Re: [PATCH 1/3] mm/slub: Only define kmalloc_large_node_hook() for NUMA systems

David Rientjes
2017-05-24T20:36:29Z

On Tue, 23 May 2017, Matthias Kaehlcke wrote:

> > diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> > index de179993e039..e1895ce6fa1b 100644
> > --- a/include/linux/compiler-clang.h
> > +++ b/include/linux/compiler-clang.h
> > @@ -15,3 +15,8 @@
> > * with any version that can compile the kernel
> > */
> > #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
> > +
> > +#ifdef inline
> > +#undef inline
> > +#define inline __attribute__((unused))
> > +#endif
>
> Thanks for the suggestion!
>
> Nothing breaks and the warnings are silenced. It seems we could use
> this if there is a stong opposition against having warnings on unused
> static inline functions in .c files.
>

It would be slightly different, it would be:

#define inline inline __attribute__((unused))

to still inline the functions, I was just seeing if there was anything
else that clang was warning about that was unrelated to a function's
inlining.

> Still I am not convinced that gcc's behavior is preferable in this
> case. True, it saves us from adding a bunch of __maybe_unused or
> #ifdefs, on the other hand the warning is a useful tool to spot truly
> unused code. So far about 50% of the warnings I looked into fall into
> this category.
>

I think gcc's behavior is a result of how it does preprocessing and is a
clearly defined and long-standing semantic given in the gcc manual
regarding -Wunused-function.

#define IS_PAGE_ALIGNED(__size) (!(__size & ((size_t)PAGE_SIZE - 1)))
static inline int is_page_aligned(size_t size)
{
return !(size & ((size_t)PAGE_SIZE - 1));
}

Gcc will not warn about either of these being unused, regardless of -Wall,
-Wunused-function, or -pedantic. Clang, correct me if I'm wrong, will
only warn about is_page_aligned().

So the argument could be made that one of the additional benefits of
static inline functions is that a subset of compilers, heavily in the
minority, will detect whether it's unused and we'll get patches that
remove them. Functionally, it would only result in LOC reduction. But,
isn't adding #ifdef's to silence the warning just adding more LOC?

I have no preference either way, I think it would be up to the person who
is maintaining the code and has to deal with the patches.