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

Wu Zhangjin
2009-11-11T13:52:04Z

Hi,

On Wed, 2009-11-11 at 13:15 +0000, Maciej W. Rozycki wrote:
> On Wed, 11 Nov 2009, Wu Zhangjin wrote:
>
> > > -mlong-calls really degrades performance. I have seen things like 6%
> > > drop in network packet forwarding rates with -mlong-calls.
> > >
> >
> > so much drop? seems only two instructions added for it: lui, addi. from
> > this view point, I think the -fno-omit-frame-pointer(add, sd, move...)
> > will also bring with much drop.
>
> 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"?

Regards,
Wu Zhangjin