[Bug rtl-optimization/65235] [4.8/4.9/5 Regression] Simplifying vec_select of vec_concat miscompiles when first element of vec_concat is const_int

[ktkachov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65235

--- Comment #2 from ktkachov at gcc dot gnu.org ---
(In reply to Aldy Hernandez from comment #1)
> Hi.  The known to work field is empty.  Is this a regression?
> 
> I can't reproduce on a cross build to --target=aarch64-linux by running cc1.
> What options causes this to fail?  Could you post a preprocessed file for
> the test?

-O2 does it for me. I didn't try anything earlier than 4.8 (since they're not
maintained) but the offending code has been there since 2005:
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3551)          rtx vec =
trueop0;
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3552)          int offset =
INTVAL (XVECEXP (trueop1, 0, 0)) * GET_MODE_SIZE (mode);
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3553) 
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3554)          /* Try to
find the element in the VEC_CONCAT.  */
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3555)          while
(GET_MODE (vec) != mode
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3556)                 &&
GET_CODE (vec) == VEC_CONCAT)
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3557)            {
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3558)             
HOST_WIDE_INT vec_size = GET_MODE_SIZE (GET_MODE (XEXP (vec, 0)));
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3559)              if
(offset < vec_size)
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3560)                vec =
XEXP (vec, 0);
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3561)              else
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3562)                {
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3563)                 
offset -= vec_size;
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3564)                  vec =
XEXP (vec, 1);
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3565)                }
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3566)              vec =
avoid_constant_pool_reference (vec);
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3567)            }
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3568) 
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3569)          if (GET_MODE
(vec) == mode)
6516cbaf (bonzini        2005-12-16 09:24:19 +0000 3570)            return vec;


and aarch64 wasn't there before 4.8...