On Thu, 17 May 2007, David Woodhouse wrote:
>
> A while ago, I played with using '-fwhole-program --combine' for
> building kernel objects -- http://lwn.net/Articles/197097/
I'm worried about three things:
- compiler stability. Quite frankly, I don't get the warm and fuzzies
about features like this that I suspect have had almost zero testing in
real life projects.
- memory use. I've seen some projects try to use various vendors
aggressive optimizations (including things like whole-program: others
have supported it for much longer than gcc has), and memory use tends
to skyrocket. Which in turn means that most users wouldn't necessarily
use it, just because it makes things so slow to compile.
(Related to that - if you used to just recompile a single file, you now
end up recompiling a whole group, so you often have a double whammy:
the compile itself is much slower, and you do a lot more of it!)
- how much of a win is this on a sane and relevant architecture?
In particular, the -fwhole-program thing tends to matter a lot more on
broken and/or uninteresting archtectures. Architectures like ia64,
where you can do real additional optimizations that matter. While these
things tend to make much less of an impact on some modern x86, which
doesn't have a lot of callee-saved registers anyway, and where function
calls aren't really all *that* expensive..
IOW, I'd like to see numbers from something like a 64-bit Core 2 build,
and both for performance and size and compile time. I think that's more
interesting than some other architectures (ia64, alpha, ppc) that have
specific issues that make function boundaries artificially more painful
than they should be.
That said, I don't think adding "__global" annotations is wrong per se. So
I don't object to the patches - I think they can be a real advantage in
the long run, and that it can even be interesting to see which functions
are "internal" to a subsystem and which ones aren't, even if the compiler
doesn't use the information.
But I'm just not very excited about plunging into using experimental gcc
features unless there is some major advantage for major architectures, and
the disadvantages are known..
Linus
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