[RFA][PR target/113666] Simplify VEC_EXTRACT from a uniform vector

[Andrew Pinski]

On Mon, Jan 12, 2026 at 7:56 PM Jeffrey Law
<jeffrey.law@oss.qualcomm.com> wrote:
>
>
>
> On 1/12/2026 2:58 PM, Andrew Pinski wrote:
>
> I think this should be closer to what I did for VEC_SHL_INSERT in
> r16-4742-gfcde4c81644aec (this was based on the review where I tried
> to do a similar thing as you did:
> https://gcc.gnu.org/pipermail/gcc-patches/2024-July/658275.html ).
>
> That is, in fold_const_call (in fold-const.cc) add VEC_EXTRACT case
> and do similar as fold_const_vec_shl_insert.
> Something like:
> ```
> static tree
> fold_const_vec_extract (tree, tree arg0, tree arg1)
> {
>   if (TREE_CODE (arg0) != VECTOR_CST)
>     return NULL_TREE;
>
>   /* vec_extract ( dup(CST), N) -> CST. */
>   if (tree elem = uniform_vector_p (arg0))
>       return elem;
>
>   return NULL_TREE;
> }
> ```
> And also  in match.pd add:
> ```
> (simplify
>  (IFN_VEC_EXTRACT (vec_duplicate @0) @1)
>   @0)
> ```
>
> I don't think we need to care about the bounds check on @1 either
> because then it would be undefined anyways.
>
> Uniform vectors can come in different forms.    They can be a VEC_DUPLICATE, VECTOR_CST or even a CONSTRUCTOR node.  uniform_vector_p knows how to handle all three cases already.  So while that solution is simpler, I suspect it's leaving good opportunities on the floor and may not even cover the regression in question (I haven't checked that).

Note your current pattern only handles VECTOR_CST because that is only
valid as an operand for the function call.
For a CONSTRUCTOR or a VEC_DUPLICATE, @0 will be a SSA_NAME as
CONSTRUCTOR/VEC_DUPLICATEs are only valid if alone in a stmt and
uniform_vector_p will return false for a SSA_NAME.

You could change the patch to be similar to what I originally did for
IFN_VEC_SHL_INSERT
(https://gcc.gnu.org/pipermail/gcc-patches/2024-July/658274.html);
though Richard had the same suggestion as I have above:
> For the case in question implementing fold_const_call would be better.


Thanks,
Andrew Pinski


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> Jeff