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Re: [range-ops] patch 01/04: types for VR_UNDEFINED and VR_VARYING




On 7/3/19 4:28 AM, Richard Biener wrote:
On Mon, Jul 1, 2019 at 10:52 AM Aldy Hernandez <aldyh@redhat.com> wrote:

As discussed before, this enforces types on undefined and varying, which
makes everything more regular, and removes some special casing
throughout range handling.

I don't like it too much given you need to introduce that "cache".

Why do VARYING or UNDEFINED need a type?  Nobody should ever need
to ask for the type of a range anyhow - the type should be always that from
the context we're looking at (the SSA name the range is associated with,
the operation we are performing on one or two ranges, etc.).

Thinking again about this it looks fundamentally wrong to associate
a type with the VARYING or UNDEFINED lattice states.

We discussed this 2 weeks ago, and it was my understanding that we had an reached an agreement on the general approach. Types on varying and undefined was the *first* thing I brought up. Explanation quoted below.

By the way, the type for varying/undefined requires no space in the value_range_base structure, as it is kept in the min/max fields which we already use for VR_RANGE/VR_ANTI_RANGE.

Aldy

https://gcc.gnu.org/ml/gcc-patches/2019-06/msg01292.html

In order to unify the APIs for value_range and irange, we'd like to make
some minor changes to value_range.  We believe most of these changes
could go in now, and would prefer so, to get broader testing and
minimize the plethora of changes we drag around on our branch.

First, introduce a type for VR_VARYING and VR_UNDEFINED.
--------------------------------------------------------

irange utilizes 0 or more sub-ranges to represent a range, and VARYING
is simply one subrange [MIN, MAX].    value_range represents this with
VR_VARYING, and since there is no type associated with it, we cannot
calculate the lower and upper bounds for the range.  There is also a
lack of canonicalness in value range in that VR_VARYING and [MIN, MAX]
are two different representations of the same value.

We tried to adjust irange to not associate a type with the empty range
[] (representing undefined), but found we were unable to perform all
operations properly.  In particular, we cannot invert an empty range.
i.e. invert ( [] ) should produce [MIN, MAX].  Again, we need to have a
type associated with this empty range.

We'd like to tweak value_range so that set_varying() and set_undefined()
both take a type, and then always set the min/max fields based on that
type.  This takes no additional memory in the structure, and is
virtually transparent to all the existing uses of value_range.

This allows:
    1)  invert to be implemented properly for both VARYING and UNDEFINED
by simply changing one to the other.
    2)  the type() method to always work without any special casing by
simply returning TREE_TYPE(min)
    3)  the new incoming bounds() routines to work trivially for these
cases as well (lbound/ubound, num_pairs(), etc).


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