Re: [PATCH 1/2] uapi: Define GENMASK_U128

[Arnd Bergmann]

On Wed, Jul 24, 2024, at 12:31, Anshuman Khandual wrote:
> --- a/include/uapi/asm-generic/bitsperlong.h
> +++ b/include/uapi/asm-generic/bitsperlong.h
> @@ -28,4 +28,8 @@
>  #define __BITS_PER_LONG_LONG 64
>  #endif
> 
> +#ifndef __BITS_PER_U128
> +#define __BITS_PER_U128 128
> +#endif

I would hope we don't need this definition. Not that it
hurts at all, but __BITS_PER_LONG_LONG was already kind
of pointless since we don't run on anything else and
__BITS_PER_U128 clearly can't have any other sensible
definition than a plain 128.

>  #define __AC(X,Y)	(X##Y)
>  #define _AC(X,Y)	__AC(X,Y)
>  #define _AT(T,X)	((T)(X))
> +#define _AC128(X)	((unsigned __int128)(X))

I just tried using this syntax and it doesn't seem to do
what you expected. gcc silently truncates the constant
to a 64-bit value here, while clang fails the build.
See also https://godbolt.org/z/rzEqra7nY
https://stackoverflow.com/questions/63328802/unsigned-int128-literal-gcc

The __GENMASK_U128() macro however seems to work correctly
since you start out with a smaller number and then shift
it after the type conversion.

     Arnd