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Re: Debug formats on Linux


On Sat, Oct 28, 2000 at 02:13:45AM +0200, jfm2@club-internet.fr wrote:
> I benchmarked -fomit-frame-pointer with gcc 2.95 and speed bonus was
> well under 10%, and I think under 5%.  I found this funny since I
> would expect an additional register woukld have a big impact on
> register-starved x86s.  But unless omit-frame-pointer gives a far
> bigger impact I would not want to go through yet another massive
> change on Linux.  I have had enough with libc4 -> libc5 -> glibc plus
> the uncompatible C++ libraries from
>  gcc-2.7 -> egcs -> gcc-2.9[56]->gcc-3

To some extent, it depends on the processor (386, 486, pentium, etc.).  Unlike
other (saner) architectures, the x86 instruction family uses a different
address format to address things via the stack register compared to the frame
pointer register.  This address format uses larger instructions, which reduces
the efficiency of the i-cache slightly.  I believe in the older processors also
executed these instructions slower due to having to use a prefix as part of the
instruction (I haven't looked in detail what the current Pentium-III and AMD
processors do with these instructions).

-- 
Michael Meissner, Red Hat, Inc.
PMB 198, 174 Littleton Road #3, Westford, Massachusetts 01886, USA
Work:	  meissner@redhat.com		phone: +1 978-486-9304
Non-work: meissner@spectacle-pond.org	fax:   +1 978-692-4482

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