Re: [PATCH v3 2/2] riscv: introduce asm/swab.h

["Arnd Bergmann" <arnd@arndb.de>]

On Thu, Apr 3, 2025, at 22:34, Ignacio Encinas wrote:
> +#define ARCH_SWAB(size) \
> +static __always_inline unsigned long __arch_swab##size(__u##size value) \
> +{									\
> +	unsigned long x = value;					\
> +									\
> +	if (riscv_has_extension_likely(RISCV_ISA_EXT_ZBB)) {            \
> +		asm volatile (".option push\n"				\
> +			      ".option arch,+zbb\n"			\
> +			      "rev8 %0, %1\n"				\
> +			      ".option pop\n"				\
> +			      : "=r" (x) : "r" (x));			\
> +		return x >> (BITS_PER_LONG - size);			\
> +	}                                                               \
> +	return  ___constant_swab##size(value);				\
> +}

I think the fallback should really just use the __builtin_bswap
helpers instead of the ___constant_swab variants. The output
would be the same, but you can skip patch 1/2.

I would also suggest dumbing down the macro a bit so you can
still find the definition with 'git grep __arch_swab64'. Ideally
just put the function body into a macro but leave the three
separate inline function definitions.

     Arnd