[llvm] [InstCombine] Propagate ninf from fptrunc when folding `fptrunc fabs(X) -> fabs(fptrunc X)` (PR #143352)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/143352

Alive2: https://alive2.llvm.org/ce/z/DWV3G3
fptrunc yields infinity when the input cannot fit in the target type. So ninf should be propagated from fptrunc. For other intrinsics, the previous check ensures that the result is never an infinity:
https://github.com/llvm/llvm-project/blob/5d3899d293e902124c3602b466031b6b799fb123/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp#L1910-L1917

Closes https://github.com/llvm/llvm-project/issues/143122.


>From 2263abc9247cbe5dbe193ae6c429b6aa202cde4c Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Mon, 9 Jun 2025 15:19:23 +0800
Subject: [PATCH 1/3] [InstCombine] Add pre-commit tests. NFC.

---
 llvm/test/Transforms/InstCombine/fpcast.ll | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/fpcast.ll b/llvm/test/Transforms/InstCombine/fpcast.ll
index d5290b572aefd..a2f14a7313ecd 100644
--- a/llvm/test/Transforms/InstCombine/fpcast.ll
+++ b/llvm/test/Transforms/InstCombine/fpcast.ll
@@ -40,6 +40,28 @@ define half @test3_fast(float %a) {
   ret half %c
 }
 
+define half @test3_both_ninf(float %a) {
+; CHECK-LABEL: @test3_both_ninf(
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half
+; CHECK-NEXT:    [[C:%.*]] = call ninf half @llvm.fabs.f16(half [[TMP1]])
+; CHECK-NEXT:    ret half [[C]]
+;
+  %b = call ninf float @llvm.fabs.f32(float %a)
+  %c = fptrunc ninf float %b to half
+  ret half %c
+}
+
+define half @test3_fabs_ninf(float %a) {
+; CHECK-LABEL: @test3_fabs_ninf(
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half
+; CHECK-NEXT:    [[C:%.*]] = call ninf half @llvm.fabs.f16(half [[TMP1]])
+; CHECK-NEXT:    ret half [[C]]
+;
+  %b = call ninf float @llvm.fabs.f32(float %a)
+  %c = fptrunc float %b to half
+  ret half %c
+}
+
 define half @fneg_fptrunc(float %a) {
 ; CHECK-LABEL: @fneg_fptrunc(
 ; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half

>From fcc4895082046f7e79fddf61b3ee274e0c5a789a Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Mon, 9 Jun 2025 15:36:04 +0800
Subject: [PATCH 2/3] [InstCombine] Drop ninf when folding `fptrunc(fabs(X)) ->
 fabs()fpfptrunc X)`

---
 llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp | 3 +++
 llvm/test/Transforms/InstCombine/fpcast.ll           | 4 ++--
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index d234a0566e191..516b8055aee4c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1926,6 +1926,9 @@ Instruction *InstCombinerImpl::visitFPTrunc(FPTruncInst &FPT) {
       CallInst *NewCI =
           CallInst::Create(Overload, {InnerTrunc}, OpBundles, II->getName());
       NewCI->copyFastMathFlags(II);
+      // A normal value may be converted to an infinity.
+      if (II->getIntrinsicID() == Intrinsic::fabs)
+        NewCI->setHasNoInfs(FPT.hasNoInfs());
       return NewCI;
     }
     }
diff --git a/llvm/test/Transforms/InstCombine/fpcast.ll b/llvm/test/Transforms/InstCombine/fpcast.ll
index a2f14a7313ecd..adc1dc5f99d6d 100644
--- a/llvm/test/Transforms/InstCombine/fpcast.ll
+++ b/llvm/test/Transforms/InstCombine/fpcast.ll
@@ -32,7 +32,7 @@ define half @test3(float %a) {
 define half @test3_fast(float %a) {
 ; CHECK-LABEL: @test3_fast(
 ; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half
-; CHECK-NEXT:    [[C:%.*]] = call half @llvm.fabs.f16(half [[TMP1]])
+; CHECK-NEXT:    [[C:%.*]] = call ninf half @llvm.fabs.f16(half [[TMP1]])
 ; CHECK-NEXT:    ret half [[C]]
 ;
   %b = call float @llvm.fabs.f32(float %a)
@@ -54,7 +54,7 @@ define half @test3_both_ninf(float %a) {
 define half @test3_fabs_ninf(float %a) {
 ; CHECK-LABEL: @test3_fabs_ninf(
 ; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half
-; CHECK-NEXT:    [[C:%.*]] = call ninf half @llvm.fabs.f16(half [[TMP1]])
+; CHECK-NEXT:    [[C:%.*]] = call half @llvm.fabs.f16(half [[TMP1]])
 ; CHECK-NEXT:    ret half [[C]]
 ;
   %b = call ninf float @llvm.fabs.f32(float %a)

>From eed2f2f3a63ffc790f2119c2140a7988c1e32790 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Mon, 9 Jun 2025 15:40:25 +0800
Subject: [PATCH 3/3] [InstCombine] Add more tests. NFC.

---
 llvm/test/Transforms/InstCombine/fpcast.ll | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/fpcast.ll b/llvm/test/Transforms/InstCombine/fpcast.ll
index adc1dc5f99d6d..2af4b85dd32eb 100644
--- a/llvm/test/Transforms/InstCombine/fpcast.ll
+++ b/llvm/test/Transforms/InstCombine/fpcast.ll
@@ -62,6 +62,17 @@ define half @test3_fabs_ninf(float %a) {
   ret half %c
 }
 
+define half @test3_fptrunc_ninf(float %a) {
+; CHECK-LABEL: @test3_fptrunc_ninf(
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half
+; CHECK-NEXT:    [[C:%.*]] = call ninf half @llvm.fabs.f16(half [[TMP1]])
+; CHECK-NEXT:    ret half [[C]]
+;
+  %b = call float @llvm.fabs.f32(float %a)
+  %c = fptrunc ninf float %b to half
+  ret half %c
+}
+
 define half @fneg_fptrunc(float %a) {
 ; CHECK-LABEL: @fneg_fptrunc(
 ; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half