[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 #143122.

Author: dtcxzyw

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;
     }
     }

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
 }
 

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

llvm/test/Transforms/InstCombine/fpcast.ll

@@ -32,14 +32,47 @@ 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)
   %c = fptrunc fast float %b to half
   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