linux-kernel - Re: [GIT PULL v2] Kbuild updates for v5.15-rc1

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Message-ID: <871r644bd2.fsf@oldenburg.str.redhat.com>
Date:   Sat, 04 Sep 2021 17:19:21 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        llvm@...ts.linux.dev, linux-toolchains@...r.kernel.org
Subject: Re: [GIT PULL v2] Kbuild updates for v5.15-rc1

* Segher Boessenkool:

> Let me quote the original mail (I had to dig it out of the archives as
> well, no nice threading, too lazy, sorry):

It still doesn't say why.  I did see a reference to fleeting reference
to <stdatomic.h> and <float.h>.

My conjecture is that the real reason is avoid atomic emulation
(softatomic?)  and softfloat code.  It's not related to <stdarg.h> at
all: this header is replaced so that GCC's include subdirectory can be
dropped from the include search path.  What I don't know if this is to
avoid obscure linker failures related to libatomic/softfloat (obviously
not great) or run-time failures (worse).

In any case, it would be nice to know what the real motivation is.

After all, <stdatomic.h> is exactly like <stdarg.h> in that it's
possible to use its functionality even without the header file.  The
__atomic builtins are even documented in the GCC manual (unlike
<stdatomic.h>), which is why some programmers prefer them over the
standard interface.  And then there's the _Atomic keyword itself, whose
use can easily result in calls to libatomic functions, too.  So blocking
<stdatomic.h> makes little sense to me.

I don't know enough about softfloat if blocking the inclusion of
<float.h> is worth it.

Thanks,
Florian