Re: [PATCH v7 04/17] tracing: add static function tracer support for MIPS

Wu Zhangjin
2009-11-11T14:27:20Z

On Wed, 2009-11-11 at 14:18 +0000, Maciej W. Rozycki wrote:
> On Wed, 11 Nov 2009, Wu Zhangjin wrote:
>
> > > No, register jumps cannot be predicted -- this is where the performance
> > > goes on any serious processor -- the two extra instructions are nothing
> > > compared to that. OTOH frame pointer calculations are pure arithmetic, so
> > > you only lose time incurred by the instructions themselves.
> >
> > Yes, I only mean the -mlong-calls and the original -mno-long-calls with
> > -pg.
> >
> > The orignal one looks like this:
> >
> > move ra, at
> > jal _mcount
> >
> > The new one with -mlong-calls looks like this:
> >
> > lui v1, HI_16BIT_OF_MCOUNT
> > addiu v1, v1, LOW_16BIT_OF_MCOUNT
> > move ra, at
> > jalr v1
> >
> > both of them have a "jump" instruciton, so, only two lui, addiu added
> > for -mlong-calls ;)
> >
> > what about the difference between that "jal _mcount" and "jalr v1"?
>
> As I say, the latter cannot be predicted and will incur a stall for any
> decent pipeline. With the former the target address of the jump can be
> calculated early and the instruction fetch unit can start feeding
> instructions from there into the pipeline even before the jump has reached
> the execution stage.
>

Get it, thanks!

Regards,
Wu Zhangjin