[Bug target/71106] MIPS: Unaligned load/store instructions are not used to access a variable with an aligned attribute

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71106

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-05-17
     Ever confirmed|0                           |1

--- Comment #2 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> Index: gcc/expr.c
> ===================================================================
> --- gcc/expr.c  (revision 236309)
> +++ gcc/expr.c  (working copy)
> @@ -4654,9 +4654,7 @@ expand_assignment (tree to, tree from, b
>  
>    /* Handle misaligned stores.  */
>    mode = TYPE_MODE (TREE_TYPE (to));
> -  if ((TREE_CODE (to) == MEM_REF
> -       || TREE_CODE (to) == TARGET_MEM_REF)
> -      && mode != BLKmode
> +  if (mode != BLKmode
>        && !mem_ref_refers_to_non_mem_p (to)
>        && ((align = get_object_alignment (to))
>           < GET_MODE_ALIGNMENT (mode))
> 
> would fix that.  Without pruning some of the "pessimistic" handling in
> get_object_alignment this will likely result in some code-gen regressions
> though.
> 
> Index: gcc/builtins.c
> ===================================================================
> --- gcc/builtins.c      (revision 236309)
> +++ gcc/builtins.c      (working copy)
> @@ -339,7 +339,7 @@ get_object_alignment_2 (tree exp, unsign
>          Do so only if get_pointer_alignment_1 did not reveal absolute
>          alignment knowledge and if using that alignment would
>          improve the situation.  */
> -      if (!addr_p && !known_alignment
> +      if (!addr_p
>           && TYPE_ALIGN (TREE_TYPE (exp)) > align)
>         align = TYPE_ALIGN (TREE_TYPE (exp));
>        else

The above change only affects indirect references as bases though, so I'm not
sure whether it will do anything here.  The expander change looks OK to me if
we want to support this kind of nonsense on strict-alignment platforms.