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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 30 Sep 2021 12:21:29 -0700
Message-ID: <CAHk-=wix+P0EtxFSEhmubWNZ++Qg1fj46e32uEKo+mzPu0jPAA@mail.gmail.com>
Subject: Re: [PATCH] modpost: add allow list for llvm IPSCCP
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Arnd Bergmann <arnd@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        llvm@lists.linux.dev
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On Thu, Sep 30, 2021 at 11:54 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Hmm. That's just a "bitmap_weight()", and that function in turn is
> __always_inline.
>
> And the *reason* it is __always_inline is that it really wants to act
> as a macro, and look at the second argument and do special things if
> it is a small constant value.

Looking around, it's not the only one. A lot of the bitmap functions
do that, but it looks like we're missing a few __always_inline cases.

I wonder if we should have a macro to generate those "do X or Y
depending on small_const_nbits()" - and have it generate
__always_inline functions.

Of course, some of those functions have more complex "check at build
time" cases, like that bitmap_clear/set() thing that has a special
case for when it just turns into "memset()"

We have a lot of these kinds of situations where we have a "generic"
function that specializes itself based on arguments. And yes, they are
often recursive, so that you need more than one level of inlining to
actually determine what the arguments are.

I don't know if we might have some way to mark these (and detect the
cases where they don't get inlined and we lose the vasy basic
optimizations).

It's kind of similar to the _Generic() thing that does different
things based on static types, it's just that it does it based on
argument ranges.

          Linus