[PATCH] D74713: [ConstantFold] fold most FP ops with undef operand to undef rather than NaN

[Sanjay Patel via Phabricator via llvm-commits]

spatel marked an inline comment as done.
spatel added inline comments.


================
Comment at: llvm/test/Transforms/InstSimplify/fp-undef.ll:218-224
 define float @fadd_undef_op1_constant(float %x) {
 ; CHECK-LABEL: @fadd_undef_op1_constant(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float undef
 ;
   %r = fadd float 2.0, undef
   ret float %r
 }
----------------
lebedev.ri wrote:
> This is not correct
> ```
> ----------------------------------------
> define float @fadd_undef_op1_constant(float %x) {
> %0:
>   %r = fadd float 2.000000, undef
>   ret float %r
> }
> =>
> define float @fadd_undef_op1_constant(float %x) {
> %0:
>   %r = fadd float 2.000000, undef
>   ret float undef
> }
> Transformation doesn't verify!
> ERROR: Value mismatch
> 
> Example:
> float %x = poison
> 
> Source:
> float %r = NaN  [based on undef value]
> 
> Target:
> float %r = NaN  [based on undef value]
> Source value: NaN
> Target value: #x84000400 (-0.000000000000?)
> 
> Summary:
>   0 correct transformations
>   1 incorrect transformations
>   0 errors
> 
> ```
Apologies for not having recent Alive installed anywhere at the moment to test this myself, but how should we interpret this output?

Is it saying that there is no value that we can assign to the undef operand that will produce the exact 0x84000400 output value?


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https://reviews.llvm.org/D74713