On Mon, 19 Sep 2022, Andrew MacLeod wrote:
> Yeah, currently the internal cache isnt really wired into the fold_using_range
> as its is suppose to be a consistent internal state. so its not currently
> expecting to work in a situation here what it thinks is global state might
> change.
>
> I figured I better go back and watch your entire VN presentation, I only
> caught the last bit live.
>
> Let me try to understand exactly what you want/need.. and let me use a simple
> example from that talk (yes I just typed it in :-).
>
> n_2 = 1
> i_4 = 0
> val_5 = 0
> <bb 3>:
> # i_1 = PHI <i_4(2), i_7(3)>
> #val_2 = PHI <val_5(2), val_6(3) >
> val_6 = val_2 + 1;
> i_7 = i_1 + 1
> if (i_7 < n_3)
> goto <bb 3>;
> else
> goto <bb 4>;
> <bb 4>
> _8 = val_6
> return _8
>
>
> In an ideal world, While you are processing bb 3 the first iteration, you
> want edge 4->3 to be unexecutable as far as ranger is concerned. You walk
> thru the block, and get to the end. and in this example, you'd be done cause
> any queries you make range would have if (2 < 1),
>
> For the case when we do need to iterate (say n_2 = 3), you need to go back and
> rewalk that block, with that edge no longer marked as unexecutable? And part
> of the problem would be that ranger's cache for bb 3 would have all those
> values from the first pass. and everything explodes.
Yep.
> It looks like you created a fur_source to manually adjust PHIs within the
> fold_stmt query to ignore edges that are not marked executable.
Yes, and use the current values from the VN lattice when looking at
statement operands.
> So now let me start with my questions.
>
> In theory, if you could tell it "kill the cache for bb3" change the
> executable state and re walk it, that would work? This clearly get s a little
> more complicated as the size of the region that is being iterated grows, so
> you would want to be able to invalidate the cache for a range of blocks.
Yep. I do have such a range of blocks nicely available. I would also
know the points when I change executability of an edge (always goes from
not executable to executable) in case that would help more.
> Thats not quite as straightforward as it might seems because the cache for
> values per basic block is indexed by ssa-name.. so you cant simply say, "kill
> bb3s value".. you need to walk all the ssa-names that have a cache object, and
> if they have an entry for bb3, kill that. But lets leave that alone for a
> minute, because that is a solvable problem.
>
> Ranger also has a globally available flag that it uses to internally flag and
> track executable state of edge:
>
> auto_edge_flag non_executable_edge_flag;
>
> The gori object which is instantiated with any ranger instance only uses that
> to determine if an edge should be processed for a value ie
>
> gori_compute::outgoing_edge_range_p (vrange &r, edge e, tree name, range_query
> &q)
> {
> if ((e->flags & m_not_executable_flag))
> {
> r.set_undefined ();
> return true;
>
> So it seems to me that if you all set/cleared this flag along with
> EDGE_EXECUTABLE (and it means the opposite by the way. TRUE means the edge
> is not_executable.. ) That at least all your queries through ranger would be
> picking up the values you are looking for. including those PHI's.
Ah, I see.
> That would then just leave you with the stale cache state to deal with? And
> if we can resolve that, would all just work? at least in theory?
In theory, yes. Besides that the use-def walking of the cache it not
wired up with fur_*
> There is a concept in the cache of staleness which is used when we recalculate
> over back edges, but its really oriented towards a model where we only ever
> move edges from executable to non-executable. Your VN approach moves them
> in the other way and I have no confidence it would work properly in that mode.
>
> Its not super efficient, but as a starting point we might be able to do
> something like
> 1 - when you enter a region call a new routine called "Mark state" which
> would push an empty bitmap over ssa-names onto a stack
> 2 - everytime the cache creates an entry, set the bit for the ssa-name in
> the bitmap at the top of the stack
> 3 - when you call "reset state", simply kill all the cache entries for any
> ssa name set in the bitmap. And also kill rangers internal global value for
> the name. THIs would in essence reset eveyrthing it knows about that ssa
> name.
