[InstCombine] Propagate FMF from fptrunc when folding fptrunc fabs(X) -> fabs(fptrunc X)

[dtcxzyw]

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:

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

Lines 1910 to 1917 in 5d3899d

	// Except for fabs, this transformation requires the input of the unary FP
	// operation to be itself an fpext from the type to which we're
	// truncating.
	if (II->getIntrinsicID() != Intrinsic::fabs) {
	FPExtInst *FPExtSrc = dyn_cast<FPExtInst>(Src);
	if (!FPExtSrc || FPExtSrc->getSrcTy() != Ty)
	break;
	}

Closes #143122.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

Changes

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:

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

Lines 1910 to 1917 in 5d3899d

	// Except for fabs, this transformation requires the input of the unary FP
	// operation to be itself an fpext from the type to which we're
	// truncating.
	if (II->getIntrinsicID() != Intrinsic::fabs) {
	FPExtInst *FPExtSrc = dyn_cast<FPExtInst>(Src);
	if (!FPExtSrc || FPExtSrc->getSrcTy() != Ty)
	break;
	}

Closes #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 --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