Messages in this thread | | | Date | Thu, 24 Aug 2006 18:48:11 +0200 (MEST) | From | Jan Engelhardt <> | Subject | Re: [PATCH 0/4] Compile kernel with -fwhole-program --combine |
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> 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 -- - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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