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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

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.


---
Full diff: https://github.com/llvm/llvm-project/pull/143352.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp (+3) 
- (modified) llvm/test/Transforms/InstCombine/fpcast.ll (+34-1) 


``````````diff
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 d5290b572aefd..2af4b85dd32eb 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)
@@ -40,6 +40,39 @@ 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 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 @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

``````````

</details>


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