Date: Tue, 6 Aug 2002 13:50:00 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Dan Nelson <dnelson@allantgroup.com> Cc: Terry Lambert <tlambert2@mindspring.com>, Darren Pilgrim <dmp@pantherdragon.org>, Jason Andresen <jandrese@mitre.org>, Dmitry Morozovsky <marck@rinet.ru>, hackers@FreeBSD.ORG Subject: Re: -fomit-frame-pointer for the world build Message-ID: <200208062050.g76Ko0AO015075@apollo.backplane.com> References: <20020802212841.R58905-100000@woozle.rinet.ru> <3D4AC526.4CD399B3@mindspring.com> <3D4C8464.A2F4775A@pantherdragon.org> <3D4CC81F.94526C8A@mindspring.com> <3D4EA466.C1289F0F@mitre.org> <3D4EF091.EA1C91D3@pantherdragon.org> <3D4F0A5F.9B76573F@mindspring.com> <3D4F58B2.E58C2865@pantherdragon.org> <3D4F751E.1A5C1327@mindspring.com> <20020806141548.GB78723@dan.emsphone.com>
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:I thought the main thing you got out of -fomit-frame-pointer was a free
:register, which is a scarce commodity on x86.
:
:--
: Dan Nelson
: dnelson@allantgroup.com
I've done considerable testing of -fomit-frame-pointer and it really
only has an effect if the program makes lots (millions) of calls to tiny,
fast procedures. This is because the push %ebp; movl %esp,%ebp is
removed from the beginning of the procedure and the 'leave' is removed
from the end of the procedure (though an addl to restore %esp has to be
added in).
GCC does not seem to be able to make use of the extra register, or if
it does it does not seem to be able to use it to any great degree.
The IA32 architecture has 6 general registers available to it
(eax, edx, ecx, ebx, esi, edi). Throwing in ebp would not make a huge
difference, nor can 8 and 16 bit specifications (e.g. %al, %ah) be
mixed together safely without a severe performance penalty on higher
end cpus.
-Matt
Matthew Dillon
<dillon@backplane.com>
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