Re: Optimizing small reads

[Kiryl Shutsemau <kirill@shutemov.name>]

On Mon, Oct 13, 2025 at 09:19:47AM -0700, Linus Torvalds wrote:
>  - both filemap_read_slow and filemap_read_fast would be 'noinline' so
> that they don't share a stack frame

clang-19 actually does pretty good job re-using stack space without
additional function call.

noinline:

../mm/filemap.c:2883:filemap_read	32	static
../mm/filemap.c:2714:filemap_read_fast	392	static
../mm/filemap.c:2763:filemap_read_slow	408	static

no modifiers:

../mm/filemap.c:2883:filemap_read	456	static

And if we increase buffer size to 1k Clang uninlines it:

../mm/filemap.c:2870:9:filemap_read	32	static
../mm/filemap.c:2714:13:filemap_read_fast	1168	static
../mm/filemap.c:2750:16:filemap_read_slow	384	static

gcc-14, on other hand, doesn't want to inline these functions, even with
'inline' specified. And '__always_inline' doesn't look good.

no modifiers / inline:

../mm/filemap.c:2883:9:filemap_read	32	static
../mm/filemap.c:2714:13:filemap_read_fast	400	static
../mm/filemap.c:2763:16:filemap_read_slow	384	static

__always_inline:

../mm/filemap.c:2883:9:filemap_read	696	static

There's room for improvement for GCC.

I am inclined leave it without modifiers. It gives reasonable result for
both compilers.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov