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

[Ingo Molnar <mingo@elte.hu>]

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

> > 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.
> 
> I'm still not sure about in-source annotations for popularity. My 
> suspicion is that it's just too workload-dependent, and a given 
> author's workload will likely be biased towards their code.

in-source annotations can do more:

- inlines could be driven by profile data: if a function is used in a 
  hot path and it's used only once, it makes sense to inline that 
  function into that hot path - because the kernel size increase will be 
  in the cold portion.

- we could drive the likely/unlikely annotations via profiling data.

OTOH i think that _most_ of the benefit (80% :-) could be achieved via 
the much simpler (and more robust) link-time-reordering solution. It is 
also alot less intrusive, and can still be presented in some plain-text 
format that can be distributed along the upstream kernel:

	linux/profiles/webserver.list
	linux/profiles/database-server.list
	linux/profiles/desktop.list
	linux/profiles/beowulf-node.list

and users could pick their profile at build time / relink time. There is 
no binary compatibility problem with such plaintext lists - they dont 
have to be fully complete, nor do they have to be fully accurate - they 
dont impact correctness in any way. In fact profiles could merge all 
architectures into one file, so they would be pretty generic as well.

	Ingo