vectorization, scheduling and aliasing

[Richard Guenther]

On Wed, Apr 14, 2010 at 8:48 AM, roy rosen <roy.1rosen@gmail.com> wrote:
> Hi All,
>
> I have implemented some vectorization features in my gcc port.
>
> In the generated code for this function I can see a scheduling problem:
>
> int xxx(int* __restrict__ a, int* __restrict__ b)
> {
>    int __restrict__ i;
>    for (i = 0; i < 8; i++)
>    {
>        a[i] = b[i];
>    }
>    return 0;
> }
>
> from asmcons:
>
> (insn 17 14 18 3 a.c:15 (set (reg:V4HI 105)
>        (mem:V4HI (reg/v/f:SI 100 [ b ]) [2 S8 A64])) 115 {*movv4hi_load} (nil))
>
> (insn 18 17 19 3 a.c:15 (set (mem:V4HI (reg/v/f:SI 99 [ a ]) [2 S8 A64])
>        (reg:V4HI 105)) 116 {*movv4hi_store} (expr_list:REG_DEAD (reg:V4HI 105)
>        (nil)))
>
> (insn 19 18 20 3 a.c:15 (set (reg:V4HI 106)
>        (mem:V4HI (plus:SI (reg/v/f:SI 100 [ b ])
>                (const_int 8 [0x8])) [2 S8 A64])) 115 {*movv4hi_load}
> (expr_list:REG_DEAD (reg/v/f:SI 100 [ b ])
>        (nil)))
>
> (insn 20 19 58 3 a.c:15 (set (mem:V4HI (plus:SI (reg/v/f:SI 99 [ a ])
>                (const_int 8 [0x8])) [2 S8 A64])
>        (reg:V4HI 106)) 116 {*movv4hi_store} (expr_list:REG_DEAD (reg:V4HI 106)
>        (expr_list:REG_DEAD (reg/v/f:SI 99 [ a ])
>            (nil))))
>
>  from sched1 - dependecies list:
>   --- Region Dependences --- b 3 bb 0
> ;;      insn  code    bb   dep  prio  cost   reservation
> ;;      ----  ----    --   ---  ----  ----   -----------
> ;;       17   115     3     0     4     1   (2_slots+lsu)       : 58 20 18
> ;;       18   116     3     1     3     1   (2_slots+lsu)       : 58 19
> ;;       19   115     3     1     2     1   (2_slots+lsu)       : 58 20
> ;;       20   116     3     2     1     1   (2_slots+lsu)       : 58
> ;;       58   101     3     4     1     1   (3_slots+pcu)       :
>
> As you can see, insn 19 is dependant on 18.
> I think that this should not happen since the arrays has __restrict__.
> It might have something to do with aliasing.
>
> Are you aware of any kind of problems regarding aliasing and vectors?
> Can you think of anything else that might be wrong in my code that
> leads gcc to think that there is a dependency between insn 18 and 19?

It completely depends on the GCC version you are using.  The
MEM_EXPRs suggest that it is maybe 4.4 or 4.5 based?  Because
they are not existant.  Try recent trunk which should work fine.

Richard.

> Thanks, Roy.
>