Re: vectorization, scheduling and aliasing

[Richard Guenther <richard dot guenther at gmail dot com>]

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.
>