[ValueTracking] Let isGuaranteedNotToBeUndefOrPoison look into branch conditions of dominating blocks' terminators

Summary:
```
  br i1 c, BB1, BB2:
BB1:
  use1(c)
BB2:
  use2(c)
```

In BB1 and BB2, c is never undef or poison because otherwise the branch would have triggered UB.

This is a resubmission of 952ad47 with crash fix of llvm/test/Transforms/LoopRotate/freeze-crash.ll.

Checked with Alive2

Reviewers: xbolva00, spatel, lebedev.ri, reames, jdoerfert, nlopes, sanjoy

Reviewed By: reames

Subscribers: jdoerfert, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75401
This commit is contained in:
Juneyoung Lee 2020-03-06 00:56:07 +09:00
parent 85ae5aa6ff
commit d7267ee194
4 changed files with 100 additions and 5 deletions

View File

@ -569,7 +569,13 @@ class Value;
/// Return true if this function can prove that V is never undef value
/// or poison value.
bool isGuaranteedNotToBeUndefOrPoison(const Value *V);
//
/// If CtxI and DT are specified this method performs flow-sensitive analysis
/// and returns true if it is guaranteed to be never undef or poison
/// immediately before the CtxI.
bool isGuaranteedNotToBeUndefOrPoison(const Value *V,
const Instruction *CtxI = nullptr,
const DominatorTree *DT = nullptr);
/// Specific patterns of select instructions we can match.
enum SelectPatternFlavor {

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@ -5360,16 +5360,16 @@ Value *llvm::SimplifyCall(CallBase *Call, const SimplifyQuery &Q) {
}
/// Given operands for a Freeze, see if we can fold the result.
static Value *SimplifyFreezeInst(Value *Op0) {
static Value *SimplifyFreezeInst(Value *Op0, const SimplifyQuery &Q) {
// Use a utility function defined in ValueTracking.
if (llvm::isGuaranteedNotToBeUndefOrPoison(Op0))
if (llvm::isGuaranteedNotToBeUndefOrPoison(Op0, Q.CxtI, Q.DT))
return Op0;
// We have room for improvement.
return nullptr;
}
Value *llvm::SimplifyFreezeInst(Value *Op0, const SimplifyQuery &Q) {
return ::SimplifyFreezeInst(Op0);
return ::SimplifyFreezeInst(Op0, Q);
}
/// See if we can compute a simplified version of this instruction.

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@ -4525,7 +4525,9 @@ bool llvm::isOverflowIntrinsicNoWrap(const WithOverflowInst *WO,
return llvm::any_of(GuardingBranches, AllUsesGuardedByBranch);
}
bool llvm::isGuaranteedNotToBeUndefOrPoison(const Value *V) {
bool llvm::isGuaranteedNotToBeUndefOrPoison(const Value *V,
const Instruction *CtxI,
const DominatorTree *DT) {
// If the value is a freeze instruction, then it can never
// be undef or poison.
if (isa<FreezeInst>(V))
@ -4558,6 +4560,30 @@ bool llvm::isGuaranteedNotToBeUndefOrPoison(const Value *V) {
return true;
}
// CxtI may be null or a cloned instruction.
if (!CtxI || !CtxI->getParent() || !DT)
return false;
// If V is used as a branch condition before reaching CtxI, V cannot be
// undef or poison.
// br V, BB1, BB2
// BB1:
// CtxI ; V cannot be undef or poison here
auto Dominator = DT->getNode(CtxI->getParent())->getIDom();
while (Dominator) {
auto *TI = Dominator->getBlock()->getTerminator();
if (auto BI = dyn_cast<BranchInst>(TI)) {
if (BI->isConditional() && BI->getCondition() == V)
return true;
} else if (auto SI = dyn_cast<SwitchInst>(TI)) {
if (SI->getCondition() == V)
return true;
}
Dominator = Dominator->getIDom();
}
return false;
}

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@ -18,3 +18,66 @@ define i32 @make_const() {
%x = freeze i32 10
ret i32 %x
}
define i1 @brcond(i1 %c, i1 %c2) {
; CHECK-LABEL: @brcond(
; CHECK-NEXT: br i1 [[C:%.*]], label [[A:%.*]], label [[B:%.*]]
; CHECK: A:
; CHECK-NEXT: br i1 [[C2:%.*]], label [[A2:%.*]], label [[B]]
; CHECK: A2:
; CHECK-NEXT: ret i1 [[C]]
; CHECK: B:
; CHECK-NEXT: ret i1 [[C]]
;
br i1 %c, label %A, label %B
A:
br i1 %c2, label %A2, label %B
A2:
%f1 = freeze i1 %c
ret i1 %f1
B:
%f2 = freeze i1 %c
ret i1 %f2
}
define i32 @brcond_switch(i32 %x) {
; CHECK-LABEL: @brcond_switch(
; CHECK-NEXT: switch i32 [[X:%.*]], label [[EXIT:%.*]] [
; CHECK-NEXT: i32 0, label [[A:%.*]]
; CHECK-NEXT: ]
; CHECK: A:
; CHECK-NEXT: ret i32 [[X]]
; CHECK: EXIT:
; CHECK-NEXT: ret i32 [[X]]
;
switch i32 %x, label %EXIT [ i32 0, label %A ]
A:
%fr1 = freeze i32 %x
ret i32 %fr1
EXIT:
%fr2 = freeze i32 %x
ret i32 %fr2
}
define i1 @brcond_noopt(i1 %c, i1 %c2) {
; CHECK-LABEL: @brcond_noopt(
; CHECK-NEXT: [[F:%.*]] = freeze i1 [[C:%.*]]
; CHECK-NEXT: call void @f1(i1 [[F]])
; CHECK-NEXT: call void @f2()
; CHECK-NEXT: br i1 [[C]], label [[A:%.*]], label [[B:%.*]]
; CHECK: A:
; CHECK-NEXT: ret i1 false
; CHECK: B:
; CHECK-NEXT: ret i1 true
;
%f = freeze i1 %c
call void @f1(i1 %f) ; cannot optimize i1 %f to %c
call void @f2() ; .. because if f2() exits, `br %c` cannot be reached
br i1 %c, label %A, label %B
A:
ret i1 0
B:
ret i1 1
}
declare void @f1(i1)
declare void @f2()