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Re: [PATCH 1/1] xen/include/xen/byteorder: Re-implement bswap/swab with compiler builtin functions
- To: Lin Liu <lin.liu@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 15 Oct 2021 10:22:53 +0200
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- Delivery-date: Fri, 15 Oct 2021 08:23:02 +0000
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On 15.10.2021 09:26, Lin Liu wrote:
> Current implementation of bswap/swab is overhaul which involves
> byte operations, compilers has builtin functions can help to
> simply the implementation
> * GCC: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
> * Clang: https://clang.llvm.org/docs/LanguageExtensions.html
>
> This commit replace the implementation with compiler builtins
__builtin_bswap{32,64}() appeared in gcc 4.3, __builtin_bswap16() in
gcc 4.8. I don't think we can use these just yet without having any
fallback. Apart from this aspect the change looks okay to me, with
just one nit:
> --- a/xen/include/xen/byteorder/swab.h
> +++ b/xen/include/xen/byteorder/swab.h
> @@ -10,166 +10,32 @@
> * to clean up support for bizarre-endian architectures.
> */
>
> -/* casts are necessary for constants, because we never know how for sure
> - * how U/UL/ULL map to __u16, __u32, __u64. At least not in a portable way.
> - */
> -#define ___swab16(x) \
> -({ \
> - __u16 __x = (x); \
> - ((__u16)( \
> - (((__u16)(__x) & (__u16)0x00ffU) << 8) | \
> - (((__u16)(__x) & (__u16)0xff00U) >> 8) )); \
> -})
> -
> -#define ___swab32(x) \
> -({ \
> - __u32 __x = (x); \
> - ((__u32)( \
> - (((__u32)(__x) & (__u32)0x000000ffUL) << 24) | \
> - (((__u32)(__x) & (__u32)0x0000ff00UL) << 8) | \
> - (((__u32)(__x) & (__u32)0x00ff0000UL) >> 8) | \
> - (((__u32)(__x) & (__u32)0xff000000UL) >> 24) )); \
> -})
> +#define __swab16(x) (__builtin_bswap16(x))
In cases like this one there's no need for the outer pair of parentheses.
Jan
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