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

["H. Peter Anvin" <hpa@zytor.com>]

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.