Re: [PATCH] i386: Fix up ix86_convert_const_wide_int_to_broadcast [PR108599]

[Uros Bizjak <ubizjak@gmail.com>]

On Tue, Jan 31, 2023 at 9:02 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> Hi!
>
> The following testcase is miscompiled.  The problem is that during
> RTL DSE we see a V4DI register is being loaded { 16, 16, 0, 0 }
> value and DSE mostly works in terms of scalar modes, so it calls
> movoi to set an OImode REG to (const_wide_int 0x100000000000000010)
> and ix86_convert_const_wide_int_to_broadcast thinks it can compute
> that value by broadcasting DImode 0x10.  While it is true that
> for TImode result the broadcast could be used, for OImode/XImode
> it can't be, because all but the lowest 2 HOST_WIDE_INTs aren't
> present (so are 0 or -1 depending on sign), not 0x10 in this case.
> The function checks if the least significant HOST_WIDE_INT elt
> of the CONST_WIDE_INT is broadcastable from QI/HI/SI/DImode and then
>   /* Check if OP can be broadcasted from VAL.  */
>   for (int i = 1; i < CONST_WIDE_INT_NUNITS (op); i++)
>     if (val != CONST_WIDE_INT_ELT (op, i))
>       return nullptr;
> That is needed of course, but nothing checks that
> CONST_WIDE_INT_NUNITS (op) isn't too small for the mode in question.
> I think if op would be 0 or -1, it ought to be never CONST_WIDE_INT,
> but CONST_INT and so we can just punt whenever the number of
> CONST_WIDE_INT elts is not the expected one.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2023-01-31  Jakub Jelinek  <jakub@redhat.com>
>
>         PR target/108599
>         * config/i386/i386-expand.cc
>         (ix86_convert_const_wide_int_to_broadcast): Return nullptr if
>         CONST_WIDE_INT_NUNITS (op) times HOST_BITS_PER_WIDE_INT isn't
>         equal to bitsize of mode.
>
>         * gcc.target/i386/avx2-pr108599.c: New test.

OK.

Thanks,
Uros.

>
> --- gcc/config/i386/i386-expand.cc.jj   2023-01-19 23:22:05.306066616 +0100
> +++ gcc/config/i386/i386-expand.cc      2023-01-30 15:33:43.418598714 +0100
> @@ -291,7 +291,9 @@ ix86_convert_const_wide_int_to_broadcast
>       broadcast only if vector broadcast is available.  */
>    if (!TARGET_AVX
>        || !CONST_WIDE_INT_P (op)
> -      || standard_sse_constant_p (op, mode))
> +      || standard_sse_constant_p (op, mode)
> +      || (CONST_WIDE_INT_NUNITS (op) * HOST_BITS_PER_WIDE_INT
> +         != GET_MODE_BITSIZE (mode)))
>      return nullptr;
>
>    HOST_WIDE_INT val = CONST_WIDE_INT_ELT (op, 0);
> --- gcc/testsuite/gcc.target/i386/avx2-pr108599.c.jj    2023-01-30 16:04:31.984429702 +0100
> +++ gcc/testsuite/gcc.target/i386/avx2-pr108599.c       2023-01-30 16:04:24.459540223 +0100
> @@ -0,0 +1,32 @@
> +/* PR target/108599 */
> +/* { dg-do run { target avx2 } } */
> +/* { dg-options "-O2 -mavx2 -mtune=skylake-avx512" } */
> +
> +#include "avx2-check.h"
> +
> +struct S { unsigned long long a, b, c, d; };
> +
> +__attribute__((noipa)) void
> +foo (unsigned long long x, unsigned long long y,
> +     unsigned long long z, unsigned long long w, const struct S s)
> +{
> +  if (s.a != x || s.b != y || s.c != z || s.d != w)
> +    abort ();
> +}
> +
> +typedef unsigned long long V __attribute__((may_alias, vector_size (4 * sizeof (unsigned long long))));
> +
> +static void
> +avx2_test (void)
> +{
> +  {
> +    struct S s;
> +    *(V *)&s = (V) { 16, 0, 0, 0 };
> +    foo (16, 0, 0, 0, s);
> +  }
> +  {
> +    struct S s;
> +    *(V *)&s = (V) { 16, 16, 0, 0 };
> +    foo (16, 16, 0, 0, s);
> +  }
> +}
>
>         Jakub
>