Re: profile-driven optimization and the linker?

[Dan Kegel <dank@kegel.com>]

Jeffrey A Law wrote:
>>http://gcc.gnu.org/news/profiledriven.html doesn't mention
>>having ld reorder sections to improve locality of reference
>>of frequently called functions.    Has that technique
>>been tried with gcc and binutils?
> 
> Yes.  I had some success with it a while back.

Was that on static executables, or dynamically linked ones?

> With ELF it's not terribly hard since we can arrange for
> each function to be put in its own section.  Thus you get
> to do function-level reordering by just twiddling a linker
> map.  In fact, that's where -ffunction-sections name from.
> 
> We had some hacks to gprof which would take profile data
> and build a linker map.  That's the code that really needs
> to be improved.  I never understood the gprof code well
> enough to do it right.

I may take a whack at this in my copious free time.
Are any such hacks online, or shall I just start from scratch?

I guess further discussion should be on the binutils mailing list.
- Dan

p.s.

I first heard of this technique about 1993 or so, when
Lee Hasiuk implemented it on MS-DOS.

http://mail.kde.org/pipermail/kde-optimize/2003-January/000108.html
describes something similar, but which only aimed at
increasing *disk* cache locality when paging in functions from
disk for the first call.

FWIW, Alan Cox mentioned the technique a couple weeks ago,
http://marc.theaimsgroup.com/?l=linux-kernel&m=109935741612533&w=2
but didn't mention any code.

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