Value-numbering essentially does such things to be able to roll-back,
it records "change objects" that can be used to remove elements added
to hash-tables efficiently for example.
> That would have to drawback of losing any information that had been calculated
> earlier, and possibly trigger some additional calculations as that value will
> now be stale elsewhere in the IL. We could make it more efficient if you
> also provided a bitmap over the basic blocks in the region (or some way of
> determining that). Then we would just kill the entries in those blocks for
> those ssa-names.
>
> As long as you weren't making any ranger queries from outside the region
> during this time, that might work.
>
> Before I go any further, are we on the same page, or are you looking for
> something different than this?
>
> You had mentioned the path_ranger model also, which effectively uses ranger
> for the values on entry to the path/region, and then uses over-rided API entry
> points to choose whether to use the local cache vector, or go to rangers
> on-entry values..
>
> This approach could also work, and one way would be to implement the
> mark/reset API. The new class would maybe instantiate an additional local
> cache from which values are set/picked up first for any queries within the
> region blocks and when you are done, throws away the cache for the region.
> This would require some tweaking from rangers primary cache lookup/set
> routine, but perhaps there is some benefit to generalizing this model and
> integrating it.. ie, a region_ranger which builds this facility on top and
> allows you to reset regions. Presumably you want to be able to "stack" these
> marks so you can push and pop regions? would also have to figure out how to
> work the relational oracle with it.
Yeah, so my thinking was that I'd keep my own range cache for in-region,
like path_ranger does, and track changes similar as to how I do for VN
so I can undo things efficiently. That way I also get to decide
how far to do the caching use-def walks.
Of course the disadvantage is duplicating quite some ranger functionality
that's already there without too much changes besides my own cache plus
ensuring using the VNs computed values and edge execuability. Maybe
it isn't too much work though.
> I can think of a couple of possible approaches for that, some more efficient
> than others. I suppose the path_ranger could even be completely replaced
> with that model. huh. it would just call mark() at the start of a path, and
> reset() when its done with the path. Might not be as efficient however. Be
> a good proof of correctness test I guess at least.
>
> Anyway, am I in the ballpark somewhere of what you are looking for? Or am I
> over engineering something? :-)
No, you basically identified the problems.
Note that when not iterating the proposed change should already work
optimally, but it's of course bad to be "better" in the weaker setting ...
The second thing would be to look at how to use conditional equivalences.
What's required to make ranger aware of them and what's required to
query them since for them the actual simplification would happen in
the folding done by value-numbering like with
if (a == b)
c = a - b;
which EVRP gets simplified for example but VN fails at this.
There's proposed enhancements to VN that might catch this as well
but I'm not too happy about the implementation. Mainly because
equivalences are a bitch.
Richard.
> Andrew
>
>
>
> On 9/19/22 07:19, Richard Biener wrote:
> > The following is an attempt to utilize ranger for simplification
> > of conditionals during value-numbering. It fails the added
> > testcase because it seems the fur source is only involved when
> > processing the stmt to be folded but not when filling the cache
> > of ranges on the operand use->def chain.
> >
> > When not iterating that's not a problem since earlier stmts
> > have operands eliminated but when not iterating or when
> > elimination is disabled the IL remains unchanged.
> >
> > When iterating it will probably do things wrong due to the lack
> > of pruning the chache for the region we unwind and when doing
> > region based VN it likely picks up stale EDGE_EXECUTABLE when
> > leaving the region over the entry because there's no way to stop
> > "local" cache prefilling when leaving the region (and just using
> > global ranges from there).
> >
> > Knowing path-range-query it's probably possible to clone all
> > the block walking and cache filling, making a special variant
> > for value-numbering but I wonder whether that's really what's
> > intended? I would probably need to implement a special
> > range_of_expr and range_of_stmt for this and keep my own cache?
> > Basically a region_range_query with wired in valueization?
