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[InstCombine] Opt phi(freeze(undef), C) -> phi(C, C) #161181

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Merged
merged 6 commits into from
Oct 1, 2025
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[InstCombine] Opt phi(freeze(undef), C) -> phi(C, C) #161181

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d1a6a71
[InstCombine] Opt phi(freeze(undef), C) -> phi(C, C)
mikael-nilsson-arm Sep 25, 2025
71665e7
Merge branch 'main' into freeze-phi-opt
mikael-nilsson-arm Sep 29, 2025
0f5ed03
Review fix 1
mikael-nilsson-arm Sep 29, 2025
e5ad36c
Review fix 2
mikael-nilsson-arm Sep 29, 2025
453332d
Review fix 3
mikael-nilsson-arm Oct 1, 2025
fe3b3ab
Review fix 4
mikael-nilsson-arm Oct 1, 2025
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31 changes: 29 additions & 2 deletions llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5169,6 +5169,7 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
// - or: pick -1
// - select's condition: if the true value is constant, choose it by making
// the condition true.
// - phi: pick the common constant across operands
// - default: pick 0
//
// Note that this transform is intentionally done here rather than
Expand All @@ -5179,9 +5180,32 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
// TODO: This could use getBinopAbsorber() / getBinopIdentity() to avoid
// duplicating logic for binops at least.
auto getUndefReplacement = [&](Type *Ty) {
Value *BestValue = nullptr;
auto pickCommonConstantFromPHI = [](PHINode &PN) -> Value * {
// phi(freeze(undef), C, C). Choose C for freeze so the PHI can be
// removed.
Constant *BestValue = nullptr;
for (Value *V : PN.incoming_values()) {
if (match(V, m_Freeze(m_Undef())))
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Is it still valid if the incoming value is another freeze undef? I have not found a counterexample yet.

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Yes, this is fine. After all, we're not actually replacing that other freeze undef here. (If we did, we'd have to be more careful about multi-use.)

continue;

Constant *C = dyn_cast<Constant>(V);
if (!C)
return nullptr;

if (!isGuaranteedNotToBeUndefOrPoison(C))
return nullptr;

if (BestValue && BestValue != C)
return nullptr;

BestValue = C;
}
return BestValue;
};

Value *NullValue = Constant::getNullValue(Ty);
for (const auto *U : I.users()) {
Value *BestValue = nullptr;
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Unused variable?

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This one is used inside the loop below, it was part of the original code.

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@mikael-nilsson-arm mikael-nilsson-arm Oct 1, 2025 •
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Seems like I have added newline between NullValue and BestValue, will undo it.

for (auto *U : I.users()) {
Value *V = NullValue;
if (match(U, m_Or(m_Value(), m_Value())))
V = ConstantInt::getAllOnesValue(Ty);
Expand All @@ -5190,6 +5214,9 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
else if (match(U, m_c_Select(m_Specific(&I), m_Value(V)))) {
if (!isGuaranteedNotToBeUndefOrPoison(V, &AC, &I, &DT))
V = NullValue;
} else if (auto *PHI = dyn_cast<PHINode>(U)) {
if (Value *MaybeV = pickCommonConstantFromPHI(*PHI))
V = MaybeV;
}

if (!BestValue)
Expand Down
274 changes: 274 additions & 0 deletions llvm/test/Transforms/InstCombine/in-freeze-phi.ll
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@@ -0,0 +1,274 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
; RUN: opt -passes=instcombine -S < %s | FileCheck %s

define i32 @phi_freeze_same_consts(i1 %c0, i1 %c1) {
; CHECK-LABEL: define i32 @phi_freeze_same_consts(
; CHECK-SAME: i1 [[C0:%.*]], i1 [[C1:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: br i1 [[C0]], label %[[BB_FREEZE:.*]], label %[[BB_OTHER:.*]]
; CHECK: [[BB_FREEZE]]:
; CHECK-NEXT: br label %[[FINAL:.*]]
; CHECK: [[BB_OTHER]]:
; CHECK-NEXT: br i1 [[C1]], label %[[CA:.*]], label %[[CB:.*]]
; CHECK: [[CA]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[CB]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[FINAL]]:
; CHECK-NEXT: ret i32 42
;
entry:
br i1 %c0, label %bb_freeze, label %bb_other

bb_freeze:
%f = freeze i32 undef
br label %final

bb_other:
br i1 %c1, label %cA, label %cB
cA:
br label %final
cB:
br label %final

final:
%phi = phi i32 [ %f, %bb_freeze ], [ 42, %cA ], [ 42, %cB ]
ret i32 %phi
}

