Re: [PATCH v2 0/9] x86: macrofying inline asm for better compilation

Josh Poimboeuf
2018-06-04T19:06:07Z

On Mon, Jun 04, 2018 at 04:21:22AM -0700, Nadav Amit wrote:
> This patch-set deals with an interesting yet stupid problem: kernel code
> that does not get inlined despite its simplicity. There are several
> causes for this behavior: "cold" attribute on __init, different function
> optimization levels; conditional constant computations based on
> __builtin_constant_p(); and finally large inline assembly blocks.
>
> This patch-set deals with the inline assembly problem. I separated these
> patches from the others (that were sent in the RFC) for easier
> inclusion. I also separated the removal of unnecessary new-lines which
> would be sent separately.
>
> The problem with inline assembly is that inline assembly is often used
> by the kernel for things that are other than code - for example,
> assembly directives and data. GCC however is oblivious to the content of
> the blocks and assumes their cost in space and time is proportional to
> the number of the perceived assembly "instruction", according to the
> number of newlines and semicolons. Alternatives, paravirt and other
> mechanisms are affected, causing code not to be inlined, and degrading
> compilation quality in general.
>
> The solution that this patch-set carries for this problem is to create
> an assembly macro, and then call it from the inline assembly block. As
> a result, the compiler sees a single "instruction" and assigns the more
> appropriate cost to the code.
>
> To avoid uglification of the code, as many noted, the macros are first
> precompiled into an assembly file, which is later assembled together
> with the the C files. This also enables to avoid duplicate
> implementation that was set before for the asm and C code. This can be
> seen in the exception table changes.
>
> Overall this patch-set slightly increases the kernel size (my build was
> done using my Ubuntu 18.04 config + localyesconfig for the record):
>
> text data bss dec hex filename
> 18140829 10224724 2957312 31322865 1ddf2f1 ./vmlinux before
> 18163608 10227348 2957312 31348268 1de562c ./vmlinux after (+0.1%)
>
> The number of static functions in the image is reduced by 379, but
> actually inlining is even better, which does not always shows in these
> numbers: a function may be inlined causing the calling function not to
> be inlined.
>
> The Makefile stuff may not be too clean. Ideas for improvements are
> welcome.
>
> v1->v2: * Compiling the macros into a separate .s file, improving
> readability (Linus)
> * Improving assembly formatting, applying most of the comments
> according to my judgment (Jan)
> * Adding exception-table, cpufeature and jump-labels
> * Removing new-line cleanup; to be submitted separately

How did you find these issues? Is there some way to find them
automatically in the future? Perhaps with a GCC plugin?

--
Josh