linux-kernel - Re: [PATCH v6] coresight: etm4x: avoid build failure with unrolled loops

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Date:   Sun, 10 Jul 2022 21:13:08 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        David Laight <David.Laight@...lab.com>,
        Tao Zhang <quic_taozha@...cinc.com>,
        Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, coresight@...ts.linaro.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <llvm@...ts.linux.dev>
Subject: Re: [PATCH v6] coresight: etm4x: avoid build failure with unrolled loops

On Sat, Jul 9, 2022 at 1:15 AM Nick Desaulniers <ndesaulniers@...gle.com> wrote:

>         ({                                                                      \
>                 u64 __val;                                                      \
>                                                                                 \
> -               if (__builtin_constant_p((offset)))                             \
> +               if (__is_constexpr((offset)))                                   \
>                         __val = read_etm4x_sysreg_const_offset((offset));       \
>                 else                                                            \
>                         __val = etm4x_sysreg_read((offset), true, (_64bit));    \
>

This is clearly better than the current version using
__builtin_constant_p(), but
I don't think it's safe in all cases, since there are expressions that
are constant
expressions to the compiler but are not valid input to the assembler.

I would prefer to see this fixed differently, but doing this one first is also
fine with me:

Acked-by: Arnd Bergmann <arnd@...db.de>

        Arnd