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[CVP] Keep ReachableCaseCount in sync with range of condition (#142302)

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https://github.com/llvm/llvm-project/pull/79993 assumes that a reachable
case must be contained by `CR`. However, it doesn't hold for some edge
cases. This patch adds additional checks to ensure `ReachableCaseCount`
is correct.

Note: Similar optimization in SCCP isn't affected by this bug because it
uses `CR` to compute `ReachableCaseCount`.

Closes https://github.com/llvm/llvm-project/issues/142286.
This commit is contained in:
Yingwei Zheng 2025-06-02 17:42:02 +08:00 committed by Tom Stellard
parent 2481e590ee
commit acf86c5c4d
2 changed files with 71 additions and 24 deletions
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59
llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
View File

@ -370,15 +370,30 @@ static bool processSwitch(SwitchInst *I, LazyValueInfo *LVI,
{ // Scope for SwitchInstProfUpdateWrapper. It must not live during
// ConstantFoldTerminator() as the underlying SwitchInst can be changed.
SwitchInstProfUpdateWrapper SI(*I);
ConstantRange CR =
LVI->getConstantRangeAtUse(I->getOperandUse(0), /*UndefAllowed=*/false);
unsigned ReachableCaseCount = 0;
for (auto CI = SI->case_begin(), CE = SI->case_end(); CI != CE;) {
ConstantInt *Case = CI->getCaseValue();
auto *Res = dyn_cast_or_null<ConstantInt>(
LVI->getPredicateAt(CmpInst::ICMP_EQ, Cond, Case, I,
/* UseBlockValue */ true));
std::optional<bool> Predicate = std::nullopt;
if (!CR.contains(Case->getValue()))
Predicate = false;
else if (CR.isSingleElement() &&
*CR.getSingleElement() == Case->getValue())
Predicate = true;
if (!Predicate) {
// Handle missing cases, e.g., the range has a hole.
auto *Res = dyn_cast_or_null<ConstantInt>(
LVI->getPredicateAt(CmpInst::ICMP_EQ, Cond, Case, I,
/* UseBlockValue=*/true));
if (Res && Res->isZero())
Predicate = false;
else if (Res && Res->isOne())
Predicate = true;
}
if (Res && Res->isZero()) {
if (Predicate && !*Predicate) {
// This case never fires - remove it.
BasicBlock *Succ = CI->getCaseSuccessor();
Succ->removePredecessor(BB);
@ -395,7 +410,7 @@ static bool processSwitch(SwitchInst *I, LazyValueInfo *LVI,
DTU.applyUpdatesPermissive({{DominatorTree::Delete, BB, Succ}});
continue;
}
if (Res && Res->isOne()) {
if (Predicate && *Predicate) {
// This case always fires. Arrange for the switch to be turned into an
// unconditional branch by replacing the switch condition with the case
// value.
@ -410,28 +425,24 @@ static bool processSwitch(SwitchInst *I, LazyValueInfo *LVI,
++ReachableCaseCount;
}
BasicBlock *DefaultDest = SI->getDefaultDest();
if (ReachableCaseCount > 1 &&
!isa<UnreachableInst>(DefaultDest->getFirstNonPHIOrDbg())) {
ConstantRange CR = LVI->getConstantRangeAtUse(I->getOperandUse(0),
/*UndefAllowed*/ false);
// The default dest is unreachable if all cases are covered.
if (!CR.isSizeLargerThan(ReachableCaseCount)) {
BasicBlock *NewUnreachableBB =
BasicBlock::Create(BB->getContext(), "default.unreachable",
BB->getParent(), DefaultDest);
new UnreachableInst(BB->getContext(), NewUnreachableBB);
// The default dest is unreachable if all cases are covered.
if (!SI->defaultDestUndefined() &&
!CR.isSizeLargerThan(ReachableCaseCount)) {
BasicBlock *DefaultDest = SI->getDefaultDest();
BasicBlock *NewUnreachableBB =
BasicBlock::Create(BB->getContext(), "default.unreachable",
BB->getParent(), DefaultDest);
new UnreachableInst(BB->getContext(), NewUnreachableBB);
DefaultDest->removePredecessor(BB);
SI->setDefaultDest(NewUnreachableBB);
DefaultDest->removePredecessor(BB);
SI->setDefaultDest(NewUnreachableBB);
if (SuccessorsCount[DefaultDest] == 1)
DTU.applyUpdates({{DominatorTree::Delete, BB, DefaultDest}});
DTU.applyUpdates({{DominatorTree::Insert, BB, NewUnreachableBB}});
if (SuccessorsCount[DefaultDest] == 1)
DTU.applyUpdates({{DominatorTree::Delete, BB, DefaultDest}});
DTU.applyUpdates({{DominatorTree::Insert, BB, NewUnreachableBB}});
++NumDeadCases;
Changed = true;
}
++NumDeadCases;
Changed = true;
}
}

36
llvm/test/Transforms/CorrelatedValuePropagation/switch.ll
View File

@ -294,6 +294,42 @@ cleanup:
ret i32 %retval.0
}
; Make sure that we don't branch into unreachable.
define void @pr142286() {
; CHECK-LABEL: define void @pr142286() {
; CHECK-NEXT: start:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: br label [[LOOP2:%.*]]
; CHECK: loop2:
; CHECK-NEXT: br label [[LOOP3:%.*]]
; CHECK: loop3:
; CHECK-NEXT: br label [[EXIT:%.*]]
; CHECK: exit:
; CHECK-NEXT: ret void
;
start:
br label %loop
loop:
%phi = phi i8 [ -1, %start ], [ 0, %loop3 ]
br label %loop2
loop2:
br label %loop3
loop3:
switch i8 %phi, label %exit [
i8 0, label %loop3
i8 1, label %loop2
i8 2, label %loop
]
exit:
ret void
}
declare i32 @call0()
declare i32 @call1()
declare i32 @call2()