Re: [PATCH 0/4] Compile kernel with -fwhole-program --combine

Jan Engelhardt
2006-08-24T16:49:52Z

> If you are compiling multiple source files, this option tells the
> driver to pass all the source files to the compiler at once (for
> those languages for which the compiler can handle this). This
> will allow intermodule analysis (IMA) to be performed by the
> compiler. Currently the only language for which this is supported
> is C. If you pass source files for multiple languages to the
> driver, using this option, the driver will invoke the compiler(s)
> that support IMA once each, passing each compiler all the source
> files appropriate for it.

Compiling files on their own (`make drivers/foo/bar.o`) seems to make
the optimization void. Sure, most people don't stop compiling in
between. Just a note

> For those languages that do not support
> IMA this option will be ignored, and the compiler will be invoked
> once for each source file in that language. If you use this
> option in conjunction with `-save-temps', the compiler will
> generate multiple pre-processed files (one for each source file),
> but only one (combined) `.o' or `.s' file.

There should be an option (in the kernel's makefile system) to disable
its use, just in case `gcc *.c` gobbles up a little more RAM
than is present.

>Using a combination of these two compiler options for building kernel
>code leads to some useful optimisation -- especially with modules which
>are made up of a bunch of incestuous C files, where none of the global
>symbols actually _need_ to be visible outside the directory they reside
>in.

For modules, we have EXPORT_SYMBOL() and any other symbols that are
'extern' but not exported are not visible to other modules.

>The same benefits can be extended to the vmlinux too, although there are
>caveats with making _everything_ static. However, it's relatively simple
>to make EXPORT_SYMBOL() automatically set the 'externally_visible'
>attribute on the symbol in question, and to introduce a new '__global'
>tag which does the same for those symbols which aren't exported to
>modules but which _are_ needed as a global symbol in vmlinux.

Does the kernel (at least for modules) really use ELF symbol visibility
(read: __attribute__((visibility(xyz))) or -fvisibility=xyz) for
lookups? I do not think so, since EXPORT_SYMBOL explicitly puts something
into __ksymtab/__kstrtab.
If visibility supports had been in GCC a long time ago, I am sure we would
not need EXPORT_SYMBOL today, or rather, would do it by use of
__attribute__() rather than a macro that ksymtabs it. Or am I possibly
misunderstanding something?

Jan Engelhardt
--