linux-kernel - Re: [PATCH v2] mips: lib: uncached: fix non-standard usage of variable 'sp'

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Date:   Fri, 11 Dec 2020 11:15:15 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Anders Roxell <anders.roxell@...aro.org>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-mips@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>
Subject: Re: [PATCH v2] mips: lib: uncached: fix non-standard usage of
 variable 'sp'

On Fri, Dec 11, 2020 at 2:24 AM Anders Roxell <anders.roxell@...aro.org> wrote:
>
> When building mips tinyconfig with clang the following warning show up:
>
> arch/mips/lib/uncached.c:45:6: warning: variable 'sp' is uninitialized when used here [-Wuninitialized]
>         if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
>             ^~
> arch/mips/lib/uncached.c:40:18: note: initialize the variable 'sp' to silence this warning
>         register long sp __asm__("$sp");
>                         ^
>                          = 0
> 1 warning generated.
>
> Rework to make an explicit inline move, instead of the non-standard use
> of specifying registers for local variables. This is what's written
> from the gcc-10 manual [1] about specifying registers for local
> variables:
>
> "6.47.5.2 Specifying Registers for Local Variables
> .................................................
> [...]
>
> "The only supported use for this feature is to specify registers for
> input and output operands when calling Extended 'asm' (*note Extended
> Asm::).  [...]".
>
> [1] https://docs.w3cub.com/gcc~10/local-register-variables
> Signed-off-by: Anders Roxell <anders.roxell@...aro.org>

Link: https://github.com/ClangBuiltLinux/linux/issues/606
Reported-by: Nathan Chancellor <natechancellor@...il.com>
Reported-by: Naresh Kamboju <naresh.kamboju@...aro.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

> ---
>  arch/mips/lib/uncached.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/mips/lib/uncached.c b/arch/mips/lib/uncached.c
> index 09d5deea747f..f80a67c092b6 100644
> --- a/arch/mips/lib/uncached.c
> +++ b/arch/mips/lib/uncached.c
> @@ -37,10 +37,12 @@
>   */
>  unsigned long run_uncached(void *func)
>  {
> -       register long sp __asm__("$sp");
>         register long ret __asm__("$2");
>         long lfunc = (long)func, ufunc;
>         long usp;
> +       long sp;
> +
> +       __asm__("move %0, $sp" : "=r" (sp));
>
>         if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
>                 usp = CKSEG1ADDR(sp);
> --
> 2.29.2
>


-- 
Thanks,
~Nick Desaulniers