Thoughts on memcmp expansion (PR43052)

[Bernd Schmidt]

On 05/13/2016 03:53 PM, Joseph Myers wrote:
> * In C: a byte is the minimal addressable unit; an N-byte object is made
> up of N "unsigned char" objects, with successive addresses in terms of
> incrementing an "unsigned char *" pointer.  A byte is at least 8 bits.
>
> * In GCC, at the level of GNU C APIs on the target, which generally
> includes built-in functions: a byte (on the target) is made of
> CHAR_TYPE_SIZE bits.  In theory this could be more than BITS_PER_UNIT, or
> that could be more than 8, though support for either of those cases would
> be very bit-rotten (and I'm not sure there ever have been targets with
> CHAR_TYPE_SIZE > BITS_PER_UNIT).  Sizes passed to memcpy and memcmp are
> sizes in units of CHAR_TYPE_SIZE bits.
>
> * In GCC, at the RTL level: a byte (on the target) is a QImode object,
> which is made of BITS_PER_UNIT bits.  (HImode is always two bytes, SImode
> four, etc., if those modes exist.)  Support for BITS_PER_UNIT being more
> than 8 is very bit-rotten.
>
> * In GCC, on the host: GCC only supports hosts (and $build) where bytes
> are 8-bit (though writing it as CHAR_BIT makes it clear that this 8 means
> the number of bits in a host byte).
>
> Internal interfaces e.g. representing the contents of strings or other
> memory on the target may not currently be well-defined except when
> BITS_PER_UNIT is 8.  Cf. e.g.
> <https://gcc.gnu.org/ml/gcc/2003-06/msg01159.html>.  But the above should
> at least give guidance as to whether BITS_PER_UNIT, CHAR_TYPE_SIZE (or
> TYPE_PRECISION (char_type_node), preferred where possible to minimize
> usage of target macros) or CHAR_BIT is logically right in a particular
> place.

Thanks. So, this would seem to suggest that BITS_PER_UNIT in 
memcmp/memcpy expansion is more accurate than a plain 8, although 
pedantically we might want to use CHAR_TYPE_SIZE? Should I adjust my 
patch to use the latter or leave these parts as-is?


Bernd