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Date: Mon, 25 Nov 2002 09:47:00 +0100
From: David Zaffiro <davzaffiro@netscape.net>
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Subject: Re: Compiling x86 with and without frame pointer
References: <19005.1037854033@kao2.melbourne.sgi.com> <20021121050607.GA1554@mark.mielke.cc> <3DDCA7C9.9040501@netscape.net> <20021121192045.GE3636@alpha.home.local>
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I can understand why not omitting framepointers generates better 
compressible code, since every function will start with:
	push   %ebp
	mov    %esp,%ebp
and end with:
	leave
	ret

But it's harder to find a reason why -fomit-frame-pointer is better 
compressible that -momit-leaf-frame-pointer (but it's probably related 
to a lot of mov's with stackpointer involved), especially since 
"-momit-leaf-frame-pointer" makes a trade-off between both other 
options: it omits framepointers for leaf functions (callees that aren't 
callers as well) and it doesn't for branch-functions.
The mixture of functions with frame-pointers and those without is 
probably causing bzip to compress less optimal.

Anyway it makes me wonder, whether kernelcompilation shouldn't be 
configurable between a "optimize for (compressed image) size" and a 
"optimize for speed" option... I'd go for speed... (and always omitting 
frame-pointers doesn't seem to as fast as omitting them only in leaf 
functions).


> Well, I tried on a 2.4.18+patches with gcc 2.95.3. bzImage is :
> 538481 bytes with -fomit-frame-pointer
> 538510 bytes with no particular flag
> 542137 bytes with -momit-leaf-frame-pointer.
> 
> So -fomit-frame-pointer shows the same as other's observation, but in this
> particular case, -momit-leaf-frame-pointer made a slightly bigger kernel.

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