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SimplifyCFG: fold unconditional branch to its predecessor if profitable.

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This patch extends FoldBranchToCommonDest to fold unconditional branches.
For unconditional branches, we fold them if it is easy to update the phi nodes 
in the common successors.

rdar://10554090


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158392 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Manman Ren 2012-06-13 05:43:29 +00:00
parent 764dc754fe
commit ee28e0fdd1
2 changed files with 213 additions and 24 deletions
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164
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -122,6 +122,47 @@ static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
return true;
}
/// isProfitableToFoldUnconditional - Return true if it is safe and profitable
/// to merge these two terminator instructions together, where SI1 is an
/// unconditional branch. PhiNodes will store all PHI nodes in common
/// successors.
///
static bool isProfitableToFoldUnconditional(BranchInst *SI1,
BranchInst *SI2,
Instruction* Cond,
SmallVectorImpl<PHINode*> &PhiNodes) {
if (SI1 == SI2) return false; // Can't merge with self!
assert(SI1->isUnconditional() && SI2->isConditional());
// We fold the unconditional branch if we can easily update all PHI nodes in
// common successors:
// 1> We have a constant incoming value for the conditional branch;
// 2> We have "Cond" as the incoming value for the unconditional branch;
// 3> SI2->getCondition() and Cond have same operands.
CmpInst *Ci2 = dyn_cast<CmpInst>(SI2->getCondition());
if (!Ci2) return false;
if (!(Cond->getOperand(0) == Ci2->getOperand(0) &&
Cond->getOperand(1) == Ci2->getOperand(1)) &&
!(Cond->getOperand(0) == Ci2->getOperand(1) &&
Cond->getOperand(1) == Ci2->getOperand(0)))
return false;
BasicBlock *SI1BB = SI1->getParent();
BasicBlock *SI2BB = SI2->getParent();
SmallPtrSet<BasicBlock*, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
if (SI1Succs.count(*I))
for (BasicBlock::iterator BBI = (*I)->begin();
isa<PHINode>(BBI); ++BBI) {
PHINode *PN = cast<PHINode>(BBI);
if (PN->getIncomingValueForBlock(SI1BB) != Cond ||
!isa<Constant>(PN->getIncomingValueForBlock(SI2BB)))
return false;
PhiNodes.push_back(PN);
}
return true;
}
/// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
/// now be entries in it from the 'NewPred' block. The values that will be
/// flowing into the PHI nodes will be the same as those coming in from
@ -1506,6 +1547,23 @@ static APInt MultiplyAndLosePrecision(APInt &A, APInt &B, APInt &C, APInt &D,
return Result;
}
/// checkCSEInPredecessor - Return true if the given instruction is available
/// in its predecessor block. If yes, the instruction will be removed.
///
bool checkCSEInPredecessor(Instruction *Inst, BasicBlock *PB) {
if (!isa<BinaryOperator>(Inst) && !isa<CmpInst>(Inst))
return false;
for (BasicBlock::iterator I = PB->begin(), E = PB->end(); I != E; I++) {
Instruction *PBI = &*I;
// Check whether Inst and PBI generate the same value.
if (Inst->isIdenticalTo(PBI)) {
Inst->replaceAllUsesWith(PBI);
Inst->eraseFromParent();
return true;
}
}
return false;
}
/// FoldBranchToCommonDest - If this basic block is simple enough, and if a
/// predecessor branches to us and one of our successors, fold the block into
@ -1513,7 +1571,36 @@ static APInt MultiplyAndLosePrecision(APInt &A, APInt &B, APInt &C, APInt &D,
bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
BasicBlock *BB = BI->getParent();
Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
Instruction *Cond = 0;
if (BI->isConditional())
Cond = dyn_cast<Instruction>(BI->getCondition());
else {
// For unconditional branch, check for a simple CFG pattern, where
// BB has a single predecessor and BB's successor is also its predecessor's
// successor. If such pattern exisits, check for CSE between BB and its
// predecessor.
if (BasicBlock *PB = BB->getSinglePredecessor())
if (BranchInst *PBI = dyn_cast<BranchInst>(PB->getTerminator()))
if (PBI->isConditional() &&
(BI->getSuccessor(0) == PBI->getSuccessor(0) ||
BI->getSuccessor(0) == PBI->getSuccessor(1))) {
for (BasicBlock::iterator I = BB->begin(), E = BB->end();
I != E; ) {
Instruction *Curr = I++;
if (isa<CmpInst>(Curr)) {
Cond = Curr;
break;
}
// Quit if we can't remove this instruction.
if (!checkCSEInPredecessor(Curr, PB))
return false;
}
}
if (Cond == 0)
return false;
}
if (Cond == 0 || (!isa<CmpInst>(Cond) && !isa<BinaryOperator>(Cond)) ||
Cond->getParent() != BB || !Cond->hasOneUse())
