[llvm] cab09e7 - [InstCombine] Propagate FMF from fptrunc when folding `fptrunc fabs(X) -> fabs(fptrunc X)` (#143352)

[via llvm-commits]

Author: Yingwei Zheng
Date: 2025-06-16T12:07:47+08:00
New Revision: cab09e76e0c4c95f44cf90bf2bf7a6eaa15b14b2

URL: https://github.com/llvm/llvm-project/commit/cab09e76e0c4c95f44cf90bf2bf7a6eaa15b14b2
DIFF: https://github.com/llvm/llvm-project/commit/cab09e76e0c4c95f44cf90bf2bf7a6eaa15b14b2.diff

LOG: [InstCombine] Propagate FMF from fptrunc when folding `fptrunc fabs(X) -> fabs(fptrunc X)` (#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.

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
    llvm/test/Transforms/InstCombine/double-float-shrink-2.ll
    llvm/test/Transforms/InstCombine/fabs.ll
    llvm/test/Transforms/InstCombine/fpcast.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 2db79228bf0e6..d4a2fe5e37ef5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1917,7 +1917,9 @@ Instruction *InstCombinerImpl::visitFPTrunc(FPTruncInst &FPT) {
       II->getOperandBundlesAsDefs(OpBundles);
       CallInst *NewCI =
           CallInst::Create(Overload, {InnerTrunc}, OpBundles, II->getName());
-      NewCI->copyFastMathFlags(II);
+      // A normal value may be converted to an infinity. It means that we cannot
+      // propagate ninf from the intrinsic. So we propagate FMF from fptrunc.
+      NewCI->copyFastMathFlags(&FPT);
       return NewCI;
     }
     }

diff  --git a/llvm/test/Transforms/InstCombine/double-float-shrink-2.ll b/llvm/test/Transforms/InstCombine/double-float-shrink-2.ll
index f2049e2813ebc..f884d2bd1ab5b 100644
--- a/llvm/test/Transforms/InstCombine/double-float-shrink-2.ll
+++ b/llvm/test/Transforms/InstCombine/double-float-shrink-2.ll
@@ -449,7 +449,7 @@ define float @test_shrink_intrin_fabs_fast_double_src(double %D) {
 ; CHECK-NEXT:    ret float [[F]]
 ;
   %E = call fast double @llvm.fabs.f64(double %D)
-  %F = fptrunc double %E to float
+  %F = fptrunc fast double %E to float
   ret float %F
 }
 
@@ -611,7 +611,7 @@ define half @test_mismatched_type_intrin_fabs_fast_double_src(double %D) {
 ; CHECK-NEXT:    ret half [[F]]
 ;
   %E = call fast double @llvm.fabs.f64(double %D)
-  %F = fptrunc double %E to half
+  %F = fptrunc fast double %E to half
   ret half %F
 }
 

diff  --git a/llvm/test/Transforms/InstCombine/fabs.ll b/llvm/test/Transforms/InstCombine/fabs.ll
index 0a22d1431b5f1..0d9374410a1d8 100644
--- a/llvm/test/Transforms/InstCombine/fabs.ll
+++ b/llvm/test/Transforms/InstCombine/fabs.ll
@@ -1522,7 +1522,7 @@ define float @test_fabs_nsz_used_by_frem(float %x) {
 define half @test_fabs_nsz_used_by_fptrunc(float %x) {
 ; CHECK-LABEL: @test_fabs_nsz_used_by_fptrunc(
 ; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[X:%.*]] to half
-; CHECK-NEXT:    [[OP:%.*]] = call nsz half @llvm.fabs.f16(half [[TMP1]])
+; CHECK-NEXT:    [[OP:%.*]] = call half @llvm.fabs.f16(half [[TMP1]])
 ; CHECK-NEXT:    ret half [[OP]]
 ;
   %cmp = fcmp oge float %x, 0.000000e+00

diff  --git a/llvm/test/Transforms/InstCombine/fpcast.ll b/llvm/test/Transforms/InstCombine/fpcast.ll
index d5290b572aefd..1a3faceebf244 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 fast 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