linux-kernel - Re: [PATCH 06/10] powerpc: remove GCC version check for UPD

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Date:   Sat, 11 Sep 2021 17:34:53 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Nathan Chancellor <nathan@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Joe Perches <joe@...ches.com>, Arnd Bergmann <arnd@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 06/10] powerpc: remove GCC version check for UPD_CONSTR



Le 11/09/2021 à 01:40, Nick Desaulniers a écrit :
> Now that GCC 5.1 is the minimum supported version, we can drop this
> workaround for older versions of GCC. This adversely affected clang,
> too.

Why do you say that GCC 5.1 is the minimum supported ?

As far as I can see, the minimum supported is still 4.9, see 
https://github.com/torvalds/linux/blob/master/Documentation/process/changes.rst

> 
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Segher Boessenkool <segher@...nel.crashing.org>
> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
> Cc: linuxppc-dev@...ts.ozlabs.org
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>   arch/powerpc/include/asm/asm-const.h | 10 ----------
>   1 file changed, 10 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/asm-const.h b/arch/powerpc/include/asm/asm-const.h
> index 0ce2368bd20f..dbfa5e1e3198 100644
> --- a/arch/powerpc/include/asm/asm-const.h
> +++ b/arch/powerpc/include/asm/asm-const.h
> @@ -12,16 +12,6 @@
>   #  define ASM_CONST(x)		__ASM_CONST(x)
>   #endif
>   
> -/*
> - * Inline assembly memory constraint
> - *
> - * GCC 4.9 doesn't properly handle pre update memory constraint "m<>"
> - *
> - */
> -#if defined(GCC_VERSION) && GCC_VERSION < 50000
> -#define UPD_CONSTR ""
> -#else
>   #define UPD_CONSTR "<>"
> -#endif

There is no point in keeping UPD_CONSTR if it becomes invariant. You 
should just replace all instances of UPD_CONSTR with <> and drop 
UPD_CONSTR completely.

Christophe