[llvm] [InstCombine] Missing optimization: fold mul (select a, b), (select b, a) to mul a, b (PR #74953)

[Nikita Popov via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -passes=instcombine -S < %s | FileCheck %s
+
+declare float @llvm.maxnum.f32(float %a, float %b)
+declare float @llvm.minnum.f32(float %a, float %b)
+declare float @llvm.maximum.f32(float %a, float %b)
+declare float @llvm.minimum.f32(float %a, float %b)
+declare i32 @llvm.smax.i32(i32 %a, i32 %b)
+declare i32 @llvm.smin.i32(i32 %a, i32 %b)
+declare i32 @llvm.umax.i32(i32 %a, i32 %b)
+declare i32 @llvm.umin.i32(i32 %a, i32 %b)
+declare i16 @llvm.sadd.sat.i16(i16 %a, i16 %b)
+declare i16 @llvm.uadd.sat.i16(i16 %a, i16 %b)
+declare {i16, i1} @llvm.sadd.with.overflow.i16(i16 %a, i16 %b)
+declare {i16, i1} @llvm.uadd.with.overflow.i16(i16 %a, i16 %b)
+declare {i16, i1} @llvm.smul.with.overflow.i16(i16 %a, i16 %b)
+declare {i16, i1} @llvm.umul.with.overflow.i16(i16 %a, i16 %b)
+declare i16 @llvm.smul.fix.i16(i16 %a, i16 %b, i32 %scale)
+declare i16 @llvm.umul.fix.i16(i16 %a, i16 %b, i32 %scale)
+declare i16 @llvm.smul.fix.sat.i16(i16 %a, i16 %b, i32 %scale)
+declare i16 @llvm.umul.fix.sat.i16(i16 %a, i16 %b, i32 %scale)
+declare float @llvm.fma.f32(float %a, float %b, float %c)
+declare float @llvm.fmuladd.f32(float %a, float %b, float %c)
+
+define i8 @fold_select_mul(i1 %c, i8 %a, i8 %b) {
+; CHECK-LABEL: define i8 @fold_select_mul(
+; CHECK-SAME: i1 [[C:%.*]], i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT:    [[RET:%.*]] = mul i8 [[B]], [[A]]
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %s0 = select i1 %c, i8 %a, i8 %b
+  %s1 = select i1 %c, i8 %b, i8 %a
+  %ret = mul i8 %s1, %s0
+  ret i8 %ret
+}
+
+define i8 @fold_select_mul_nsw_nuw_preserve(i1 %c, i8 %a, i8 %b) {
+; CHECK-LABEL: define i8 @fold_select_mul_nsw_nuw_preserve(
+; CHECK-SAME: i1 [[C:%.*]], i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT:    [[RET:%.*]] = mul i8 [[B]], [[A]]
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nikic wrote:

Hm, for some reason the flags here still don't get preserved...

https://github.com/llvm/llvm-project/pull/74953