define i32 @phi_freeze_mixed_consts(i1 %c0, i1 %c1) {
; CHECK-LABEL: define i32 @phi_freeze_mixed_consts(
; CHECK-SAME: i1 [[C0:%.*]], i1 [[C1:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: br i1 [[C0]], label %[[BB_FREEZE:.*]], label %[[BB_OTHER:.*]]
; CHECK: [[BB_FREEZE]]:
; CHECK-NEXT: br label %[[FINAL:.*]]
; CHECK: [[BB_OTHER]]:
; CHECK-NEXT: br i1 [[C1]], label %[[CA:.*]], label %[[CB:.*]]
; CHECK: [[CA]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[CB]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[FINAL]]:
; CHECK-NEXT: [[PHI:%.*]] = phi i32 [ 0, %[[BB_FREEZE]] ], [ 42, %[[CA]] ], [ 7, %[[CB]] ]
; CHECK-NEXT: ret i32 [[PHI]]
;
entry:
br i1 %c0, label %bb_freeze, label %bb_other

bb_freeze:
%f = freeze i32 undef
br label %final

bb_other:
br i1 %c1, label %cA, label %cB
cA:
br label %final
cB:
br label %final

final:
%phi = phi i32 [ %f, %bb_freeze ], [ 42, %cA ], [ 7, %cB ]
ret i32 %phi
}

define i32 @phi_freeze_with_nonconst_incoming(i32 %x, i1 %c0, i1 %c1) {
; CHECK-LABEL: define i32 @phi_freeze_with_nonconst_incoming(
; CHECK-SAME: i32 [[X:%.*]], i1 [[C0:%.*]], i1 [[C1:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: br i1 [[C0]], label %[[BB_FREEZE:.*]], label %[[BB_OTHER:.*]]
; CHECK: [[BB_FREEZE]]:
; CHECK-NEXT: br label %[[FINAL:.*]]
; CHECK: [[BB_OTHER]]:
; CHECK-NEXT: br i1 [[C1]], label %[[CA:.*]], label %[[CB:.*]]
; CHECK: [[CA]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[CB]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[FINAL]]:
; CHECK-NEXT: [[PHI:%.*]] = phi i32 [ 0, %[[BB_FREEZE]] ], [ [[X]], %[[CA]] ], [ 13, %[[CB]] ]
; CHECK-NEXT: ret i32 [[PHI]]
;
entry:
br i1 %c0, label %bb_freeze, label %bb_other

bb_freeze:
%f = freeze i32 undef
br label %final

bb_other:
br i1 %c1, label %cA, label %cB
cA:
br label %final
cB:
br label %final

final:
%phi = phi i32 [ %f, %bb_freeze ], [ %x, %cA ], [ 13, %cB ]
ret i32 %phi
}

define <4 x i8> @phi_freeze_vector(i1 %c0, i1 %c1) {
; CHECK-LABEL: define <4 x i8> @phi_freeze_vector(
; CHECK-SAME: i1 [[C0:%.*]], i1 [[C1:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: br i1 [[C0]], label %[[BB_FREEZE:.*]], label %[[BB_OTHER:.*]]
; CHECK: [[BB_FREEZE]]:
; CHECK-NEXT: br label %[[FINAL:.*]]
; CHECK: [[BB_OTHER]]:
; CHECK-NEXT: br i1 [[C1]], label %[[CA:.*]], label %[[CB:.*]]
; CHECK: [[CA]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[CB]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[FINAL]]:
; CHECK-NEXT: ret <4 x i8> splat (i8 9)
;
entry:
br i1 %c0, label %bb_freeze, label %bb_other

bb_freeze:
%f = freeze <4 x i8> undef
br label %final

bb_other:
br i1 %c1, label %cA, label %cB

cA:
br label %final

cB:
br label %final

final:
%phi = phi <4 x i8> [ %f, %bb_freeze ],
[<i8 9, i8 9, i8 9, i8 9>, %cA ],
[<i8 9, i8 9, i8 9, i8 9>, %cB ]
ret <4 x i8> %phi
}

