Re: [PATCH, rs6000] Enable vector compare for 16-byte memory equality compare [PR111449]

[Richard Sandiford <richard.sandiford@arm.com>]

HAO CHEN GUI <guihaoc@linux.ibm.com> writes:
> Kewen and Richard,
>   Thanks for your comments. Please let me clarify it.
>
> 在 2023/9/27 19:10, Richard Sandiford 写道:
>> Yeah, I agree there doesn't seem to be a good reason to exclude vectors.
>> Sorry to dive straight into details, but maybe we should have something
>> called bitwise_mode_for_size that tries to use integer modes where possible,
>> but falls back to vector modes otherwise.  That mode could then be used
>> for copying, storing, bitwise ops, and equality comparisons (if there
>> is appropriate optabs support).
>
>   The vector mode is not supported for compare_by_pieces and move_by_pieces.
> But it is supported for set_by_pieces and clear_by_pieces. The help function
> widest_fixed_size_mode_for_size returns vector mode when qi_vector is set to
> true.
>
> static fixed_size_mode
> widest_fixed_size_mode_for_size (unsigned int size, bool qi_vector)

Ah, had forgotten about that function.

>
> I tried to enable qi_vector for compare_by_pieces. It can pick up a vector
> mode (eg. V16QImode) and works on some cases. But it fails on a constant
> string case.
>
> int compare (const char* s1)
> {
>   return __builtin_memcmp_eq (s1, "__GCC_HAVE_DWARF2_CFI_ASM", 16);
> }
>
> As the second op is a constant string, it calls builtin_memcpy_read_str to
> build the string. Unfortunately, the inner function doesn't support
> vector mode.
>
>   /* The by-pieces infrastructure does not try to pick a vector mode
>      for memcpy expansion.  */
>   return c_readstr (rep + offset, as_a <scalar_int_mode> (mode),
>                     /*nul_terminated=*/false);
>
> Seems by-pieces infrastructure itself supports vector mode, but low level
> functions do not.

That looks easily solvable though.  I've posted a potential fix as:

   https://gcc.gnu.org/pipermail/gcc-patches/2023-September/631595.html

Is that the only blocker to doing this in generic code?

Thanks,
Richard

>
> I think there are two ways enable vector mode for compare_by_pieces.
> One is to modify the by-pieces infrastructure . Another is to enable it
> by cmpmem expand. The expand is target specific and be flexible.
>
> What's your opinion?
>
> Thanks
> Gui Haochen