[InstCombine] Always propagate FMF from fptrunc

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -1917,10 +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.
-      if (II->getIntrinsicID() == Intrinsic::fabs)
-        NewCI->setHasNoInfs(FPT.hasNoInfs());
+      // 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/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

@@ -32,7 +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 ninf 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)