[Bug target/70429] Wrong code with -O1.

["jakub at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70429

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.9.4

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Kirill Yukhin from comment #4)
> Seems like combiner performs invalid reassociation. This trivial addition to
> Jakub's PR70222 fix makes test work:
> 
> --- a/gcc/combine.c
> +++ b/gcc/combine.c
> @@ -10526,7 +10526,7 @@ simplify_shift_const_1 (enum rtx_code code,
> machine_mode result_mode,
>             {
>               /* For ((unsigned) (cstULL >> count)) >> cst2 we have to make
>                  sure the result will be masked.  See PR70222.  */
> -             if (code == LSHIFTRT
> +             if ((code == LSHIFTRT || code == ASHIFTRT)
>                   && mode != result_mode
>                   && !merge_outer_ops (&outer_op, &outer_const, AND,
>                                        GET_MODE_MASK (result_mode)

That looks wrong.
Consider following testcase:
__attribute__((noinline, noclone)) int
foo (int a)
{
  return (int) (0x14ff6e2207db5d1fLL >> a) >> 4;
}

int
main ()
{
  if (sizeof (int) != 4 || sizeof (long long) != 8 || __CHAR_BIT__ != 8)
    return 0;
  if (foo (1) != 0x3edae8 || foo (2) != -132158092)
    __builtin_abort ();
  return 0;
}
With your above patch, it will try to combine:
Trying 6, 7 -> 8:
Failed to match this instruction:
(parallel [
        (set (reg:SI 91)
            (and:SI (subreg:SI (ashiftrt:DI (const_int 94565568266221009
[0x14ff6e2207db5d1])
                        (subreg:QI (reg/v:SI 90 [ a ]) 0)) 0)
                (const_int 268435455 [0xfffffff])))
        (clobber (reg:CC 17 flags))
    ])
Failed to match this instruction:
(set (reg:SI 91)
    (and:SI (subreg:SI (ashiftrt:DI (const_int 94565568266221009
[0x14ff6e2207db5d1])
                (subreg:QI (reg/v:SI 90 [ a ]) 0)) 0)
        (const_int 268435455 [0xfffffff])))
and only because there is no such insn on x86, it will not miscompile it.
ASHIFTRT needs corresponding sign extension, not AND.