Re: [PATCH] i386: Fix up <insn><dwi>3_doubleword_lowpart [PR112523]

[Uros Bizjak <ubizjak@gmail.com>]

On Tue, Nov 14, 2023 at 1:07 PM Jakub Jelinek <jakub@redhat.com> wrote:
>
> Hi!
>
> On Sun, Nov 12, 2023 at 09:03:42PM -0000, Roger Sayle wrote:
> > This patch improves register pressure during reload, inspired by PR 97756.
> > Normally, a double-word right-shift by a constant produces a double-word
> > result, the highpart of which is dead when followed by a truncation.
> > The dead code calculating the high part gets cleaned up post-reload, so
> > the issue isn't normally visible, except for the increased register
> > pressure during reload, sometimes leading to odd register assignments.
> > Providing a post-reload splitter, which clobbers a single wordmode
> > result register instead of a doubleword result register, helps (a bit).
>
> Unfortunately this broke bootstrap on i686-linux, broke all ACATS tests
> on x86_64-linux as well as miscompiled e.g. __floattisf in libgcc there
> as well.
>
> The bug is that shrd{l,q} instruction expects the low part of the input
> to be the same register as the output, rather than the high part as the
> patch implemented.
>   split_double_mode (<DWI>mode, &operands[1], 1, &operands[1], &operands[3]);
> sets operands[1] to the lo_half and operands[3] to the hi_half, so if
> operands[0] is not the same register as operands[1] (rather than [3]) after
> RA, we should during splitting move operands[1] into operands[0].
>
> Your testcase:
>
> > #define MASK60 ((1ul << 60) - 1)
> > unsigned long foo (__uint128_t n)
> > {
> >   unsigned long a = n & MASK60;
> >   unsigned long b = (n >> 60);
> >   b = b & MASK60;
> >   unsigned long c = (n >> 120);
> >   return a+b+c;
> > }
>
> still has the same number of instructions.
>
> Bootstrapped/regtested on x86_64-linux (where it e.g. turns
>                 === acats Summary ===
> -# of unexpected failures       2328
> +# of expected passes           2328
> +# of unexpected failures       0
> and fixes gcc.dg/torture/fp-int-convert-*timode.c FAILs as well)
> and i686-linux (where it previously didn't bootstrap, but compared to
> Friday evening's bootstrap the testresults are ok), ok for trunk?
>
> 2023-11-14  Jakub Jelinek  <jakub@redhat.com>
>
>         PR target/112523
>         PR ada/112514
>         * config/i386/i386.md (<insn><dwi>3_doubleword_lowpart): Move
>         operands[1] aka low part of input rather than operands[3] aka high
>         part of input to output if not the same register.

OK.

Thanks,
Uros.

>
> --- gcc/config/i386/i386.md.jj  2023-11-14 08:10:18.932549803 +0100
> +++ gcc/config/i386/i386.md     2023-11-14 09:31:05.565019207 +0100
> @@ -14825,8 +14825,8 @@ (define_insn_and_split "<insn><dwi>3_dou
>  {
>    split_double_mode (<DWI>mode, &operands[1], 1, &operands[1], &operands[3]);
>    operands[4] = GEN_INT ((<MODE_SIZE> * BITS_PER_UNIT) - INTVAL (operands[2]));
> -  if (!rtx_equal_p (operands[0], operands[3]))
> -    emit_move_insn (operands[0], operands[3]);
> +  if (!rtx_equal_p (operands[0], operands[1]))
> +    emit_move_insn (operands[0], operands[1]);
>  })
>
>  (define_insn "x86_64_shrd"
>
>
>         Jakub
>