lkml.org 
[lkml]   [2006]   [Sep]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] Linux Kernel Markers 0.14 for 2.6.17
* Jeremy Fitzhardinge (jeremy@goop.org) wrote:
> Mathieu Desnoyers wrote:
> >Hi,
> >
> >Constructing on Jeremy Fitzhardinge's comments about gcc optimizations, I
> >rewrote (once more) the markers mechanism so that the optimized mode does
> >not
> >jump between two different inline asm. Instead, the optimized version uses
> >a
> >load immediate (in assembly) that will be used by a test to decide of a
> >branch
> >(in C).
> >
>
> I should have spelled out my point a bit more. If you've got a flag
> you're just testing, couldn't you just do:
>
> if (__mark_enabled_##name)
> (*__mark_func)(...);
>
> and do without the asms or the section?
>

Because a supplementary memory read is added on the critical path with a normal
flag test. The assembly can provide an immediate value without any need of
memory read from the data section.

To change the behavior of the program, I just have to change the immediate value
in the movb instruction.

However, the non-optimized generic version does exactly this : it simply tests a
flag loaded from memory. It can be very useful on embedded systems where the
code is in read-only memory.

Regards,

Mathieu


OpenPGP public key: http://krystal.dyndns.org:8080/key/compudj.gpg
Key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-09-27 00:31    [from the cache]
©2003-2014 Jasper Spaans. Advertise on this site