[boost] Re: shared_ptr on ppc?

From: Alexander Terekhov (terekhov_at_[hidden])
Date: 2005-04-06 12:15:58

Peter Dimov wrote:
> 
> Alexander Terekhov wrote:
> > and
> >
> > asm long atomic_decrement_strong( register long * pw ) {

[... loop -> loop1+loop2 ...]

> but it's either suboptimal (more than one sync) or incorrect (missing sync),
> I think. It needs a state machine.

And how is this

 asm long atomic_decrement_strong( register long * pw ) {

      <load-reserved>
      <add -1>
      <branch if zero to acquire>
      {lw}sync

    loop1:

      <store-conditional>
      <branch if !failed to done>

    loop2:

      <load-reserved>
      <add -1>
      <branch if !zero to loop1>

    acquire:

      <store-conditional>
      <branch if failed to loop2>
      isync

    done:

      <...>
  }

incorrrect or suboptimal?

> 
> loop0:
> 
> lwarx
> add -1
> beq acquire-without-sync
> 
> sync
> 
> loop1:
> 
> stwcx.
> beq+ done
> 
> loop2:
> 
> lwarx
> add -1
> bne loop1
> 
> acquire-with-sync:
> 
> stwcx.
> bne- loop2
> isync
> blr
> 
> acquire-without-sync:
> 
> stwcx.
> bne- loop0
> isync
> 
> done:
> 
> blr

I must be missing something, but it looks to me that you have 
way too much branching and isync-ing.

regards,
alexander.