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Re: About global var reg optimization

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);

- 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.


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