Re: [PATCH] RISCV/bitops: Use Zbb to provide arch-optimised bitops

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 3/7/25 1:12 PM, Andrew Cooper wrote:

> On 07/03/2025 12:01 pm, Jan Beulich wrote:
> > On 07.03.2025 12:50, Oleksii Kurochko wrote:
> > > On 3/6/25 9:19 PM, Andrew Cooper wrote:
> > > > On 05/03/2025 7:34 am, Jan Beulich wrote:
> > > > > I was actually hoping to eliminate BITS_PER_LONG at some point, in favor
> > > > > of using sizeof(long) * BITS_PER_BYTE. (Surely in common code we could
> > > > > retain a shorthand of that name, if so desired, but I see no reason why
> > > > > each arch would need to provide all three BITS_PER_{BYTE,INT,LONG}.)
> > > > >             
> > > > The concern is legibility and clarity.
> > > >
> > > > This:
> > > >
> > > >      ((x) ? 32 - __builtin_clz(x) : 0)
> > > >
> > > > is a clear _expression_ in a way that this:
> > > >
> > > >      ((x) ? (sizeof(int) * BITS_PER_BYTE) - __builtin_clz(x) : 0)
> > > >
> > > > is not.  The problem is the extra binary _expression_, and this:
> > > >
> > > >      ((x) ? BITS_PER_INT - __builtin_clz(x) : 0)
> > > >
> > > > is still clear, because the reader doesn't have to perform a multiply to
> > > > just to figure out what's going on.
> > > >
> > > >
> > > > It is definitely stupid to have each architecture provide their own
> > > > BITS_PER_*.  The compiler is in a superior position to provide those
> > > > details, and it should be in a common location.
> > > >
> > > > I don't particularly mind how those constants are derived, but one key
> > > > thing that BITS_PER_* can do which sizeof() can't is be used in #ifdef/etc.
> > > >           
> > > What about moving them to xen/config.h? (if it isn't the best one place, any suggestion which is better?)
> > >
> > > #define BYTES_PER_INT  (1 << INT_BYTEORDER)
> > > #define BITS_PER_INT  (BYTES_PER_INT << 3)
> > >
> > > #define BYTES_PER_LONG (1 << LONG_BYTEORDER)
> > > #define BITS_PER_LONG (BYTES_PER_LONG << 3)
> > > #define BITS_PER_BYTE 8
> > >         
> >       
> The *_BYTEORDER's are useless indirection.  BITS_PER_* should be defined
> straight up, and this will simplify quite a lot of preprocessing.
>     
Could we really drop *_BYTEORDER?

For example, LONG_BYTEORDER for Arm could be either 2 or 3 depends on Arm32 or Arm64 is used.

> > > Also, it seems like the follwoing could be moved there too:
> > >
> > > #define POINTER_ALIGN  BYTES_PER_LONG
> > >         
> > This one is likely fine to move.
> >
> > > #define BITS_PER_LLONG 64
> > >         
> > This one is only fine to move imo when converted to
> >
> > #define BITS_PER_LONG (BYTES_PER_LLONG << 3)
> >       
> Erm?  That's mixing long and long long types.  Presuming that's an
> errant L, then sure.
>
> > > #define BITS_PER_BYTE 8
> > >         
> > Personally I'd rather leave this per-arch. The others can truly be derived;
> > this one can't be. If we centralize, imo we should also convert the " << 3"
> > to " * BITS_PER_BYTE".
> >       
> It is highly unlikely that Xen will ever run on a system where CHAR_BIT
> isn't 8.
>
> So I suggest it stays central to reduce complexity.  If an arch ever
> needs to change it, the complexity can be added then.
>     
Does it make sense to ifdef that? Or, at least, before defintion of BITS_PER_BYTE something like:
#if CHAR_BIT != 8
#error "CHAR_BIT isn't 8"
#endif

~ Oleksii