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

[Matt Arsenault via llvm-commits]

================
@@ -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) {
+; CHECK-LABEL: @mul_pos_zero_nnan_ninf(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[RET:%.*]] = call float @llvm.copysign.f32(float 0.000000e+00, float [[A:%.*]])
+; CHECK-NEXT:    ret float [[RET]]
+;
+entry:
+  %ret = fmul float %a, 0.000000e+00
+  ret float %ret
+}
+
+define float @mul_pos_zero_nnan(float nofpclass(nan) %a) {
+; CHECK-LABEL: @mul_pos_zero_nnan(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[RET:%.*]] = fmul float [[A:%.*]], 0.000000e+00
+; CHECK-NEXT:    ret float [[RET]]
+;
+entry:
+  %ret = fmul float %a, 0.000000e+00
+  ret float %ret
+}
+
+define float @mul_pos_zero_nnan_ninf_fmf(float nofpclass(nan) %a) {
+; CHECK-LABEL: @mul_pos_zero_nnan_ninf_fmf(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[RET:%.*]] = fmul ninf float [[A:%.*]], 0.000000e+00
+; CHECK-NEXT:    ret float [[RET]]
+;
+entry:
+  %ret = fmul ninf float %a, 0.000000e+00
+  ret float %ret
+}
+
+define float @mul_neg_zero_nnan_ninf(float nofpclass(inf nan) %a) {
+; CHECK-LABEL: @mul_neg_zero_nnan_ninf(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = fneg float [[A:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = call float @llvm.copysign.f32(float 0.000000e+00, float [[TMP0]])
+; CHECK-NEXT:    ret float [[RET]]
+;
+entry:
+  %ret = fmul float %a, -0.000000e+00
+  ret float %ret
+}
+
+define float @mul_neg_zero_nnan_fmf(float %a) {
+; CHECK-LABEL: @mul_neg_zero_nnan_fmf(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = fneg nnan float [[A:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = call nnan float @llvm.copysign.f32(float 0.000000e+00, float [[TMP0]])
+; CHECK-NEXT:    ret float [[RET]]
+;
+entry:
+  %ret = fmul nnan float %a, -0.000000e+00
+  ret float %ret
+}
+
+define float @mul_neg_zero_nnan_ninf_fmf(float nofpclass(inf nan) %a) {
+; CHECK-LABEL: @mul_neg_zero_nnan_ninf_fmf(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = fneg nnan ninf float [[A:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = call nnan ninf float @llvm.copysign.f32(float 0.000000e+00, float [[TMP0]])
+; CHECK-NEXT:    ret float [[RET]]
+;
+entry:
+  %ret = fmul nnan ninf float %a, -0.000000e+00
+  ret float %ret
+}
+
+; poison propagates through vector elements
+
+define <2 x float> @mul_neg_zero_nnan_ninf_vec(<2 x float> nofpclass(inf nan) %a) {
+; CHECK-LABEL: @mul_neg_zero_nnan_ninf_vec(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = fneg <2 x float> [[A:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = call <2 x float> @llvm.copysign.v2f32(<2 x float> <float 0.000000e+00, float poison>, <2 x float> [[TMP0]])
+; CHECK-NEXT:    ret <2 x float> [[RET]]
+;
+entry:
+  %ret = fmul <2 x float> %a, <float -0.0, float poison>
----------------
arsenm wrote:

test mixed +0/-0/poison? 

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