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

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 3/25/25 5:46 PM, Jan Beulich wrote:

> On 25.03.2025 17:35, Oleksii Kurochko wrote:
> > 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.
> >       
> The can still #define BITS_PER_LONG to 32 or 64 respectively, can't they?
>     
Yes, but then if we want to move it to xen/config.h then it will be needed to:
in Arm's asm/config.h to have something like:
  #ifdef CONFIG_ARM_32
      #define BITS_PER_LONG 32
  #endif

and then in xen/config.h
  #ifndef BITS_PER_LONG
      #define BITS_PER_LONG 64
  #endif

But I wanted to not have #ifndef BITS_PER_LONG in xen/config.h. (or using CONFIG_ARM_32 in xen/config.h)

If it is okay to have this #ifndef in xen/config.h then we can do in that way.

> > > > > 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
> >       
> Where would CHAR_BIT come from? Oh, perhaps you mean __CHAR_BIT__? If all
> compilers we can build with supply that value, we could indeed centrally
> use either
>
> #define BITS_PER_BYTE __CHAR_BIT__
>
> or
>
> #define BITS_PER_BYTE 8
> #if BITS_PER_BYTE != __CHAR_BIT__
> # error "..."
> #endif
>     
Sorry, I meant __CHAR_BIT__.

Thanks.

~ Oleksii