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

[HAO CHEN GUI <guihaoc@linux.ibm.com>]

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)

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.

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