Re: i386 lazy pmap switching in trap.c

[yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi)]

> On Sun, Feb 10, 2008 at 02:49:50AM +0900, YAMAMOTO Takashi wrote:
> > 
> > > Although I'd also put a 'cmp' and 'jz' after the call earlier, which
> > > is likely to be benefitialon athlons, but not P4 - which will predict
> > > the backwards jump as taken if the branch isn't in the branch cache.
> > 
> > do you mean the following?
> > 
> >         1:                                              ; \
> >         cmpl    $0, CPUVAR(WANT_PMAPLOAD)               ; \
> >         jz      1f                                      ; \
> >         call    _C_LABEL(pmap_load)                     ; \
> >         cmpl    $0, CPUVAR(WANT_PMAPLOAD)               ; \
> >         jz      1f                                      ; \
> >         jmp     1b                                      ; \
> >         1:
> 
> That version is bad, both amd and intel processors will mispredict
> the second 'jz'.
> Adding the second 'cmpl' is probably a gain (followed by a 'jnz').
> But the intel processors will predict the backwards conditional
> jump as taken (assuming is isn't in the brach cache).
> (amd processors predict all conditional jumps as 'not taken').

i don't think that version is good either.  i was merely trying to
understand what you meant by "put a 'cmp' and 'jz' after the call earlier".

> But if we assume that the pmap is loaded most of the time, what you
> actually need is:
> 
>       cmpl    $0, CPUVAR(WANT_PMAPLOAD)
>       jnz     999f
>     998:
>       # rest of function
> 
>     999:
>       call    _C_LABEL(pmap_load)
>       jmp     988b
> 
> 
>       David
> 
> -- 
> David Laight: david%l8s.co.uk@localhost

i'm confused.  weren't you talking about s/_RETRY//?

YAMAMOTO Takashi