On 5/29/19 9:11 AM, Richard Biener wrote:
On Tue, May 28, 2019 at 4:41 PM Jeff Law <l...@redhat.com> wrote:
This aspect of symbolics would be handled by a relational/equivalence
processing engine that would be follow on work. Using the same basic
model as ranges, each tree code is taught to understand the relation
between its operands, and then we can answer equivalency and relational
accurately as well. It would be available for any pass to use
independent of ranges. I will expound upon that a bit in the future
directions section.
While I agree that symbolic ranges are a complication and that most
cases it currently handles are not "value-range" things I do not agree
with the idea that we can simply remove handling them and deal
with the fallout in some later point in the future. Eric may also be
able to show you "real" symbolic value-range magic.
Note that symbolic ranges are already restricted to PLUS_EXPR
and MINUS_EXPR (and NEGATE_EXPR I think). There are
also "symbolic" (non-integer constant) ranges like [&a, &a].
I've never seen [&a, &MEM[&a + 4]] but we wouldn't reject those
I think.
You may have noticed that EVRP does not use symbolic ranges.
Btw, I probably misremembered this part - it is equivalences that
EVRP doesn't use. equivalences are the "other half" of relations
besides symbolic ranges.
I do get confused by this assertion... From the EVRP dumps :
Intersecting
int [b_6(D), b_6(D)] EQUIVALENCES: { a_5(D) b_6(D) } (2 elements)
and
VARYING
to
int [b_6(D), b_6(D)] EQUIVALENCES: { a_5(D) b_6(D) } (2 elements)
Intersecting
int [a_5(D), a_5(D)] EQUIVALENCES: { a_5(D) b_6(D) } (2 elements)
and
VARYING
and
pushing new range for c_7(D): int [b_6(D), b_6(D)] EQUIVALENCES: {
b_6(D) c_7(D) } (2 elements)
Visiting stmt if (a_5(D) == c_7(D))
it certainly does seem to have symbolics in the range.. ( [b_6, b_6]
) and it does use equivalencies... presumably thats why its in the
range, to help track them.. regardless, thats is how it gets cases we
dont right now like
if (a == b && b == c)
if (a == c)
It may not have symbolic expressions like [0, b_6 + 1] as end points
like VRP does, or maybe it doesn't resolve them to anything other than
varying, but it does appear to have symbolics? Perhaps it resolves them
immediately, but it does appear to use some partial bits of the symbolic
machinery.
We'd like to make this consistent everywhere and track the equivalencies
outside of the range via a separate mechanism.
As already said I'd like to see VRP go but obstackles are
symbolic ranges and jump-threading (with Jeff promising
to handle the jump-threading part in the past).
Right. FWIW, one of the follow-on items to this work is Aldy's
improvements to backwards jump threading which utilizes the ranger
framework -- the primary purpose of that work is to eliminate the need
for jump threading in VRP.
But without relations it won't catch most of the cases...
Richard.
its catching what it quite a few new things.... and the
relations/equivalencies are coming... I haven't heard of any issues
regarding equivalencies, but Aldy's the one that been working with it,
so he can speak to it better.
Andrew
