessor "
<< (*PI)->getName() << "\n");
return false;
}
}
} else {
Value* Val = PN->getIncomingValueForBlock(BB);
for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end();
PI != PE; PI++) {
// See if the incoming value for the common predecessor is equal to the
// one for BB, in which case this phi node will not prevent the merging
// of the block.
if (Val != PN->getIncomingValueForBlock(*PI)) {
DEBUG(errs() << "Can't fold, phi node " << PN->getName() << " in "
<< Succ->getName() << " is conflicting with regard to common "
<< "predecessor " << (*PI)->getName() << "\n");
return false;
}
}
}
}
return true;
}
/// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an
/// unconditional branch, and contains no instructions other than PHI nodes,
/// potential debug intrinsics and the branch. If possible, eliminate BB by
/// rewriting all the predecessors to branch to the successor block and return
/// true. If we can't transform, return false.
bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) {
// We can't eliminate infinite loops.
BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0);
if (BB == Succ) return false;
// Check to see if merging these blocks would cause conflicts for any of the
// phi nodes in BB or Succ. If not, we can safely merge.
if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
// Check for cases where Succ has multiple predecessors and a PHI node in BB
// has uses which will not disappear when the PHI nodes are merged. It is
// possible to handle such cases, but difficult: it requires checking whether
// BB dominates Succ, which is non-trivial to calculate in the case where
// Succ has multiple predecessors. Also, it requires checking whether
// constructing the necessary self-referential PHI node doesn't intoduce any
// conflicts; this isn't too difficult, but the previous code for doing this
// was incorrect.
//
// Note that if this check finds a live use, BB dominates Succ, so BB is
// something like a loop pre-header (or rarely, a part of an irreducible CFG);
// folding the branch isn't profitable in that case anyway.
if (!Succ->getSinglePredecessor()) {
BasicBlock::iterator BBI = BB->begin();
while (isa<PHINode>(*BBI)) {
for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end();
UI != E; ++UI) {
if (PHINode* PN = dyn_cast<PHINode>(*UI)) {
if (PN->getIncomingBlock(UI) != BB)
return false;
} else {
return false;
}
}
++BBI;
}
}
DEBUG(errs() << "Killing Trivial BB: \n" << *BB);
if (isa<PHINode>(Succ->begin())) {
// If there is more than one pred of succ, and there are PHI nodes in
// the successor, then we need to add incoming edges for the PHI nodes
//
const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
// Loop over all of the PHI nodes in the successor of BB.
for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
Value *OldVal = PN->removeIncomingValue(BB, false);
assert(OldVal && "No entry in PHI for Pred BB!");
// If this incoming value is one of the PHI nodes in BB, the new entries
// in the PHI node are the entries from the old PHI.
if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
PHINode *OldValPN = cast<PHINode>(OldVal);
for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
// Note that, since we are merging phi nodes and BB and Succ might
// have common predecessors, we could end up with a phi node with
// identical incoming branches. This will be cleaned up later (and
// will trigger asserts if we try to clean it up now, without also
// simplifying the corresponding conditional branch).
PN->addIncoming(OldValPN->getIncomingValue(i),
OldValPN->getIncomingBlock(i));
} else {
// Add an incoming value for each of the new incoming values.
for (unsigned i = 0, e = BBPreds.size(); i != e; ++i)
PN->addIncoming(OldVal, BBPreds[i]);
}
}
}
while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
if (Succ->getSinglePredecessor()) {
// BB is the only predecessor of Succ, so Succ will end up with exactly
// the same predecessors BB had.
Succ->getInstList().splice(Succ->begin(),
BB->getInstList(), BB->begin());
} else {
// We explicitly check for such uses in CanPropagatePredecessorsForPHIs.
assert(PN->use_empty() && "There shouldn't be any uses here!");
PN->eraseFromParent();
}
}
// Everything that jumped to BB now goes to Succ.
BB->replaceAllUsesWith(Succ);
if (!Succ->hasName()) Succ->takeName(BB);
BB->eraseFromParent(); // Delete the old basic block.
return true;
}
/// OnlyUsedByDbgIntrinsics - Return true if the instruction I is only used
/// by DbgIntrinsics. If DbgInUses is specified then the vector is filled
/// with the DbgInfoIntrinsic that use the instruction I.

166
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -78,166 +78,6 @@ static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
PN->addIncoming(PN->getIncomingValueForBlock(ExistPred), NewPred);
}
/// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
/// almost-empty BB ending in an unconditional branch to Succ, into succ.
///
/// Assumption: Succ is the single successor for BB.
///
static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
DEBUG(errs() << "Looking to fold " << BB->getName() << " into "
<< Succ->getName() << "\n");
// Shortcut, if there is only a single predecessor it must be BB and merging
// is always safe
if (Succ->getSinglePredecessor()) return true;
// Make a list of the predecessors of BB
typedef SmallPtrSet<BasicBlock*, 16> BlockSet;
BlockSet BBPreds(pred_begin(BB), pred_end(BB));
// Use that list to make another list of common predecessors of BB and Succ
BlockSet CommonPreds;
for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);
PI != PE; ++PI)
if (BBPreds.count(*PI))
CommonPreds.insert(*PI);
// Shortcut, if there are no common predecessors, merging is always safe
if (CommonPreds.empty())
return true;
// Look at all the phi nodes in Succ, to see if they present a conflict when
// merging these blocks
for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
// If the incoming value from BB is again a PHINode in
// BB which has the same incoming value for *PI as PN does, we can
// merge the phi nodes and then the blocks can still be merged
PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB));
if (BBPN && BBPN->getParent() == BB) {
for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end();
PI != PE; PI++) {
if (BBPN->getIncomingValueForBlock(*PI)
!= PN->getIncomingValueForBlock(*PI)) {
DEBUG(errs() << "Can't fold, phi node " << PN->getName() << " in "
<< Succ->getName() << " is conflicting with "
<< BBPN->getName() << " with regard to common predecessor "
<< (*PI)->getName() << "\n");
return false;
}
}
} else {
Value* Val = PN->getIncomingValueForBlock(BB);
for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end();
PI != PE; PI++) {
// See if the incoming value for the common predecessor is equal to the
// one for BB, in which case this phi node will not prevent the merging
// of the block.
if (Val != PN->getIncomingValueForBlock(*PI)) {
DEBUG(errs() << "Can't fold, phi node " << PN->getName() << " in "
<< Succ->getName() << " is conflicting with regard to common "
<< "predecessor " << (*PI)->getName() << "\n");
return false;
}
}
}
}
return true;
}
/// TryToSimplifyUncondBranchFromEmptyBlock - BB contains an unconditional
/// branch to Succ, and contains no instructions other than PHI nodes and the
/// branch. If possible, eliminate BB.
static bool TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
BasicBlock *Succ) {
// Check to see if merging these blocks would cause conflicts for any of the
// phi nodes in BB or Succ. If not, we can safely merge.
if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
// Check for cases where Succ has multiple predecessors and a PHI node in BB
// has uses which will not disappear when the PHI nodes are merged. It is
// possible to handle such cases, but difficult: it requires checking whether
// BB dominates Succ, which is non-trivial to calculate in the case where
// Succ has multiple predecessors. Also, it requires checking whether
// constructing the necessary self-referential PHI node doesn't intoduce any
// conflicts; this isn't too difficult, but the previous code for doing this
// was incorrect.
//
// Note that if this check finds a live use, BB dominates Succ, so BB is
// something like a loop pre-header (or rarely, a part of an irreducible CFG);
// folding the branch isn't profitable in that case anyway.
if (!Succ->getSinglePredecessor()) {
BasicBlock::iterator BBI = BB->begin();
while (isa<PHINode>(*BBI)) {
for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end();
UI != E; ++UI) {
if (PHINode* PN = dyn_cast<PHINode>(*UI)) {
if (PN->getIncomingBlock(UI) != BB)
return false;
} else {
return false;
}
}
++BBI;
}
}
DEBUG(errs() << "Killing Trivial BB: \n" << *BB);
if (isa<PHINode>(Succ->begin())) {
// If there is more than one pred of succ, and there are PHI nodes in
// the successor, then we need to add incoming edges for the PHI nodes
//
const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
// Loop over all of the PHI nodes in the successor of BB.
for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
Value *OldVal = PN->removeIncomingValue(BB, false);
assert(OldVal && "No entry in PHI for Pred BB!");
// If this incoming value is one of the PHI nodes in BB, the new entries
// in the PHI node are the entries from the old PHI.
if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
PHINode *OldValPN = cast<PHINode>(OldVal);
for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
// Note that, since we are merging phi nodes and BB and Succ might
// have common predecessors, we could end up with a phi node with
// identical incoming branches. This will be cleaned up later (and
// will trigger asserts if we try to clean it up now, without also
// simplifying the corresponding conditional branch).
PN->addIncoming(OldValPN->getIncomingValue(i),
OldValPN->getIncomingBlock(i));
} else {
// Add an incoming value for each of the new incoming values.
for (unsigned i = 0, e = BBPreds.size(); i != e; ++i)
PN->addIncoming(OldVal, BBPreds[i]);
}
}
}
while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
if (Succ->getSinglePredecessor()) {
// BB is the only predecessor of Succ, so Succ will end up with exactly
// the same predecessors BB had.
Succ->getInstList().splice(Succ->begin(),
BB->getInstList(), BB->begin());
} else {
// We explicitly check for such uses in CanPropagatePredecessorsForPHIs.
assert(PN->use_empty() && "There shouldn't be any uses here!");
PN->eraseFromParent();
}
}
// Everything that jumped to BB now goes to Succ.
BB->replaceAllUsesWith(Succ);
if (!Succ->hasName()) Succ->takeName(BB);
BB->eraseFromParent(); // Delete the old basic block.
return true;
}
/// GetIfCondition - Given a basic block (BB) with two predecessors (and
/// presumably PHI nodes in it), check to see if the merge at this block is due
@ -1983,13 +1823,11 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
if (BI->isUnconditional()) {
BasicBlock::iterator BBI = BB->getFirstNonPHI();
BasicBlock *Succ = BI->getSuccessor(0);
// Ignore dbg intrinsics.
while (isa<DbgInfoIntrinsic>(BBI))
++BBI;
if (BBI->isTerminator() && // Terminator is the only non-phi instruction!
Succ != BB) // Don't hurt infinite loops!
if (TryToSimplifyUncondBranchFromEmptyBlock(BB, Succ))
if (BBI->isTerminator()) // Terminator is the only non-phi instruction!
if (TryToSimplifyUncondBranchFromEmptyBlock(BB))
return true;
} else { // Conditional branch