Re: [PATCH] Linux Kernel Markers 0.14 for 2.6.17

From: Mathieu Desnoyers
Date: Tue Sep 26 2006 - 18:28:23 EST

* Jeremy Fitzhardinge (jeremy@xxxxxxxx) 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.



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