return false;
@ -1565,7 +1652,7 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
// Finally, don't infinitely unroll conditional loops.
BasicBlock *TrueDest = BI->getSuccessor(0);
BasicBlock *FalseDest = BI->getSuccessor(1);
BasicBlock *FalseDest = (BI->isConditional()) ? BI->getSuccessor(1) : 0;
if (TrueDest == BB || FalseDest == BB)
return false;
@ -1576,13 +1663,19 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
// Check that we have two conditional branches. If there is a PHI node in
// the common successor, verify that the same value flows in from both
// blocks.
if (PBI == 0 || PBI->isUnconditional() || !SafeToMergeTerminators(BI, PBI))
SmallVector<PHINode*, 4> PHIs;
if (PBI == 0 || PBI->isUnconditional() ||
(BI->isConditional() &&
!SafeToMergeTerminators(BI, PBI)) ||
(!BI->isConditional() &&
!isProfitableToFoldUnconditional(BI, PBI, Cond, PHIs)))
continue;
// Determine if the two branches share a common destination.
Instruction::BinaryOps Opc;
bool InvertPredCond = false;
if (BI->isConditional()) {
if (PBI->getSuccessor(0) == TrueDest)
Opc = Instruction::Or;
else if (PBI->getSuccessor(1) == FalseDest)
@ -1593,6 +1686,10 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
Opc = Instruction::Or, InvertPredCond = true;
else
continue;
} else {
if (PBI->getSuccessor(0) != TrueDest && PBI->getSuccessor(1) != TrueDest)
continue;
}
// Ensure that any values used in the bonus instruction are also used
// by the terminator of the predecessor. This means that those values
@ -1668,10 +1765,12 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
New->takeName(Cond);
Cond->setName(New->getName()+".old");
if (BI->isConditional()) {
Instruction *NewCond =
cast<Instruction>(Builder.CreateBinOp(Opc, PBI->getCondition(),
New, "or.cond"));
PBI->setCondition(NewCond);
if (PBI->getSuccessor(0) == BB) {
AddPredecessorToBlock(TrueDest, PredBlock, BB);
PBI->setSuccessor(0, TrueDest);
@ -1680,13 +1779,64 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
AddPredecessorToBlock(FalseDest, PredBlock, BB);
PBI->setSuccessor(1, FalseDest);
}
} else {
// Update PHI nodes in the common successors.
for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
ConstantInt *PBI_C = dyn_cast<ConstantInt>(
PHIs[i]->getIncomingValueForBlock(PBI->getParent()));
assert(PBI_C->getType()->isIntegerTy(1));
Instruction *MergedCond = 0;
if (PBI->getSuccessor(0) == TrueDest) {
// Create (PBI_Cond and PBI_C) or (!PBI_Cond and BI_Value)
// PBI_C is true: PBI_Cond or (!PBI_Cond and BI_Value)
// is false: !PBI_Cond and BI_Value
Instruction *NotCond =
cast<Instruction>(Builder.CreateNot(PBI->getCondition(),
"not.cond"));
MergedCond =
cast<Instruction>(Builder.CreateBinOp(Instruction::And,
NotCond, New,
"and.cond"));
if (PBI_C->isOne())
MergedCond =
cast<Instruction>(Builder.CreateBinOp(Instruction::Or,
PBI->getCondition(), MergedCond,
"or.cond"));
} else {
// Create (PBI_Cond and BI_Value) or (!PBI_Cond and PBI_C)
// PBI_C is true: (PBI_Cond and BI_Value) or (!PBI_Cond)
// is false: PBI_Cond and BI_Value
MergedCond =
cast<Instruction>(Builder.CreateBinOp(Instruction::And,
PBI->getCondition(), New,
"and.cond"));
if (PBI_C->isOne()) {
Instruction *NotCond =
cast<Instruction>(Builder.CreateNot(PBI->getCondition(),
"not.cond"));
MergedCond =
cast<Instruction>(Builder.CreateBinOp(Instruction::Or,
NotCond, MergedCond,
"or.cond"));
}
}
// Update PHI Node.
PHIs[i]->setIncomingValue(PHIs[i]->getBasicBlockIndex(PBI->getParent()),
MergedCond);
}
// Change PBI from Conditional to Unconditional.
BranchInst *New_PBI = BranchInst::Create(TrueDest, PBI);
EraseTerminatorInstAndDCECond(PBI);
PBI = New_PBI;
}
// TODO: If BB is reachable from all paths through PredBlock, then we
// could replace PBI's branch probabilities with BI's.
// Merge probability data into PredBlock's branch.
APInt A, B, C, D;
if (ExtractBranchMetadata(PBI, C, D) && ExtractBranchMetadata(BI, A, B)) {
if (PBI->isConditional() && BI->isConditional() &&
ExtractBranchMetadata(PBI, C, D) && ExtractBranchMetadata(BI, A, B)) {
// Given IR which does:
// bbA:
// br i1 %x, label %bbB, label %bbC
@ -2772,6 +2922,12 @@ bool SimplifyCFGOpt::SimplifyUncondBranch(BranchInst *BI, IRBuilder<> &Builder){
return true;
}
// If this basic block is ONLY a compare and a branch, and if a predecessor
// branches to us and our successor, fold the comparison into the
// predecessor and use logical operations to update the incoming value
// for PHI nodes in common successor.
if (FoldBranchToCommonDest(BI))
return SimplifyCFG(BB) | true;
return false;
}

