[llvm] [InstCombine] Fix flag propagation in `foldSelectIntoOp` (PR #162003)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/162003

Alive2: https://alive2.llvm.org/ce/z/NwVqpL
Closes https://github.com/llvm/llvm-project/issues/161634.


>From 97168183a32b2cd782411e3befc8c8316958fe5b Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Sun, 5 Oct 2025 17:41:33 +0800
Subject: [PATCH 1/2] [InstCombine] Add pre-commit tests. NFC.

---
 .../select-binop-foldable-floating-point.ll   | 44 +++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll b/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll
index 253bc9e784c2f..00c4ef93e919e 100644
--- a/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll
+++ b/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll
@@ -23,6 +23,50 @@ define float @select_fpclass_fadd(i1 %cond, float nofpclass(nan) %A, float %B) {
   ret float %D
 }
 
+define float @select_fpclass_fadd_ninf1(i1 %cond, float nofpclass(nan) %A, float %B) {
+; CHECK-LABEL: @select_fpclass_fadd_ninf1(
+; CHECK-NEXT:    [[C:%.*]] = select i1 [[COND:%.*]], float [[B:%.*]], float -0.000000e+00
+; CHECK-NEXT:    [[D:%.*]] = fadd float [[A:%.*]], [[C]]
+; CHECK-NEXT:    ret float [[D]]
+;
+  %C = fadd ninf float %A, %B
+  %D = select i1 %cond, float %C, float %A
+  ret float %D
+}
+
+define float @select_fpclass_fadd_ninf2(i1 %cond, float nofpclass(nan) %A, float %B) {
+; CHECK-LABEL: @select_fpclass_fadd_ninf2(
+; CHECK-NEXT:    [[C:%.*]] = select ninf i1 [[COND:%.*]], float [[B:%.*]], float -0.000000e+00
+; CHECK-NEXT:    [[D:%.*]] = fadd float [[A:%.*]], [[C]]
+; CHECK-NEXT:    ret float [[D]]
+;
+  %C = fadd float %A, %B
+  %D = select ninf i1 %cond, float %C, float %A
+  ret float %D
+}
+
+define float @select_fpclass_fadd_ninf3(i1 %cond, float nofpclass(nan) %A, float %B) {
+; CHECK-LABEL: @select_fpclass_fadd_ninf3(
+; CHECK-NEXT:    [[C:%.*]] = select ninf i1 [[COND:%.*]], float [[B:%.*]], float -0.000000e+00
+; CHECK-NEXT:    [[D:%.*]] = fadd ninf float [[A:%.*]], [[C]]
+; CHECK-NEXT:    ret float [[D]]
+;
+  %C = fadd ninf float %A, %B
+  %D = select ninf i1 %cond, float %C, float %A
+  ret float %D
+}
+
+define float @select_fpclass_fadd_nnan_ninf(i1 %cond, float nofpclass(nan) %A, float %B) {
+; CHECK-LABEL: @select_fpclass_fadd_nnan_ninf(
+; CHECK-NEXT:    [[C:%.*]] = select nnan ninf i1 [[COND:%.*]], float [[B:%.*]], float -0.000000e+00
+; CHECK-NEXT:    [[D:%.*]] = fadd float [[A:%.*]], [[C]]
+; CHECK-NEXT:    ret float [[D]]
+;
+  %C = fadd float %A, %B
+  %D = select nnan ninf i1 %cond, float %C, float %A
+  ret float %D
+}
+
 define float @select_nnan_fadd(i1 %cond, float %A, float %B) {
 ; CHECK-LABEL: @select_nnan_fadd(
 ; CHECK-NEXT:    [[C:%.*]] = select nnan i1 [[COND:%.*]], float [[B:%.*]], float -0.000000e+00

>From b1fbcdf188bfc3eeb10718fa9bec811800a08bdc Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Sun, 5 Oct 2025 18:30:54 +0800
Subject: [PATCH 2/2] [InstCombine] Fix flag propagation in `foldSelectIntoOp`

---
 llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp        | 5 ++++-
 .../InstCombine/select-binop-foldable-floating-point.ll      | 2 +-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 8f60e506e8a33..fb4aaa6992702 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -544,8 +544,11 @@ Instruction *InstCombinerImpl::foldSelectIntoOp(SelectInst &SI, Value *TrueVal,
 
     Value *NewSel = Builder.CreateSelect(SI.getCondition(), Swapped ? C : OOp,
                                          Swapped ? OOp : C, "", &SI);
-    if (isa<FPMathOperator>(&SI))
+    if (isa<FPMathOperator>(&SI)) {
       cast<Instruction>(NewSel)->setFastMathFlags(FMF);
+      if (!TVI->hasNoInfs() && !FMF.noNaNs())
+        cast<Instruction>(NewSel)->setHasNoInfs(false);
+    }
     NewSel->takeName(TVI);
     BinaryOperator *BO =
         BinaryOperator::Create(TVI->getOpcode(), FalseVal, NewSel);
diff --git a/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll b/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll
index 00c4ef93e919e..8b5e02acfb02e 100644
--- a/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll
+++ b/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll
@@ -36,7 +36,7 @@ define float @select_fpclass_fadd_ninf1(i1 %cond, float nofpclass(nan) %A, float
 
 define float @select_fpclass_fadd_ninf2(i1 %cond, float nofpclass(nan) %A, float %B) {
 ; CHECK-LABEL: @select_fpclass_fadd_ninf2(
-; CHECK-NEXT:    [[C:%.*]] = select ninf i1 [[COND:%.*]], float [[B:%.*]], float -0.000000e+00
+; CHECK-NEXT:    [[C:%.*]] = select i1 [[COND:%.*]], float [[B:%.*]], float -0.000000e+00
 ; CHECK-NEXT:    [[D:%.*]] = fadd float [[A:%.*]], [[C]]
 ; CHECK-NEXT:    ret float [[D]]
 ;