define i32 @multi_use_one_folds_one_not_zero(i1 %c0, i1 %c1, i1 %c2) {
; CHECK-LABEL: define i32 @multi_use_one_folds_one_not_zero(
; CHECK-SAME: i1 [[C0:%.*]], i1 [[C1:%.*]], i1 [[C2:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: br i1 [[C0]], label %[[BB_OTHER3:.*]], label %[[CC1:.*]]
; CHECK: [[BB_OTHER3]]:
; CHECK-NEXT: br label %[[MID:.*]]
; CHECK: [[CC1]]:
; CHECK-NEXT: br i1 [[C1]], label %[[CA:.*]], label %[[CB:.*]]
; CHECK: [[CA]]:
; CHECK-NEXT: br label %[[MID]]
; CHECK: [[CB]]:
; CHECK-NEXT: br label %[[MID]]
; CHECK: [[MID]]:
; CHECK-NEXT: [[PHI_FOLD:%.*]] = phi i32 [ 0, %[[BB_OTHER3]] ], [ 1, %[[CA]] ], [ 1, %[[CB]] ]
; CHECK-NEXT: br i1 [[C2]], label %[[BB_FREEZE2:.*]], label %[[CD:.*]]
; CHECK: [[BB_FREEZE2]]:
; CHECK-NEXT: br label %[[FINAL:.*]]
; CHECK: [[BB_OTHER2:.*:]]
; CHECK-NEXT: br i1 true, label %[[CA]], label %[[CB]]
; CHECK: [[CC:.*:]]
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[CD]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[FINAL]]:
; CHECK-NEXT: ret i32 [[PHI_FOLD]]
;
entry:
%f = freeze i32 undef
br i1 %c0, label %bb_freeze, label %bb_other
bb_freeze:
br label %mid
bb_other:
br i1 %c1, label %cA, label %cB
cA:
br label %mid
cB:
br label %mid
mid:
%phi_no_fold = phi i32 [ %f, %bb_freeze ], [ 1, %cA ], [ 1, %cB ]
br i1 %c2, label %bb_freeze2, label %cD
bb_freeze2:
br label %final
bb_other2:
br i1 %c1, label %cA, label %cB
cC:
br label %final
cD:
br label %final
final:
%phi_fold = phi i32 [ %f, %bb_freeze2 ], [ 0, %cC ], [ 0, %cD ]
%a = add i32 %phi_fold, %phi_no_fold
ret i32 %a
}

define i32 @phi_freeze_poison(i1 %c0, i1 %c1) {
; CHECK-LABEL: define i32 @phi_freeze_poison(
; CHECK-SAME: i1 [[C0:%.*]], i1 [[C1:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: br i1 [[C0]], label %[[BB_FREEZE:.*]], label %[[BB_OTHER:.*]]
; CHECK: [[BB_FREEZE]]:
; CHECK-NEXT: br label %[[FINAL:.*]]
; CHECK: [[BB_OTHER]]:
; CHECK-NEXT: br i1 [[C1]], label %[[CA:.*]], label %[[CB:.*]]
; CHECK: [[CA]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[CB]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[FINAL]]:
; CHECK-NEXT: ret i32 0
;
entry:
br i1 %c0, label %bb_freeze, label %bb_other

bb_freeze:
%f = freeze i32 undef
br label %final

bb_other:
br i1 %c1, label %cA, label %cB
cA:
br label %final
cB:
br label %final

final:
%phi = phi i32 [ %f, %bb_freeze ], [ poison, %cA ], [ poison, %cB ]
ret i32 %phi
}

define <2 x i32> @phi_freeze_poison_vec(i1 %c0, i1 %c1) {
; CHECK-LABEL: define <2 x i32> @phi_freeze_poison_vec(
; CHECK-SAME: i1 [[C0:%.*]], i1 [[C1:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: br i1 [[C0]], label %[[BB_FREEZE:.*]], label %[[BB_OTHER:.*]]
; CHECK: [[BB_FREEZE]]:
; CHECK-NEXT: br label %[[FINAL:.*]]
; CHECK: [[BB_OTHER]]:
; CHECK-NEXT: br i1 [[C1]], label %[[CA:.*]], label %[[CB:.*]]
; CHECK: [[CA]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[CB]]:
; CHECK-NEXT: br label %[[FINAL]]
; CHECK: [[FINAL]]:
; CHECK-NEXT: [[PHI:%.*]] = phi <2 x i32> [ zeroinitializer, %[[BB_FREEZE]] ], [ <i32 poison, i32 1>, %[[CA]] ], [ <i32 poison, i32 1>, %[[CB]] ]
; CHECK-NEXT: ret <2 x i32> [[PHI]]
;
entry:
br i1 %c0, label %bb_freeze, label %bb_other

bb_freeze:
%f = freeze <2 x i32> undef
br label %final

bb_other:
br i1 %c1, label %cA, label %cB
cA:
br label %final
cB:
br label %final

final:
%phi = phi <2 x i32> [ %f, %bb_freeze ], [ <i32 poison, i32 1>, %cA ], [ <i32 poison, i32 1>, %cB ]
ret <2 x i32> %phi
}