[llvm] [InstCombine] Fix FMF propagation in `foldFCmpFSubIntoFCmp` (PR #161539)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/161539

Proof: https://alive2.llvm.org/ce/z/orSP-S
Closes https://github.com/llvm/llvm-project/issues/161525.


>From fe707567fe038d1e8c32715382ba2a1a3531619e Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Wed, 1 Oct 2025 23:24:11 +0800
Subject: [PATCH 1/2] [InstCombine] Add pre-commit tests. NFC.

---
 llvm/test/Transforms/InstCombine/fcmp.ll | 40 ++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/fcmp.ll b/llvm/test/Transforms/InstCombine/fcmp.ll
index 119cffd73c662..0e63a39861e7a 100644
--- a/llvm/test/Transforms/InstCombine/fcmp.ll
+++ b/llvm/test/Transforms/InstCombine/fcmp.ll
@@ -1812,6 +1812,46 @@ define i1 @fcmp_ule_fsub_const(float %x, float %y) {
   ret i1 %cmp
 }
 
+define i1 @fcmp_ninf_ule_fsub_const(float %x, float %y) {
+; CHECK-LABEL: @fcmp_ninf_ule_fsub_const(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ninf ule float [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %fs = fsub float %x, %y
+  %cmp = fcmp ninf ule float %fs, 0.000000e+00
+  ret i1 %cmp
+}
+
+define i1 @fcmp_nnan_ule_fsub_const(float %x, float %y) {
+; CHECK-LABEL: @fcmp_nnan_ule_fsub_const(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp nnan ule float [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %fs = fsub float %x, %y
+  %cmp = fcmp nnan ule float %fs, 0.000000e+00
+  ret i1 %cmp
+}
+
+define i1 @fcmp_ule_fsub_ninf_const(float %x, float %y) {
+; CHECK-LABEL: @fcmp_ule_fsub_ninf_const(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ule float [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %fs = fsub ninf float %x, %y
+  %cmp = fcmp ule float %fs, 0.000000e+00
+  ret i1 %cmp
+}
+
+define i1 @fcmp_ule_fsub_nnan_const(float %x, float %y) {
+; CHECK-LABEL: @fcmp_ule_fsub_nnan_const(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ule float [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %fs = fsub nnan float %x, %y
+  %cmp = fcmp ule float %fs, 0.000000e+00
+  ret i1 %cmp
+}
+
 define i1 @fcmp_ugt_fsub_const(float %x, float %y) {
 ; CHECK-LABEL: @fcmp_ugt_fsub_const(
 ; CHECK-NEXT:    [[FS:%.*]] = fsub float [[X:%.*]], [[Y:%.*]]

>From ba95bd5f238878a9338378fdcf0d59a4c4322c6a Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Wed, 1 Oct 2025 23:26:27 +0800
Subject: [PATCH 2/2] [InstCombine] Fix FMF propagation in
 `foldFCmpFSubIntoFCmp`

---
 llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp | 3 +++
 llvm/test/Transforms/InstCombine/fcmp.ll                | 6 +++---
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index e4cb457499ef5..60da48e1d95d0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -8527,6 +8527,9 @@ static Instruction *foldFCmpFSubIntoFCmp(FCmpInst &I, Instruction *LHSI,
             DenormalMode::getIEEE()) {
       CI.replaceOperand(I, 0, X);
       CI.replaceOperand(I, 1, Y);
+      I.setHasNoInfs(LHSI->hasNoInfs());
+      if (LHSI->hasNoNaNs())
+        I.setHasNoNaNs(true);
       return &I;
     }
     break;
diff --git a/llvm/test/Transforms/InstCombine/fcmp.ll b/llvm/test/Transforms/InstCombine/fcmp.ll
index 0e63a39861e7a..d94e78c55a375 100644
--- a/llvm/test/Transforms/InstCombine/fcmp.ll
+++ b/llvm/test/Transforms/InstCombine/fcmp.ll
@@ -1814,7 +1814,7 @@ define i1 @fcmp_ule_fsub_const(float %x, float %y) {
 
 define i1 @fcmp_ninf_ule_fsub_const(float %x, float %y) {
 ; CHECK-LABEL: @fcmp_ninf_ule_fsub_const(
-; CHECK-NEXT:    [[CMP:%.*]] = fcmp ninf ule float [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ule float [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %fs = fsub float %x, %y
@@ -1834,7 +1834,7 @@ define i1 @fcmp_nnan_ule_fsub_const(float %x, float %y) {
 
 define i1 @fcmp_ule_fsub_ninf_const(float %x, float %y) {
 ; CHECK-LABEL: @fcmp_ule_fsub_ninf_const(
-; CHECK-NEXT:    [[CMP:%.*]] = fcmp ule float [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ninf ule float [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %fs = fsub ninf float %x, %y
@@ -1844,7 +1844,7 @@ define i1 @fcmp_ule_fsub_ninf_const(float %x, float %y) {
 
 define i1 @fcmp_ule_fsub_nnan_const(float %x, float %y) {
 ; CHECK-LABEL: @fcmp_ule_fsub_nnan_const(
-; CHECK-NEXT:    [[CMP:%.*]] = fcmp ule float [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp nnan ule float [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %fs = fsub nnan float %x, %y