vectorization, scheduling and aliasing

[roy rosen]

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?

Thanks, Roy.