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Re: [patch gimplifier]: Make sure TRUTH_NOT_EXPR has boolean_type_node type and argument
On Mon, May 16, 2011 at 3:45 PM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Mon, 16 May 2011, Richard Guenther wrote:
>
>> > I think conversion _to_ BOOLEAN_TYPE shouldn't be useless, on the
>> > grounds that it requires booleanization (at least conceptually), i.e.
>> > conversion to a set of two values (no matter the precision or size)
>> > based on the outcome of comparing the RHS value with
>> > false_pre_image(TREE_TYPE(RHS)).
>> >
>> > Conversion _from_ BOOLEAN_TYPE can be regarded as useless, as the
>> > conversions from false or true into false_pre_image or true_pre_image
>> > always is simply an embedding of 0 or 1/-1 (depending on target type
>> > signedness). ?And if the BOOLEAN_TYPE and the LHS have same signedness
>> > the bit representation of boolean_true_type is (or should be) the same
>> > as the one converted to LHS (namely either 1 or -1).
>>
>> Sure, that would probably be enough to prevent non-BOOLEAN_TYPEs be used
>> where BOOLEAN_TYPE nodes were used before. ?It still will cause an
>> artificial conversion from a single-bit bitfield read to a bool.
>
> Not if you're special casing single-bit conversions (on the grounds that a
> booleanization from two-valued set to a different two-valued set of
> the same signedness will not actually require a comparison). ?I think it's
> better to be very precise in our base predicates than to add various hacks
> over the place to care for imprecision.
Or require a 1-bit integral type for TRUTH_* operands only (which ensures
the two-valueness which is what we really want). That can be done
by either fixing the frontends to make boolean_type_node have 1-bit
precision or to build a middle-end private type with that constraints
(though that's the more difficult route as we still do not have a strong
FE - middle-end hand-off point, and it certainly is not the gimplifier).
Long term all the global trees should be FE private and the middle-end
should have its own set.
Richard.
>
> Ciao,
> Michael.