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Message-ID: <61eb467389f24216b77aaedb85006153@AcuMS.aculab.com>
Date: Mon, 30 Sep 2024 15:29:26 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Thomas Weißschuh' <linux@...ssschuh.net>, "Willy
Tarreau" <w@....eu>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] tools/nolibc: compiler: add macro __nolibc_fallthrough
From: Thomas Weißschuh
> Sent: 30 September 2024 06:35
>
> Recent version of GCC and clang gained -Wimplicit-fallthrough,
> warning about implicit fall-through between switch labels.
> As nolibc does not control the compilation flags, this can trigger
> warnings for when built by the user.
> Make use of the "fallthrough" attribute to explicitly annotate the
> expected fall-throughs and silence the warning.
>
> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> ---
> Changes in v2:
> - Add a do-while loop to improve compiler compatibility
> - Link to v1: https://lore.kernel.org/r/20240929-nolibc-fallthrough-v1-1-5ee07ea9a683@weissschuh.net
...
>
> +#if __nolibc_has_attribute(fallthrough)
> +# define __nolibc_fallthrough do { } while (0); __attribute__((fallthrough))
> +#else
> +# define __nolibc_fallthrough do { } while (0)
> +#endif /* __nolibc_has_attribute(fallthrough) */
> +
> #endif /* _NOLIBC_COMPILER_H */
> diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
> index c968dbbc4ef8137e237b859bf18a6d2970230cbf..3892034198dd566d21a5cc0a9f67cf097d428393 100644
> --- a/tools/include/nolibc/stdio.h
> +++ b/tools/include/nolibc/stdio.h
> @@ -15,6 +15,7 @@
> #include "stdarg.h"
> #include "stdlib.h"
> #include "string.h"
> +#include "compiler.h"
>
> #ifndef EOF
> #define EOF (-1)
> @@ -264,7 +265,7 @@ int vfprintf(FILE *stream, const char *fmt, va_list args)
> case 'p':
> *(out++) = '0';
> *(out++) = 'x';
> - /* fall through */
> + __nolibc_fallthrough;
> default: /* 'x' and 'p' above */
Doesn't this break any old tools that would have parsed the /* fall though */
comment (or any of its variants)?
If you move the ; into the define the 'old' definition can be empty.
And then it is possible that:
case x:
xxxxx;
/* fall though */
__nolibc_fallthough
case y:
will be processed correctly be all tools.
I know I had to lower the warning level for one of our kernel drivers.
But I've forgotten why - that code has to go through a lot of
compilers - including Microsofts.
David
> u64toh_r(v, out);
> break;
>
> ---
> base-commit: e7ed343658792771cf1b868df061661b7bcc5cef
> change-id: 20240929-nolibc-fallthrough-ba822a4a9255
>
> Best regards,
> --
> Thomas Weißschuh <linux@...ssschuh.net>
>
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