[Bug tree-optimization/35653] [4.3/4.4 Regression]: gcc-4.3 -O3/-ftree-vectorize regression: incorrect code generation

["pinskia at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #7 from pinskia at gmail dot com  2008-03-23 16:50 -------
Subject: Re:  [4.3/4.4 Regression]: gcc-4.3 -O3/-ftree-vectorize regression:
incorrect code generation

This code violates c/c++ aliasing rules.

Sent from my iPhone

On Mar 23, 2008, at 6:33, "victork at gcc dot gnu dot org"
<gcc-bugzilla@gcc.gnu.org 
 > wrote:

>
>
> ------- Comment #6 from victork at gcc dot gnu dot org  2008-03-23  
> 13:33 -------
> Here is AN even more reduced example which demonstrates the problem:
> int main()
> {
>  char buf[256];
>  char *dest;
>  int i;
>
>  dest = &buf[2];
>  for (i = 0; i < 32; i++)
>  {
>    *(unsigned *)dest = 0;
>    dest += 4;
>  }
>
>  return buf[2];
> }
>
> gcc -O3 t.c  && ./a.out
> Segmentation fault
>
>
> The problem is that vectorizer is assuming that the access was  
> aligned to the
> the size of the element before vectorization. A loop peeling only  
> handles
> misalignment in multiples of element size.
> In this case, vectorizer could check the actual alignment in compile  
> time and
> prevent vectorization.  (Though the assignment of a constant still  
> can be
> vectorized if value of constant is adjusted accordingly).
> In general case, when alignment of the non-vectorized access is  
> unknown, a fix
> of this bug would require an addition of a run-time test.
>
>
> -- 
>
> victork at gcc dot gnu dot org changed:
>
>           What    |Removed                     |Added
> --- 
> --- 
> ----------------------------------------------------------------------
>         AssignedTo|unassigned at gcc dot gnu   |victork at gcc dot  
> gnu dot
>                   |dot org                     |org
>             Status|NEW                         |ASSIGNED
>   Last reconfirmed|2008-03-21 17:47:58         |2008-03-23 13:33:08
>               date|                            |
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35653
>


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35653