linux-kernel - Re: [PATCH 1/2] uapi: Define GENMASK

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Message-ID: <7d075af2-e94e-4201-9d5d-35fd53124c4c@arm.com>
Date: Wed, 24 Jul 2024 17:29:02 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Yury Norov <yury.norov@...il.com>,
 Rasmus Villemoes <linux@...musvillemoes.dk>,
 Linux-Arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 1/2] uapi: Define GENMASK_U128



On 7/24/24 16:33, Arnd Bergmann wrote:
> 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.

Agreed, although this just followed __BITS_PER_LONG_LONG.
But sure __BITS_PER_U128 can be plain 128.

So would you like to have #ifndef __BITS_PER_LONG_LONG dropped here 
as well ? But should that be folded or in a separate patch ?

> 
>>  #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

But numbers passed into _AC128() are smaller in the range [128..0].
Hence the truncation might not be problematic in this context ? OR
could it be ?

> to a 64-bit value here, while clang fails the build.

Should this be disabled for CC_IS_CLANG ?

> See also https://godbolt.org/z/rzEqra7nY
> https://stackoverflow.com/questions/63328802/unsigned-int128-literal-gcc

So unless the value in there is beyond 64 bits, it should be good ?
OR am I missing something.

> 
> 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.

_U128() never receives anything beyond [127..0] range. So then this
should be good ?