[llvm] [ValueTracking] Take PHI's poison-generating flags into account (PR #161530)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/161530

ninf/nnan in the phi node may produce poison values. They should be considered in `isGuaranteedNotToBeUndefOrPoison`.
Closes https://github.com/llvm/llvm-project/issues/161524.


>From 78a1575301761e2fab0b29b235c5b943b084e0dc Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Wed, 1 Oct 2025 22:31:59 +0800
Subject: [PATCH 1/2] [InstCombine] Add pre-commit tests. NFC.

---
 .../test/Transforms/InstCombine/freeze-phi.ll | 28 +++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/freeze-phi.ll b/llvm/test/Transforms/InstCombine/freeze-phi.ll
index cdc9a5efe5933..36a5444c90350 100644
--- a/llvm/test/Transforms/InstCombine/freeze-phi.ll
+++ b/llvm/test/Transforms/InstCombine/freeze-phi.ll
@@ -212,3 +212,31 @@ D:
   %y.fr = freeze i32 %y
   ret i32 %y.fr
 }
+
+; Make sure that fmf in phi node is dropped when freeze get folded.
+
+define float @pr161524(float noundef %arg) {
+; CHECK-LABEL: @pr161524(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[COND:%.*]] = tail call i1 @llvm.is.fpclass.f32(float [[ARG:%.*]], i32 144)
+; CHECK-NEXT:    br i1 [[COND]], label [[IF_THEN:%.*]], label [[IF_EXIT:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    [[FADD:%.*]] = fadd float [[ARG]], 1.000000e+00
+; CHECK-NEXT:    br label [[IF_EXIT]]
+; CHECK:       if.exit:
+; CHECK-NEXT:    [[RET:%.*]] = phi ninf float [ [[FADD]], [[IF_THEN]] ], [ [[ARG]], [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret float [[RET]]
+;
+entry:
+  %cond = tail call i1 @llvm.is.fpclass.f32(float %arg, i32 144)
+  br i1 %cond, label %if.then, label %if.exit
+
+if.then:
+  %fadd = fadd float %arg, 1.0
+  br label %if.exit
+
+if.exit:
+  %ret = phi ninf float [ %fadd, %if.then ], [ %arg, %entry ]
+  %ret.fr = freeze float %ret
+  ret float %ret.fr
+}

>From 7e71b3f0256f14424bf28c95daed3772ae485431 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Wed, 1 Oct 2025 22:47:48 +0800
Subject: [PATCH 2/2] [ValueTracking] Take PHI's poison-generating flags into
 account

---
 llvm/lib/Analysis/ValueTracking.cpp           | 35 ++++++++++---------
 .../test/Transforms/InstCombine/freeze-phi.ll |  2 +-
 2 files changed, 19 insertions(+), 18 deletions(-)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 6f11b250cf21f..09a8fbea065ac 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -7651,25 +7651,26 @@ static bool isGuaranteedNotToBeUndefOrPoison(
         return true;
     }
 
-    if (const auto *PN = dyn_cast<PHINode>(V)) {
-      unsigned Num = PN->getNumIncomingValues();
-      bool IsWellDefined = true;
-      for (unsigned i = 0; i < Num; ++i) {
-        if (PN == PN->getIncomingValue(i))
-          continue;
-        auto *TI = PN->getIncomingBlock(i)->getTerminator();
-        if (!isGuaranteedNotToBeUndefOrPoison(PN->getIncomingValue(i), AC, TI,
-                                              DT, Depth + 1, Kind)) {
-          IsWellDefined = false;
-          break;
+    if (!::canCreateUndefOrPoison(Opr, Kind,
+                                  /*ConsiderFlagsAndMetadata=*/true)) {
+      if (const auto *PN = dyn_cast<PHINode>(V)) {
+        unsigned Num = PN->getNumIncomingValues();
+        bool IsWellDefined = true;
+        for (unsigned i = 0; i < Num; ++i) {
+          if (PN == PN->getIncomingValue(i))
+            continue;
+          auto *TI = PN->getIncomingBlock(i)->getTerminator();
+          if (!isGuaranteedNotToBeUndefOrPoison(PN->getIncomingValue(i), AC, TI,
+                                                DT, Depth + 1, Kind)) {
+            IsWellDefined = false;
+            break;
+          }
         }
-      }
-      if (IsWellDefined)
+        if (IsWellDefined)
+          return true;
+      } else if (all_of(Opr->operands(), OpCheck))
         return true;
-    } else if (!::canCreateUndefOrPoison(Opr, Kind,
-                                         /*ConsiderFlagsAndMetadata*/ true) &&
-               all_of(Opr->operands(), OpCheck))
-      return true;
+    }
   }
 
   if (auto *I = dyn_cast<LoadInst>(V))
diff --git a/llvm/test/Transforms/InstCombine/freeze-phi.ll b/llvm/test/Transforms/InstCombine/freeze-phi.ll
index 36a5444c90350..62bb9dc31b76b 100644
--- a/llvm/test/Transforms/InstCombine/freeze-phi.ll
+++ b/llvm/test/Transforms/InstCombine/freeze-phi.ll
@@ -224,7 +224,7 @@ define float @pr161524(float noundef %arg) {
 ; CHECK-NEXT:    [[FADD:%.*]] = fadd float [[ARG]], 1.000000e+00
 ; CHECK-NEXT:    br label [[IF_EXIT]]
 ; CHECK:       if.exit:
-; CHECK-NEXT:    [[RET:%.*]] = phi ninf float [ [[FADD]], [[IF_THEN]] ], [ [[ARG]], [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[RET:%.*]] = phi float [ [[FADD]], [[IF_THEN]] ], [ [[ARG]], [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    ret float [[RET]]
 ;
 entry: