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[JumpThreading] Simplify Instructions first in ComputeValueKnownInPre…
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…decessors()

This change tries to find more opportunities to thread over basic blocks.

llvm-svn: 261981
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Haicheng Wu committed Feb 26, 2016
1 parent 63cecb3 commit 5539f85
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Showing 2 changed files with 85 additions and 20 deletions.
55 changes: 35 additions & 20 deletions llvm/lib/Transforms/Scalar/JumpThreading.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -152,6 +152,7 @@ namespace {
bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
PredValueInfo &Result,
ConstantPreference Preference,
bool &Changed,
Instruction *CxtI = nullptr);
bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
ConstantPreference Preference,
Expand Down Expand Up @@ -395,10 +396,9 @@ static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
///
/// This returns true if there were any known values.
///
bool JumpThreading::
ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
ConstantPreference Preference,
Instruction *CxtI) {
bool JumpThreading::ComputeValueKnownInPredecessors(
Value *V, BasicBlock *BB, PredValueInfo &Result,
ConstantPreference Preference, bool &Changed, Instruction *CxtI) {
// This method walks up use-def chains recursively. Because of this, we could
// get into an infinite loop going around loops in the use-def chain. To
// prevent this, keep track of what (value, block) pairs we've already visited
Expand All @@ -410,6 +410,16 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
// stack pops back out again.
RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB));

// Simplify the instruction before inferring the value.
Instruction *I = dyn_cast<Instruction>(V);
if (I && !isa<PHINode>(I))
if (auto *NewV = SimplifyInstruction(I, BB->getModule()->getDataLayout())) {
I->replaceAllUsesWith(NewV);
I->eraseFromParent();
V = NewV;
Changed = true;
}

// If V is a constant, then it is known in all predecessors.
if (Constant *KC = getKnownConstant(V, Preference)) {
for (BasicBlock *Pred : predecessors(BB))
Expand All @@ -420,7 +430,7 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,

// If V is a non-instruction value, or an instruction in a different block,
// then it can't be derived from a PHI.
Instruction *I = dyn_cast<Instruction>(V);
I = dyn_cast<Instruction>(V);
if (!I || I->getParent() != BB) {

// Okay, if this is a live-in value, see if it has a known value at the end
Expand Down Expand Up @@ -475,9 +485,9 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
if (I->getOpcode() == Instruction::Or ||
I->getOpcode() == Instruction::And) {
ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
WantInteger, CxtI);
WantInteger, Changed, CxtI);
ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals,
WantInteger, CxtI);
WantInteger, Changed, CxtI);

if (LHSVals.empty() && RHSVals.empty())
return false;
Expand Down Expand Up @@ -512,8 +522,8 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
if (I->getOpcode() == Instruction::Xor &&
isa<ConstantInt>(I->getOperand(1)) &&
cast<ConstantInt>(I->getOperand(1))->isOne()) {
ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result,
WantInteger, CxtI);
ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result, WantInteger,
Changed, CxtI);
if (Result.empty())
return false;

Expand All @@ -531,7 +541,7 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
PredValueInfoTy LHSVals;
ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals,
WantInteger, CxtI);
WantInteger, Changed, CxtI);

// Try to use constant folding to simplify the binary operator.
for (const auto &LHSVal : LHSVals) {
Expand Down Expand Up @@ -608,7 +618,7 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) {
PredValueInfoTy LHSVals;
ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
WantInteger, CxtI);
WantInteger, Changed, CxtI);

for (const auto &LHSVal : LHSVals) {
Constant *V = LHSVal.first;
Expand All @@ -631,7 +641,7 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
PredValueInfoTy Conds;
if ((TrueVal || FalseVal) &&
ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
WantInteger, CxtI)) {
WantInteger, Changed, CxtI)) {
for (auto &C : Conds) {
Constant *Cond = C.first;

Expand Down Expand Up @@ -1180,8 +1190,10 @@ bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
return false;

PredValueInfoTy PredValues;
if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference, CxtI))
return false;
bool Changed = false;
if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference,
Changed, CxtI))
return Changed;

assert(!PredValues.empty() &&
"ComputeValueKnownInPredecessors returned true with no values");
Expand Down Expand Up @@ -1239,7 +1251,7 @@ bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,

// If all edges were unthreadable, we fail.
if (PredToDestList.empty())
return false;
return Changed;

// Determine which is the most common successor. If we have many inputs and
// this block is a switch, we want to start by threading the batch that goes
Expand Down Expand Up @@ -1272,7 +1284,8 @@ bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
getSuccessor(GetBestDestForJumpOnUndef(BB));

// Ok, try to thread it!
return ThreadEdge(BB, PredsToFactor, MostPopularDest);
Changed |= ThreadEdge(BB, PredsToFactor, MostPopularDest);
return Changed;
}

/// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
Expand Down Expand Up @@ -1343,12 +1356,13 @@ bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {

PredValueInfoTy XorOpValues;
bool isLHS = true;
bool Changed = false;
if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
WantInteger, BO)) {
WantInteger, Changed, BO)) {
assert(XorOpValues.empty());
if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
WantInteger, BO))
return false;
WantInteger, Changed, BO))
return Changed;
isLHS = false;
}

Expand Down Expand Up @@ -1406,7 +1420,8 @@ bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
}

// Try to duplicate BB into PredBB.
return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
Changed |= DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
return Changed;
}


Expand Down
50 changes: 50 additions & 0 deletions llvm/test/Transforms/JumpThreading/basic.ll
View file
Original file line number Diff line number Diff line change
Expand Up @@ -476,6 +476,56 @@ exit1:
; CHECK: }
}


;;; Verify that we can handle constraint propagation through cast.
define i32 @test16(i1 %cond) {
Entry:
; CHECK-LABEL: @test16(
br i1 %cond, label %Merge, label %F1

; CHECK: Entry:
; CHECK-NEXT: br i1 %cond, label %F2, label %Merge

F1:
%v1 = call i32 @f1()
br label %Merge

Merge:
%B = phi i32 [1, %Entry], [%v1, %F1]
%M = icmp eq i32 %B, 0
%M1 = zext i1 %M to i32
%N = icmp eq i32 %M1, 1
br i1 %N, label %T2, label %F2

; CHECK: Merge:
; CHECK-NOT: phi
; CHECK-NEXT: %v1 = call i32 @f1()

T2:
%Q = zext i1 %M to i32
ret i32 %Q

F2:
ret i32 %B
; CHECK: F2:
; CHECK-NEXT: phi i32
}

;;; Just check that ComputeValueKnownInPredecessors() does not return true with
;;; no values and triggers the assert in ProcessThreadableEdges().
define i32 @test17() {
entry:
%A = add i32 0, 1
%B = icmp eq i32 %A, 0
br i1 %B, label %T, label %F
T:
%v1 = call i32 @f1()
ret i32 %v1
F:
%v2 = call i32 @f2()
ret i32 %v2
}

; In this test we check that block duplication is inhibited by the presence
; of a function with the 'noduplicate' attribute.

Expand Down

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