linux-kernel - Re: [PATCH] powerpc: Avoid clang warnings around setjmp and longjmp

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Date:   Tue, 10 Sep 2019 11:37:54 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Segher Boessenkool <segher@...nel.crashing.org>,
        David Laight <David.Laight@...lab.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "# 3.4.x" <stable@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] powerpc: Avoid clang warnings around setjmp and longjmp

On Wed, Sep 11, 2019 at 04:30:38AM +1000, Michael Ellerman wrote:
> Nathan Chancellor <natechancellor@...il.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@...il.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