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make jump threading on a phi with undef inputs happen.

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llvm-svn: 83754
This commit is contained in:
Chris Lattner 2009-10-11 04:18:15 +00:00
parent a9bbfdf699
commit 525c0272aa
2 changed files with 86 additions and 30 deletions
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82
lib/Transforms/Scalar/JumpThreading.cpp
View File

@ -220,6 +220,28 @@ static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
return Size;
}
/// GetBestDestForBranchOnUndef - If we determine that the specified block ends
/// in an undefined jump, decide which block is best to revector to.
///
/// Since we can pick an arbitrary destination, we pick the successor with the
/// fewest predecessors. This should reduce the in-degree of the others.
///
static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
TerminatorInst *BBTerm = BB->getTerminator();
unsigned MinSucc = 0;
BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
// Compute the successor with the minimum number of predecessors.
unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
TestBB = BBTerm->getSuccessor(i);
unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
if (NumPreds < MinNumPreds)
MinSucc = i;
}
return MinSucc;
}
/// ProcessBlock - If there are any predecessors whose control can be threaded
/// through to a successor, transform them now.
bool JumpThreading::ProcessBlock(BasicBlock *BB) {
@ -269,31 +291,20 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
}
// If the terminator is branching on an undef, we can pick any of the
// successors to branch to. Since this is arbitrary, we pick the successor
// with the fewest predecessors. This should reduce the in-degree of the
// others.
// successors to branch to. Let GetBestDestForJumpOnUndef decide.
if (isa<UndefValue>(Condition)) {
TerminatorInst *BBTerm = BB->getTerminator();
unsigned MinSucc = 0;
BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
// Compute the successor with the minimum number of predecessors.
unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
TestBB = BBTerm->getSuccessor(i);
unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
if (NumPreds < MinNumPreds)
MinSucc = i;
}
unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
// Fold the branch/switch.
TerminatorInst *BBTerm = BB->getTerminator();
for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
if (i == MinSucc) continue;
if (i == BestSucc) continue;
BBTerm->getSuccessor(i)->removePredecessor(BB);
}
DEBUG(errs() << " In block '" << BB->getName()
<< "' folding undef terminator: " << *BBTerm);
BranchInst::Create(BBTerm->getSuccessor(MinSucc), BBTerm);
BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
BBTerm->eraseFromParent();
return true;
}
@ -684,22 +695,32 @@ bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
}
/// ProcessJumpOnPHI - We have a conditional branch of switch on a PHI node in
/// ProcessJumpOnPHI - We have a conditional branch or switch on a PHI node in
/// the current block. See if there are any simplifications we can do based on
/// inputs to the phi node.
///
bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) {
// See if the phi node has any constant values. If so, we can determine where
// the corresponding predecessor will branch.
ConstantInt *PredCst = 0;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i))))
// See if the phi node has any constant integer or undef values. If so, we
// can determine where the corresponding predecessor will branch.
Constant *PredCst = 0;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *PredVal = PN->getIncomingValue(i);
if (ConstantInt *CI = dyn_cast<ConstantInt>(PredVal)) {
PredCst = CI;
break;
}
if (UndefValue *UV = dyn_cast<UndefValue>(PredVal)) {
PredCst = UV;
break;
}
}
// If no incoming value has a constant, we don't know the destination of any
// predecessors.
if (PredCst == 0)
if (PredCst == 0) {
return false;
}
// See if the cost of duplicating this block is low enough.
BasicBlock *BB = PN->getParent();
@ -714,14 +735,19 @@ bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) {
// that will act the same.
BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
TerminatorInst *BBTerm = BB->getTerminator();
// Next, figure out which successor we are threading to.
BasicBlock *SuccBB;
if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
SuccBB = BI->getSuccessor(PredCst ==
ConstantInt::getFalse(PredBB->getContext()));
if (isa<UndefValue>(PredCst)) {
// If the branch was going off an undef from PredBB, pick an arbitrary dest.
SuccBB = BBTerm->getSuccessor(GetBestDestForJumpOnUndef(BB));
} else if (BranchInst *BI = dyn_cast<BranchInst>(BBTerm))
SuccBB = BI->getSuccessor(cast<ConstantInt>(PredCst)->isZero());
else {
SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst));
SwitchInst *SI = cast<SwitchInst>(BBTerm);
SuccBB = SI->getSuccessor(SI->findCaseValue(cast<ConstantInt>(PredCst)));
}
// Ok, try to thread it!

34
test/Transforms/JumpThreading/basic.ll
View File

@ -1,6 +1,4 @@
; RUN: opt < %s -jump-threading -S | FileCheck %s
; There should be no uncond branches left.
; RUN: opt < %s -jump-threading -S | not grep {br label}
declare i32 @f1()
declare i32 @f2()
@ -75,3 +73,35 @@ T1:
F1:
ret i32 17
}
define i32 @test4(i1 %cond, i1 %cond2) {
; CHECK: @test4
br i1 %cond, label %T1, label %F1
T1:
; CHECK: %v1 = call i32 @f1()
; CHECK-NEXT: br label %T
%v1 = call i32 @f1()
br label %Merge
F1:
%v2 = call i32 @f2()
; CHECK: %v2 = call i32 @f2()
; CHECK-NEXT: br i1 %cond2,
br label %Merge
Merge:
%A = phi i1 [undef, %T1], [%cond2, %F1]
%B = phi i32 [%v1, %T1], [%v2, %F1]
br i1 %A, label %T2, label %F2
T2:
call void @f3()
ret i32 %B
F2:
ret i32 %B
}