Re: wide-int, rtl

[Richard Sandiford <rdsandiford at googlemail dot com>]

Kenneth Zadeck <zadeck@naturalbridge.com> writes:
>> +	/* One could argue that GET_MODE_PRECISION (TYPE_MODE (type))
>> +	   should always be the same as TYPE_PRECISION (type).
>> +	   However, it is not.  Since we are converting from tree to
>> +	   rtl, we have to expose this ugly truth here.  */
>> +	temp = immed_wide_int_const (wide_int::from
>> +				       (exp,
>> +					GET_MODE_PRECISION (TYPE_MODE (type)),
>> +					TYPE_SIGN (type)),
>> +				     TYPE_MODE (type));
>> +	return temp;
>> +      }
>>
>> I don't really see how one could argue that, given that there are much fewer
>> modes than possible type precisions, so please rewrite the comment, e.g.:
>>
>> "Given that TYPE_PRECISION (type) is not always equal to
>> GET_MODE_PRECISION (TYPE_MODE (type)), we need to extend from the former
>> to the latter according to the signedness of the type".
>>
>> What about a fast track where the precisions are indeed equal?
>>
>
> There is not really a faster track here.    you still are starting with 
> a tree and converting to an rtx.   All that the default one would do 
> would be to access the types precision and sign and use that.

FWIW it would be:

	temp = immed_wide_int_const (exp, TYPE_MODE (type));

But it's hard to tell whether it would buy much.  It didn't show up as
a hot spot when I was doing performance measurements before.

>> --- a/gcc/machmode.def
>> +++ b/gcc/machmode.def
>> @@ -229,6 +229,9 @@ UACCUM_MODE (USA, 4, 16, 16); /* 16.16 */
>>   UACCUM_MODE (UDA, 8, 32, 32); /* 32.32 */
>>   UACCUM_MODE (UTA, 16, 64, 64); /* 64.64 */
>>   
>> +/* Should be overridden by EXTRA_MODES_FILE if wrong.  */
>> +#define MAX_BITS_PER_UNIT 8
>> +
>>
>> What is it for?  It's not documented at all.
>>
> This requires some discussion as to the direction we want to go. This is 
> put in so that in gen_modes we can compute MAX_BITSIZE_MODE_ANY_INT and 
> MAX_BITSIZE_MODE_ANY_MODE.    The problem is that during genmodes we do 
> have access to BITS_PER_UNIT.    These two computed symbols are then 
> used as compile time constants in other parts of the compiler to 
> allocate data structures that are guaranteed to be large enough.
>
> Richard Sandiford put this in so we would preserve the ability to build 
> a multi-targetted compiler where the targets had different values for 
> BITS_PER_UNIT.   So one possibility is that we add some documentation to 
> this effect.

Sorry, I forgot yesterday an important detail behind why this seemed
like a good thing.  I think there was a strong feeling (from me and others)
that wide-int.h shouldn't depend on tm.h.  If we make wide-int.h depend
on tm.h then basically all the compiler does.

So as it stands we can't use BITS_PER_UNIT directly.  Having a
MAX_BITS_PER_UNIT for "all compiled-in targets" (which obviously
as things stand is exactly one) seemed like a reasonable abstraction.

Alternatively we could say that BITS_PER_UNIT is really part of the
definition of QImode and move it to the modes.def file.

Thanks,
Richard