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Date: Thu, 19 Nov 2009 11:50:05 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: rostedt@goodmis.org
CC: David Daney <ddaney@caviumnetworks.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Haley <aph@redhat.com>,
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       Fr??d??ric Weisbecker <fweisbec@gmail.com>,
       Peter Zijlstra <peterz@infradead.org>, jakub@redhat.com,
       gcc@gcc.gnu.org
Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions
References: <alpine.LFD.2.00.0911181933540.24119@localhost.localdomain>	 <tip-887a29f59b93cf54e21814869a4ab6e80b6fa623@git.kernel.org>	 <20091119072040.GA23579@elte.hu>	 <alpine.LFD.2.00.0911191053390.24119@localhost.localdomain>	 <alpine.LFD.2.00.0911191423190.24119@localhost.localdomain>	 <alpine.LFD.2.00.0911191607440.24119@localhost.localdomain>	 <alpine.LFD.2.00.0911190934070.2793@localhost.localdomain>	 <1258653562.22249.682.camel@gandalf.stny.rr.com>	 <84fc9c000911191003t244eb864o3d5b355ab5485f@mail.gmail.com>	 <4B058CCD.8050605@redhat.com>	 <alpine.LFD.2.00.0911191039470.2793@localhost.localdomain>	 <alpine.LFD.2.00.0911191041471.2793@localhost.localdomain>	 <alpine.LFD.2.00.0911191047350.2793@localhost.localdomain>	 <4B05982B.6060200@caviumnetworks.com> <1258658886.22249.874.camel@gandalf.stny.rr.com>
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On 11/19/2009 11:28 AM, Steven Rostedt wrote:
> 
> 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.
> 

For a modern ABI it is better described as:

	%eax, %edx, %ecx	- argument/return/scratch registers
	%ebx, %esi, %edi	- saved registers
	%esp			- stack pointer
	%ebp			- frame pointer (saved)

> 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__
> 		[...]
> 

Calling the profiler immediately at the entry point is clearly the more
sane option.  It means the ABI is well-defined, stable, and independent
of what the actual function contents are.  It means that ABI isn't the
normal C ABI (the __fentry__ function would have to preserve all
registers), but that's fine...

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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