[llvm] [InstCombine] Fold `fmul X, -0.0` into `copysign(0.0, -X)` (PR #85772)

[Matt Arsenault via llvm-commits]

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@@ -1268,13 +1268,95 @@ define half @mul_zero(half %x) {
   ret half %r
 }
 
-; TODO: This could be fneg+copysign.
-
 define half @mul_negzero_nnan(half %x) {
 ; CHECK-LABEL: @mul_negzero_nnan(
-; CHECK-NEXT:    [[R:%.*]] = fmul nnan half [[X:%.*]], 0xH8000
+; CHECK-NEXT:    [[TMP1:%.*]] = fneg nnan half [[X:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = call nnan half @llvm.copysign.f16(half 0xH0000, half [[TMP1]])
 ; CHECK-NEXT:    ret half [[R]]
 ;
   %r = fmul nnan half %x, -0.0
   ret half %r
 }
+
+define float @mul_pos_zero_nnan_ninf(float nofpclass(inf nan) %a) {
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arsenm wrote:

The no-infs aren't relevant throughout? 

https://github.com/llvm/llvm-project/pull/85772