[Bug rtl-optimization/49088] Combine fails to properly handle zero-extension and signed int constant

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49088

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.05.20 19:42:51
     Ever Confirmed|0                           |1

--- Comment #4 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2011-05-20 
19:42:51 UTC ---
> combine turns it into
> 
> (insn 18 17 10 2 (set (reg:DI 73) 
>         (const_int 4294967238 [0xffffffc6])) x.i:14 62 {*movdi_internal_rex64}
>      (nil))
> 
> (insn 11 10 12 2 (set (reg:DI 68)
>         (plus:DI (reg/f:DI 20 frame)
>             (reg:DI 73))) x.i:14 247 {*lea_1}
>      (expr_list:REG_DEAD (reg:DI 73)
>         (nil)))
> 
> "const_int -64" is signed, not unsigned.

RTL constants are always sign-extended for their mode but RTL objects,
including constants, have no specific signedness, except for a few exceptions
like the SUBREG_PROMOTED_UNSIGNED_P flag.

That being said, the transformation looks indeed invalid.