[PATCH] [i386] Optimize vec_perm_expr to match vpmov{dw,qd,wb}.

Richard Sandiford richard.sandiford@arm.com
Fri Aug 13 09:03:41 GMT 2021

Jakub Jelinek via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> On Fri, Aug 13, 2021 at 09:42:00AM +0800, Hongtao Liu wrote:
>> > So, I wonder if your new routine shouldn't be instead done after
>> > in ix86_expand_vec_perm_const_1 after vec_perm_1 among other 2 insn cases
>> > and handle the other vpmovdw etc. cases in combine splitters (see that we
>> > only use low half or quarter of the result and transform whatever
>> > permutation we've used into what we want).
>> >
>> Got it, i'll try that way.
> Note, IMHO the ultimate fix would be to add real support for the
> __builtin_shufflevector -1 indices (meaning I don't care what will be in
> that element, perhaps narrowed down to an implementation choice of
> any element of the input vector(s) or 0).
> As VEC_PERM_EXPR is currently used for both perms by variable permutation
> vector and constant, I think we'd need to introduce VEC_PERM_CONST_EXPR,
> which would be exactly like VEC_PERM_EXPR, except that the last operand
> would be required to be a VECTOR_CST and that all ones element would mean
> something different, the I don't care behavior.
> The GIMPLE side would be fairly easy, except that there should be some
> optimizations eventually, like when only certain subset of elements of
> a vector are used later, we can mark the other elements as don't care.

Another alternative I'd wondered about was keeping a single tree code,
but adding an extra operand with a “care/don't care” mask.  I think
that would fit with variable-length vectors better.


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