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

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Message-ID: <87mufcypf5.fsf@mpe.ellerman.id.au>
Date:   Wed, 11 Sep 2019 04:30:38 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Segher Boessenkool <segher@...nel.crashing.org>
Cc:     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

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