Re: [PATCH v2] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for gcc >= 10

[Linus Torvalds]

On Mon, May 11, 2020 at 5:04 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> Not inlining as aggressively is not necessarily a bad thing. It can
> be, of course. But I've actually also done gcc bugreports about gcc
> inlining too much, and generating _worse_ code as a result (ie
> inlinging things that were behind an "if (unlikely())" test, and
> causing the likely path to grow a stack fram and stack spills as a
> result).

In case people care, the bugzilla case I mentioned is this one:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49194

with example code on why it's actively wrong to inline.

Obviously, in the kernel, we can fix the obvious cases with "noinline"
and "always_inline", but those take care of the outliers.  Having a
compiler that does reasonably well by default is a good thing, and
that very much includes *not* inlining mindlessly.

                  Linus