On Nov 24, 2010, at 6:45 AM, Richard Guenther wrote:
> On Tue, Nov 23, 2010 at 9:09 PM, Joern Rennecke <amyl...@spamcop.net> wrote:
>> If we changed BITS_PER_UNIT into an ordinary piece-of-data 'hook', this
>> would not only cost a data load from the target vector, but would also
>> inhibit optimizations that replace division / modulo / multiply with shift
>> or mask operations.
>> So maybe we should look into having a few functional hooks that do common
>> operations, i.e.
>> bits_in_units x / BITS_PER_UNIT
>> bits_in_units_ceil (x + BITS_PER_UNIT - 1) / BITS_PER_UNIT
>> bit_unit_remainder x % BITS_PER_UNIT
>> units_in_bits x * BITS_PER_UNIT
>>
>> Although we currently have some HOST_WIDE_INT uses, I hope using
>> unsigned HOST_WIDE_INT as the argument / return type will generally work.
>>
>> tree.h also defines BITS_PER_UNIT_LOG, which (or its hook equivalent)
>> should probably be used in all the places that use
>> exact_log_2 (BITS_PER_UNIT), and, if it could be relied upon to exist, we
>> could also use it as a substitute for the above hooks. However, this seems
>> a bit iffy - we'd permanently forgo the possibility to have 6 / 7 / 36
>> bit etc. units.
>>
>> Similar arrangements could be made for BITS_PER_WORD and UNITS_PER_WORD,
>> although these macros seem not quite so prevalent in the tree optimizers.
>
> Well. Some things really ought to stay as macros. You can always
> error out if a multi-target compiler would have conflicts there at
> configure time.
That seems reasonable especially since BITS_PER_UNIT is likely to be consistent
(and 8) in any multi-target compiler.
paul
