Re: [PATCH] i386: Fix up ix86_expand_vector_init_general [PR105123]

[Uros Bizjak <ubizjak@gmail.com>]

On Sat, Apr 2, 2022 at 11:25 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> Hi!
>
> The following testcase is miscompiled on ia32.
> The problem is that at -O0 we end up with:
>   vector(4) short unsigned int _1;
>   short unsigned int u.0_3;
> ...
>   _1 = {u.0_3, u.0_3, u.0_3, u.0_3};
> statement (dead) which is wrongly expanded.
> elt is (subreg:HI (reg:SI 83 [ u.0_3 ]) 0), tmp_mode SImode,
> so after convert_mode we start with word (reg:SI 83 [ u.0_3 ]).
> The intent is to manually broadcast that value to 2 SImode parts,
> but because we pass word as target to expand_simple_binop, it will
> overwrite (reg:SI 83 [ u.0_3 ]) and we end up with 0:
>    10: {r83:SI=r83:SI<<0x10;clobber flags:CC;}
>    11: {r83:SI=r83:SI|r83:SI;clobber flags:CC;}
>    12: {r83:SI=r83:SI<<0x10;clobber flags:CC;}
>    13: {r83:SI=r83:SI|r83:SI;clobber flags:CC;}
>    14: clobber r110:V4HI
>    15: r110:V4HI#0=r83:SI
>    16: r110:V4HI#4=r83:SI
> as the two ors do nothing and two shifts each by 16 left shift it all
> away.
> The following patch fixes that by using NULL_RTX target, so we expand it as
>    10: {r110:SI=r83:SI<<0x10;clobber flags:CC;}
>    11: {r111:SI=r110:SI|r83:SI;clobber flags:CC;}
>    12: {r112:SI=r83:SI<<0x10;clobber flags:CC;}
>    13: {r113:SI=r112:SI|r83:SI;clobber flags:CC;}
>    14: clobber r114:V4HI
>    15: r114:V4HI#0=r111:SI
>    16: r114:V4HI#4=r113:SI
> instead.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> Another possibility would be to pass NULL_RTX only when word == elt
> and word otherwise, where word would necessarily be a pseudo from the first
> shift after passing NULL_RTX there once or pass NULL_RTX for the shift and
> word for ior.
>
> 2022-04-02  Jakub Jelinek  <jakub@redhat.com>
>
>         PR target/105123
>         * config/i386/i386-expand.cc (ix86_expand_vector_init_general): Avoid
>         using word as target for expand_simple_binop when doing ASHIFT and
>         IOR.
>
>         * gcc.target/i386/pr105123.c: New test.

OK.

Thanks,
Uros.

>
> --- gcc/config/i386/i386-expand.cc.jj   2022-03-19 13:52:53.000000000 +0100
> +++ gcc/config/i386/i386-expand.cc      2022-04-01 16:51:27.253154191 +0200
> @@ -15830,9 +15830,9 @@ quarter:
>               else
>                 {
>                   word = expand_simple_binop (tmp_mode, ASHIFT, word, shift,
> -                                             word, 1, OPTAB_LIB_WIDEN);
> +                                             NULL_RTX, 1, OPTAB_LIB_WIDEN);
>                   word = expand_simple_binop (tmp_mode, IOR, word, elt,
> -                                             word, 1, OPTAB_LIB_WIDEN);
> +                                             NULL_RTX, 1, OPTAB_LIB_WIDEN);
>                 }
>             }
>
> --- gcc/testsuite/gcc.target/i386/pr105123.c.jj 2022-04-01 16:56:44.549625810 +0200
> +++ gcc/testsuite/gcc.target/i386/pr105123.c    2022-04-01 16:56:33.569782511 +0200
> @@ -0,0 +1,22 @@
> +/* PR target/105123 */
> +/* { dg-do run { target sse2_runtime } } */
> +/* { dg-options "-msse2" } */
> +/* { dg-additional-options "-mtune=i686" { target ia32 } } */
> +
> +typedef unsigned short __attribute__((__vector_size__ (4 * sizeof (unsigned short)))) V;
> +
> +V
> +foo (unsigned short u, V v)
> +{
> +  return __builtin_shuffle (u * v, v);
> +}
> +
> +int
> +main ()
> +{
> +  V x = foo (1, (V) { 0, 1, 2, 3 });
> +  for (unsigned i = 0; i < 4; i++)
> +    if (x[i] != i)
> +      __builtin_abort ();
> +  return 0;
> +}
>
>         Jakub
>