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Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions
From: Steven Rostedt <rostedt@goodmis.org>
Reply-To: rostedt@goodmis.org
To: David Daney <ddaney@caviumnetworks.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
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On Thu, 2009-11-19 at 11:10 -0800, David Daney wrote:
> Linus Torvalds wrote:

> For the MIPS port of GCC and Linux I recently added the 
> -mmcount-ra-address switch.  It causes the location of the return 
> address (on the stack) to be passed to mcount in a scratch register.

Hehe, scratch register on i686 ;-)

i686 has no extra regs. It just has:

%eax, %ebx, %ecx, %edx - as the general purpose regs
%esp - stack
%ebp - frame pointer
%edi, %esi - counter regs

That's just 8 regs, and half of those are special.

> 
> Perhaps something similar could be done for x86.  It would make this 
> patching of the return location more reliable at the expense of more 
> code at the mcount invocation site.

I rather not put any more code in the call site.

> 
> For the MIPS case the code size doesn't increase, as it is done in the 
> delay slot of the call instruction, which would otherwise be a nop.

I showed in a previous post what the best would be for x86. That is just
calling mcount at the very beginning of the function. The return address
is automatically pushed onto the stack.

Perhaps we could create another profiler? Instead of calling mcount,
call a new function: __fentry__ or something. Have it activated with
another switch. This could make the performance of the function tracer
even better without all these exceptions.

	<function>:
		call __fentry__
		[...]

	
-- Steve


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