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

mikael-nilsson-arm · 2025-09-29T11:47:46Z

Try to choose a value for freeze that enables the PHI to be replaced with its input constants if they are equal.

Change-Id: I29fd2663a5c0e4e3a6d98a2c9536f515a12947a5

llvmbot · 2025-09-29T11:48:19Z

@llvm/pr-subscribers-llvm-transforms

Author: None (mikael-nilsson-arm)

Changes

Change-Id: I29fd2663a5c0e4e3a6d98a2c9536f515a12947a5

Full diff: https://github.com/llvm/llvm-project/pull/161181.diff

2 Files Affected:

(modified) llvm/lib/Transforms/InstCombine/InstructionCombining.cpp (+44-1)
(added) llvm/test/Transforms/InstCombine/in-freeze-phi.ll (+238)

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index f0ddd5ca94c5a..8e4bb38740117 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -5179,8 +5179,51 @@ 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;
+      Constant *C = nullptr;
+      for (Value *V : PN.incoming_values()) {
+        if (match(V, m_Freeze(m_Undef())))
+          continue;
+
+        if (!isa<Constant>(V))
+          return nullptr;
+
+        C = cast<Constant>(V);
+
+        if (BestValue && BestValue != C)
+          return nullptr;
+
+        BestValue = C;
+      }
+      return BestValue;
+    };
+
     Value *NullValue = Constant::getNullValue(Ty);
+
+    bool OnlyPHIUsers =
+        all_of(I.users(), [](const User *U) { return isa<PHINode>(U); });
+    if (OnlyPHIUsers) {
+      Value *BestValue = nullptr;
+      for (auto *U : I.users()) {
+        Value *V = pickCommonConstantFromPHI(*cast<PHINode>(U));
+        if (!V)
+          continue;
+
+        if (!BestValue) {
+          BestValue = V;
+        } else if (BestValue && BestValue == NullValue) {
+          BestValue = NullValue;
+        }
+      }
+
+      if (BestValue)
+        return BestValue;
+    }
+
+    Value *BestValue = nullptr;
     for (const auto *U : I.users()) {
       Value *V = NullValue;
       if (match(U, m_Or(m_Value(), m_Value())))
diff --git a/llvm/test/Transforms/InstCombine/in-freeze-phi.ll b/llvm/test/Transforms/InstCombine/in-freeze-phi.ll
new file mode 100644
index 0000000000000..261aafa179cb7
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/in-freeze-phi.ll
@@ -0,0 +1,238 @@
+; 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(i1 %c0, i1 %c1) {
+; CHECK-LABEL: define i32 @multi_use_one_folds_one_not(
+; 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 %[[MID:.*]]
+; CHECK:       [[BB_OTHER]]:
+; 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:    [[A:%.*]] = phi i32 [ 2, %[[BB_FREEZE]] ], [ 4, %[[CA]] ], [ 3, %[[CB]] ]
+; CHECK-NEXT:    ret i32 [[A]]
+;
+entry:
+  br i1 %c0, label %bb_freeze, label %bb_other
+bb_freeze:
+  %f = freeze i32 undef
+  br label %mid
+bb_other:
+  br i1 %c1, label %cA, label %cB
+cA:
+  br label %mid
+cB:
+  br label %mid
+mid:
+  %phi_fold  = phi i32 [ %f, %bb_freeze ], [ 1, %cA ], [ 1, %cB ]
+  %phi_nofld = phi i32 [ %f, %bb_freeze ], [ 3,  %cA ], [ 2, %cB ]
+  %a = add i32 %phi_fold, %phi_nofld
+  ret i32 %a
+}
+
+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
+}

mikael-nilsson-arm · 2025-09-29T11:51:01Z

Alive proof:

define i32 @src(i1 %c0, i1 %c1) {
entry:
br i1 %c0, label %bb_freeze, label %bb_other

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

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

merge:
  %phi = phi i32 [ %f, %bb_freeze ], [ 42, %cA ], [ 42, %cB ]
ret i32 %phi
}
define i32 @tgt(i1 %c0, i1 %c1) {
entry:
br i1 %c0, label %bb_freeze, label %bb_other

bb_freeze:
br label %merge

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

merge:
ret i32 42
}

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

Change-Id: I40d22b4b8f34bcc12b2d5f76784dd3c42d00422d

github-actions · 2025-09-29T15:24:03Z

✅ With the latest revision this PR passed the undef deprecator.

Change-Id: I1e2f175cc22c3e0190f2e8f0ad8196a35e53af71

dtcxzyw · 2025-09-30T16:08:32Z

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

+      Constant *BestValue = nullptr;
+      Constant *C = nullptr;
+      for (Value *V : PN.incoming_values()) {
+        if (match(V, m_Freeze(m_Undef())))


Is it still valid if the incoming value is another freeze undef? I have not found a counterexample yet.

