From: Nathan Chancellor <natechancellor@gmail.com>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
LKML <linux-kernel@vger.kernel.org>,
"# 3.4.x" <stable@vger.kernel.org>,
clang-built-linux <clang-built-linux@googlegroups.com>,
David Laight <David.Laight@aculab.com>,
Paul Mackerras <paulus@samba.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH] powerpc: Avoid clang warnings around setjmp and longjmp
Date: Tue, 10 Sep 2019 11:37:54 -0700 [thread overview]
Message-ID: <20190910183754.GA42190@archlinux-threadripper> (raw)
In-Reply-To: <87mufcypf5.fsf@mpe.ellerman.id.au>
On Wed, Sep 11, 2019 at 04:30:38AM +1000, Michael Ellerman wrote:
> Nathan Chancellor <natechancellor@gmail.com> writes:
> > On Wed, Sep 04, 2019 at 08:01:35AM -0500, Segher Boessenkool wrote:
> >> On Wed, Sep 04, 2019 at 08:16:45AM +0000, David Laight wrote:
> >> > From: Nathan Chancellor [mailto:natechancellor@gmail.com]
> >> > > Fair enough so I guess we are back to just outright disabling the
> >> > > warning.
> >> >
> >> > Just disabling the warning won't stop the compiler generating code
> >> > that breaks a 'user' implementation of setjmp().
> >>
> >> Yeah. I have a patch (will send in an hour or so) that enables the
> >> "returns_twice" attribute for setjmp (in <asm/setjmp.h>). In testing
> >> (with GCC trunk) it showed no difference in code generation, but
> >> better save than sorry.
> >>
> >> It also sets "noreturn" on longjmp, and that *does* help, it saves a
> >> hundred insns or so (all in xmon, no surprise there).
> >>
> >> I don't think this will make LLVM shut up about this though. And
> >> technically it is right: the C standard does say that in hosted mode
> >> setjmp is a reserved name and you need to include <setjmp.h> to access
> >> it (not <asm/setjmp.h>).
> >
> > It does not fix the warning, I tested your patch.
> >
> >> So why is the kernel compiled as hosted? Does adding -ffreestanding
> >> hurt anything? Is that actually supported on LLVM, on all relevant
> >> versions of it? Does it shut up the warning there (if not, that would
> >> be an LLVM bug)?
> >
> > It does fix this warning because -ffreestanding implies -fno-builtin,
> > which also solves the warning. LLVM has supported -ffreestanding since
> > at least 3.0.0. There are some parts of the kernel that are compiled
> > with this and it probably should be used in more places but it sounds
> > like there might be some good codegen improvements that are disabled
> > with it:
> >
> > https://lore.kernel.org/lkml/CAHk-=wi-epJZfBHDbKKDZ64us7WkF=LpUfhvYBmZSteO8Q0RAg@mail.gmail.com/
>
> For xmon.c and crash.c I think using -ffreestanding would be fine.
> They're both crash/debug code, so we don't care about minor optimisation
> differences. If anything we don't want the compiler being too clever
> when generating that code.
>
> cheers
I will send a v2 later today along with another patch to fix this
warning and another build error.
Cheers,
Nathan
prev parent reply other threads:[~2019-09-10 18:39 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-12 2:32 [PATCH] powerpc: Avoid clang warnings around setjmp and longjmp Nathan Chancellor
2019-08-12 5:37 ` Christophe Leroy
2019-08-12 16:55 ` Nathan Chancellor
2019-08-12 17:35 ` Nick Desaulniers
2019-08-27 0:12 ` Nathan Chancellor
2019-08-28 13:43 ` Michael Ellerman
2019-08-28 17:53 ` Nathan Chancellor
2019-08-28 18:01 ` Nick Desaulniers
2019-08-28 18:45 ` Nathan Chancellor
2019-08-28 22:16 ` Nick Desaulniers
2019-08-29 8:32 ` Segher Boessenkool
2019-08-29 9:59 ` David Laight
2019-09-03 5:55 ` Nathan Chancellor
2019-09-03 19:31 ` Segher Boessenkool
2019-09-04 0:24 ` Nathan Chancellor
2019-09-04 8:16 ` David Laight
2019-09-04 13:01 ` Segher Boessenkool
2019-09-04 23:15 ` Nathan Chancellor
2019-09-10 18:30 ` Michael Ellerman
2019-09-10 18:37 ` Nathan Chancellor [this message]
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