Re: BUG: GCC-4.4.x changes the function frame on some functions

[Ingo Molnar]

* Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> On Thu, 2009-11-19 at 19:47 +0100, Ingo Molnar wrote:
> > * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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.

In that case we could also fix up static callsites to this address as 
well (to jump +5 bytes into the function) and avoid the NOP as well in 
most cases. (That would in essence merge any slow-path function epilogue 
with the mcount cal instruction in terms of I$ footprint - i.e. it would 
be an even lower overhead feature.)

If only the kernel had its own compiler.

	Ingo
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