On Thu, 2009-11-19 at 19:47 +0100, Ingo Molnar wrote:
> * Linus Torvalds <torva...@linux-foundation.org> wrote:
>
> > Admittedly, anybody who compiles with -pg probably doesn't care deeply
> > about smaller and more efficient code, since the mcount call overhead
> > tends to make the thing moot anyway, but it really looks like a
> > win-win situation to just fix the mcount call sequence regardless.
>
> Just a sidenote: due to dyn-ftrace, which patches out all mcounts during
> bootup to be NOPs (and opt-in patches them in again if someone runs the
> function tracer), the cost is not as large as one would have it with say
> -pg based user-space profiling.
>
> It's not completely zero-cost as the pure NOPs balloon the i$ footprint
> a bit and GCC generates different code too in some cases. But it's
> certainly good enough that it's generally pretty hard to prove overhead
> via micro or macro benchmarks that the patched out mcounts call sites
> are there.
And frame pointers do add a little overhead as well. Too bad the mcount
ABI wasn't something like this:
<function>:
call mcount
[...]
This way, the function address for mcount would have been (%esp) and the
parent address would be 4(%esp). Mcount would work without frame
pointers and this whole mess would also become moot.
-- Steve
