Re: [patch 00/2] improve .text size on gcc 4.0 and newer compilers

Ingo Molnar
2006-01-05T22:59:09Z

* Matt Mackall <mpm@selenic.com> wrote:

> > I don't believe it is actually all _that_ volatile. Yes, it would be a
> > huge issue _initially_, but the incremental effects shouldn't be that big,
> > or there is something wrong with the approach.
>
> No, perhaps not. But it would be nice in theory for people to be able
> to do things like profile their production system and relink. And
> having to touch hundreds of files to do it would be painful.

we can (almost) do that: via -ffunction-sections. It does seem to work
on both the gcc and the ld side. [i tried to use this for --gc-sections
to save more space, but that ld option seems unstable, i couldnt link a
bootable image. -ffunction-sections itself seems to work fine in gcc4.]

i think all that is needed to reorder the functions is a build-time
generated ld script, which is generated off the 'popularity list'.

so i think the two concepts could nicely co-exist: in-source annotations
help us maintain the popularity list, -ffunction-sections allows us to
reorder at link time. In fact such a kernel could be shipped in
'unlinked' state, and could be relinked based on per-system profiling
data. As long as we have KALLSYMS, it's not even a big debuggability
issue.

the branch-likelyhood thing is a separate issue from function-cohesion,
but could be handled by the same concept: if there was a
--ffunction-unlikely-sections option in gcc (there's none currently,
AFAICS), then those could be reordered in a smart way too. (We wouldnt
get the 8-byte relative jumps back though, gcc would always have to
assume that the jump is far away.)

Ingo