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Date: Wed, 24 May 2023 17:28:35 -0400
From: Rich Felker <dalias@...c.org>
To: Jens Gustedt <Jens.Gustedt@...ia.fr>
Cc: musl@...ts.openwall.com
Subject: Re: [C23 new stdlib 2/4] C23: add the memalignment function
On Wed, May 24, 2023 at 09:38:52PM +0200, Jens Gustedt wrote:
> The name is reserved, so we don't need to protect it via a feature
> macro or a weak symbol.
> ---
> include/stdlib.h | 3 +++
> src/stdlib/memalignment.c | 6 ++++++
> 2 files changed, 9 insertions(+)
> create mode 100644 src/stdlib/memalignment.c
>
> diff --git a/include/stdlib.h b/include/stdlib.h
> index 7800074d..68993c04 100644
> --- a/include/stdlib.h
> +++ b/include/stdlib.h
> @@ -178,6 +178,9 @@ long double strtold_l(const char *__restrict, char **__restrict, struct __locale
> typedef int once_flag;
> void call_once(once_flag *, void (*)(void));
>
> +size_t (memalignment)(const void *);
> +#define memalignment(P) (((size_t)1)<<__builtin_ctzll((unsigned long long)P))
> +
> #ifdef __cplusplus
> }
> #endif
> diff --git a/src/stdlib/memalignment.c b/src/stdlib/memalignment.c
> new file mode 100644
> index 00000000..58b68954
> --- /dev/null
> +++ b/src/stdlib/memalignment.c
> @@ -0,0 +1,6 @@
> +#include <stdlib.h>
> +
> +size_t (memalignment)(const void *p)
> +{
> + return memalignment(p);
> +}
> --
> 2.34.1
There's a more efficient implementation of this function which does
not depend on __builtin_ctzll (which we don't): p^(p-1)&p
I'm not clear that there's a really good motivation for having it
implemented in the header. It kinda falls under the "maybe do it
because it's easy" case, but once you switch to the non-GNUC-specific
version, it needs a static inline function to avoid multiple
evaluation, and then it gets to the point of "this is really too much
code to have in a header" (which is more an issue of avoiding
"creative" content in headers than anything else).
Rich
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