Re: [Qemu-devel] [PATCH v6 3/3] tcg: Optimize qemu_ld/st by generating slow paths at the end of a block

[Yeongkyoon Lee]

On 2012년 10월 18일 08:44, Richard Henderson wrote:
> On 2012-10-16 17:23, Yeongkyoon Lee wrote:
> > +    /* Code generation of qemu_ld/st's slow path calling MMU helper
> > +
> > +       PRE_PROC ...
> > +       call MMU helper
> > +       jmp POST_PROC (2b) : short forward jump <- GETRA()
> > +       jmp next_code (5b) : dummy long backward jump which is never executed
> > +       POST_PROC ... : do post-processing <- GETRA() + 7
> > +       jmp next_code : jump to the code corresponding to next IR of qemu_ld/st
> > +    */
> Is this jump over jump really any better than passing next_code
> as another function argument?
>
> In 32-bit mode
>         push $next_code
> In 64-bit mode
>         leaq next_code(%rip),%r8
>
>
> r~

Only one advantage is no fragmentation of MMU helpers, that is, we will 
still have the same helper prototypes.
In my opinion, the performance degradation of using jmp instead of push 
or something, is negligible because it is executed on slow path.