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.)

dtcxzyw · 2025-09-30T16:09:17Z

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

+        if (!isa<Constant>(V))
+          return nullptr;
+
+        if (!isGuaranteedNotToBeUndefOrPoison(V, &AC, &I, &DT))


Suggested change

if (!isGuaranteedNotToBeUndefOrPoison(V, &AC, &I, &DT))

if (!isGuaranteedNotToBeUndefOrPoison(V))

Context information is unused for constants.

Probably worth noting that we can also do this for non-constants, though that needs a domination check.

dtcxzyw

LGTM.

Change-Id: If87e6cbe36d008805ccc14e77660d245eaa64ed9

nikic · 2025-10-01T07:03:33Z

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

+      // phi(freeze(undef), C, C). Choose C for freeze so the PHI can be
+      // removed.
+      Constant *BestValue = nullptr;
+      Constant *C = nullptr;


C is not used across iterations, so please combine it with the initialization in the loop.

nikic · 2025-10-01T07:03:49Z

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

+        if (match(V, m_Freeze(m_Undef())))
+          continue;
+
+        if (!isa<Constant>(V))


Prefer dyn_cast over isa + cast?

nikic · 2025-10-01T07:04:14Z

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

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


Unused variable?

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

Seems like I have added newline between NullValue and BestValue, will undo it.

nikic · 2025-10-01T07:04:58Z

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

          V = NullValue;
+      } else if (auto *PHI = dyn_cast<PHINode>(U)) {
+        Value *MaybeV = pickCommonConstantFromPHI(*PHI);
+        if (MaybeV)


You can combine the if + variable declaration.

Change-Id: I97f0ba5e8c41518f5cdf1322cfb5e3ec6681025e

nikic

LGTM

nikic · 2025-10-01T07:47:56Z

Can you please add a description to the PR?

mikael-nilsson-arm · 2025-10-01T08:01:07Z

I updated the description.

Try to choose a value for freeze that enables the PHI to be replaced with its input constants if they are equal.

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

d1a6a71

Change-Id: I29fd2663a5c0e4e3a6d98a2c9536f515a12947a5

mikael-nilsson-arm requested a review from nikic as a code owner September 29, 2025 11:47

llvmbot added llvm:instcombine Covers the InstCombine, InstSimplify and AggressiveInstCombine passes llvm:transforms labels Sep 29, 2025

Merge branch 'main' into freeze-phi-opt

71665e7

nikic reviewed Sep 29, 2025

View reviewed changes

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp Outdated Show resolved Hide resolved

dtcxzyw reviewed Sep 29, 2025

View reviewed changes

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp Outdated Show resolved Hide resolved

Review fix 1

0f5ed03

Change-Id: I40d22b4b8f34bcc12b2d5f76784dd3c42d00422d

Review fix 2

e5ad36c

Change-Id: I1e2f175cc22c3e0190f2e8f0ad8196a35e53af71

dtcxzyw mentioned this pull request Sep 30, 2025

Task submission dtcxzyw/llvm-opt-benchmark#1312

Open

zyw-bot mentioned this pull request Sep 30, 2025

pre-commit: PR161181 dtcxzyw/llvm-opt-benchmark#2877

Closed

dtcxzyw reviewed Sep 30, 2025

View reviewed changes

dtcxzyw mentioned this pull request Sep 30, 2025

Fuzz PR161181 dtcxzyw/llvm-fuzz-service#139

Closed

dtcxzyw approved these changes Oct 1, 2025

View reviewed changes

Review fix 3

453332d

Change-Id: If87e6cbe36d008805ccc14e77660d245eaa64ed9

nikic reviewed Oct 1, 2025

View reviewed changes

Review fix 4

fe3b3ab

Change-Id: I97f0ba5e8c41518f5cdf1322cfb5e3ec6681025e

nikic approved these changes Oct 1, 2025

View reviewed changes

nikic merged commit 6958633 into llvm:main Oct 1, 2025
9 checks passed

mikael-nilsson-arm deleted the freeze-phi-opt branch October 1, 2025 08:15

aemerson mentioned this pull request Oct 16, 2025

[clang-reorder-fields] Check for flexible array member (#160262) #163697

Closed

	if (!isGuaranteedNotToBeUndefOrPoison(V, &AC, &I, &DT))
	if (!isGuaranteedNotToBeUndefOrPoison(V))

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

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

Uh oh!

Conversation

mikael-nilsson-arm commented Sep 29, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

llvmbot commented Sep 29, 2025

Uh oh!

mikael-nilsson-arm commented Sep 29, 2025

Uh oh!

Uh oh!

Uh oh!

github-actions bot commented Sep 29, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

dtcxzyw left a comment

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

mikael-nilsson-arm Oct 1, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

nikic left a comment

Choose a reason for hiding this comment

Uh oh!

nikic commented Oct 1, 2025

Uh oh!

mikael-nilsson-arm commented Oct 1, 2025

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

mikael-nilsson-arm commented Sep 29, 2025 •

edited

Loading

github-actions bot commented Sep 29, 2025 •

edited

Loading

mikael-nilsson-arm Oct 1, 2025 •

edited

Loading