Re: [PATCH] coresight: etm4x: work around clang-12+ build failure

[Arnd Bergmann <arnd@kernel.org>]

On Thu, Feb 25, 2021 at 10:23 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> On Thu, Feb 25, 2021 at 8:45 AM Mathieu Poirier
> <mathieu.poirier@linaro.org> wrote:
> >
> > Good morning,
> >
> > On Thu, Feb 25, 2021 at 10:42:58AM +0100, Arnd Bergmann wrote:
> > > From: Arnd Bergmann <arnd@arndb.de>
> > >
> > > clang-12 fails to build the etm4x driver with -fsanitize=array-bounds:
>
> Is a sanitizer enabled, that would trap on OOB?
>
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/1310

As described over there, this happens only with the array-bounds
sanitizer.

Actually, looking at it again now, in the reduced test case, the inline
assembly is not even parsable, it only works because it never gets
emitted without  -fsanitize=array-bounds, and the alternative
code path is used in

#define read_etm4x_sysreg_offset(offset, _64bit)
         \
        ({
         \
                u64 __val;
         \

         \
                if (__builtin_constant_p((offset)))
         \
                        __val =
read_etm4x_sysreg_const_offset((offset));       \
                else
         \
                        __val = etm4x_sysreg_read((offset), true,
(_64bit));    \
                __val;
         \
         })

read_etm4x_sysreg_const_offset() eventually turns into something
like

asm("msr_s " __stringify(offset));

so the offset has to be something that can be parsed by the
assembler. __builtin_constant_p() checks that it is a constant
value at compile-time, and in this case it can be because there
is a small upper bound and clang just unrolls the loop.

I don't think there is an alternative to __builtin_constant_p()
that can be used to decide if the argument is something that
can be used as a constant expression in an inline assembly.

            Arnd