[llvm] [InstCombine] Extend fcmp+select folding to minnum/maxnum intrinsics (PR #112088)

[Alexey Bader via llvm-commits]

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@@ -127,13 +127,12 @@ define float @clamp_float_fast_unordered_nonstrict_minmax(float %x) {
 ; Some more checks with fast
 
 ; (X > 1.0) ? min(x, 255.0) : 1.0
-; That did not match because select was in inverse order.
 define float @clamp_test_1(float %x) {
 ; CHECK-LABEL: @clamp_test_1(
-; CHECK-NEXT:    [[INNER_CMP_INV:%.*]] = fcmp fast oge float [[X:%.*]], 2.550000e+02
-; CHECK-NEXT:    [[INNER_SEL:%.*]] = select fast i1 [[INNER_CMP_INV]], float 2.550000e+02, float [[X]]
-; CHECK-NEXT:    [[R1:%.*]] = call fast float @llvm.maxnum.f32(float [[INNER_SEL]], float 1.000000e+00)
-; CHECK-NEXT:    ret float [[R1]]
+; CHECK-NEXT:    [[INNER_SEL:%.*]] = call fast float @llvm.minnum.f32(float [[X:%.*]], float 2.550000e+02)
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bader wrote:

It happens due to this transformation: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp#L3773-L3794. The first pass of InstCombine propagates `fcmp` fast math flags to the `select` instruction.

Considering that all operands of `select` are coming from `fcmp`, the propagation is valid. In general, it should be the union of fast math flags of both instructions. 

https://github.com/llvm/llvm-project/pull/112088