33
test/Transforms/SimplifyCFG/branch-fold.ll
View File

@ -17,3 +17,36 @@ b:
c:
ret void
}
; rdar://10554090
define zeroext i1 @test2(i64 %i0, i64 %i1) nounwind uwtable readonly ssp {
entry:
; CHECK: test2
; CHECK: br i1
%and.i.i = and i64 %i0, 281474976710655
%and.i11.i = and i64 %i1, 281474976710655
%or.cond = icmp eq i64 %and.i.i, %and.i11.i
br i1 %or.cond, label %c, label %a
a:
; CHECK: br
%shr.i4.i = lshr i64 %i0, 48
%and.i5.i = and i64 %shr.i4.i, 32767
%shr.i.i = lshr i64 %i1, 48
%and.i2.i = and i64 %shr.i.i, 32767
%cmp9.i = icmp ult i64 %and.i5.i, %and.i2.i
br i1 %cmp9.i, label %c, label %b
b:
; CHECK-NOT: br
%shr.i13.i9 = lshr i64 %i1, 48
%and.i14.i10 = and i64 %shr.i13.i9, 32767
%shr.i.i11 = lshr i64 %i0, 48
%and.i11.i12 = and i64 %shr.i.i11, 32767
%phitmp = icmp uge i64 %and.i14.i10, %and.i11.i12
br label %c
c:
%o2 = phi i1 [ false, %a ], [ %phitmp, %b ], [ false, %entry ]
ret i1 %o2
}