Re: About global var reg optimization

[Jeff Law <law at redhat dot com>]

On 04/03/2017 09:42 AM, Aurelien Buhrig wrote:
> Hello !
>
> I would like to use a fixed global register (here, as an applicative
> stack pointer) and would like gcc (6.3 / private backend) to actually
> optimize the following code using postinc/predec addressing mode on this
> global fixed reg itself (a4 here) :
>
> register int *ptr asm ("a4");
> void  pushint (int a) {  *ptr++ = a; }
>
> - The optimized tree looks like:
> pushint (int a)
> {
>   int * ptr.0_2;
>   int * _3;
>
>   <bb 2>:
>   ptr.0_2 = ptr;
>   _3 = ptr.0_2 + 2;
>   ptr = _3;
>   *ptr.0_2 = a_5(D);
>   return;
> }
>
> - And the RTL expand looks like:
> (set (reg/v:HI 26 [ a ]) (reg:HI 0 r0 [ a ]))
> (set (reg/f:SI 24 [ ptr.0_2 ]) (reg/v:SI 12 a4 [ ptr ]))
> (set (reg/v:SI 12 a4 [ ptr ]) (plus:SI (reg/f:SI 24 [ ptr.0_2 ])
> (const_int 2)))
> (set (mem:HI (reg/f:SI 24 [ ptr.0_2 ])) (reg/v:HI 26 [ a ]))
>
> Firstly, the global fixed reg (a4) is flagged volatile, but maybe it is
> normal for a user var?
On a reg the /v flag means it is a user variable.  The meaning of the 
various flags is documented in the developer manual.


May the volatile flag prevent the autoincdec pass
> to use a4 as postinc reg?
Nope.  Again, the volatile flag really means it's a user variable.

> Then, ptr is spilled into pseudo reg 24 which is used as base reg, which
> seems to prevent the autoincdec pass to use a4.
>
> Any idea how I could optimize such a code ?
You'd have to dig into the auto-inc pass to see why it doesn't see the 
autoincrement opportunity.  This looks like a pre-inc to me.

Jeff