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 [off-list ref] writes:

quoted

On Wed, Sep 04, 2019 at 08:01:35AM -0500, Segher Boessenkool wrote:

quoted

On Wed, Sep 04, 2019 at 08:16:45AM +0000, David Laight wrote:

quoted

From: Nathan Chancellor [mailto:natechancellor@gmail.com]

quoted

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.

quoted

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/ (local)

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