> >
> > * tree-ssa-sccvn.cc (...): ...
> > ---
> > gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-100.c | 15 ++++
> > gcc/tree-ssa-sccvn.cc | 77 ++++++++++++++++++---
> > gcc/tree-ssa-sccvn.h | 3 +-
> > 3 files changed, 83 insertions(+), 12 deletions(-)
> > create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-100.c
> >
> > diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-100.c
> > b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-100.c
> > new file mode 100644
> > index 00000000000..f479a322b70
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-100.c
> > @@ -0,0 +1,15 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-O2 -fdump-tree-fre1" } */
> > +
> > +int foo (int *p, int *q)
> > +{
> > + if (*p > 0 && *q > 0)
> > + {
> > + int v = *p + *q;
> > + if (v > 0)
> > + return *q;
> > + }
> > + return 0;
> > +}
> > +
> > +/* { dg-final { scan-tree-dump-times "known outgoing edge" 1 "fre1" } } */
> > diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc
> > index 74b8d8d18ef..2753200c2c2 100644
> > --- a/gcc/tree-ssa-sccvn.cc
> > +++ b/gcc/tree-ssa-sccvn.cc
> > @@ -73,6 +73,7 @@ along with GCC; see the file COPYING3. If not see
> > #include "fold-const-call.h"
> > #include "ipa-modref-tree.h"
> > #include "ipa-modref.h"
> > +#include "gimple-range.h"
> > #include "tree-ssa-sccvn.h"
> >
> > /* This algorithm is based on the SCC algorithm presented by Keith
> > @@ -360,10 +361,11 @@ static vn_ssa_aux_t last_pushed_avail;
> > correct. */
> > static vn_tables_t valid_info;
> >
> > +tree (*vn_valueize) (tree);
> >
> > /* Valueization hook for simplify_replace_tree. Valueize NAME if it is
> > an SSA name, otherwise just return it. */
> > -tree (*vn_valueize) (tree);
> > +
> > static tree
> > vn_valueize_for_srt (tree t, void* context ATTRIBUTE_UNUSED)
> > {
> > @@ -380,6 +382,36 @@ vn_valueize_for_srt (tree t, void* context
> > ATTRIBUTE_UNUSED)
> > return res;
> > }
> >
> > +class vn_fur : public fur_depend
> > +{
> > +public:
> > + vn_fur (gimple *s, gori_compute *g, range_query *q)
> > + : fur_depend (s, g, q) {}
> > + virtual bool get_operand (vrange &, tree);
> > + virtual bool get_phi_operand (vrange &, tree, edge);
> > +};
> > +
> > +bool
> > +vn_fur::get_operand (vrange &r, tree op)
> > +{
> > + return fur_depend::get_operand (r, vn_valueize (op));
> > +}
> > +
> > +bool
> > +vn_fur::get_phi_operand (vrange &r, tree op, edge e)
> > +{
> > + // ??? Check region boundary, avoid walking ourside of the region
> > + // somehow? Possibly when initializing the ranger object?
> > + // or can/should we simply return 'false' when we arrive with
> > + // e being the entry edge? What about for non-PHIs? Can we
> > + // somehow force to only use the global range and stop caching
> > + // after that?
> > + if (e->flags & EDGE_EXECUTABLE)
> > + return fur_depend::get_phi_operand (r, vn_valueize (op), e);
> > + r.set_undefined ();
> > + return true;
> > +}
> > +
> >
> > /* This represents the top of the VN lattice, which is the universal
> > value. */
> > @@ -7292,12 +7324,12 @@ eliminate_with_rpo_vn (bitmap inserted_exprs)
> >
> > static unsigned
> > do_rpo_vn_1 (function *fn, edge entry, bitmap exit_bbs,
> > - bool iterate, bool eliminate, vn_lookup_kind kind);
> > + bool iterate, bool eliminate, vn_lookup_kind kind, gimple_ranger
> > *);
> >
> > void
> > run_rpo_vn (vn_lookup_kind kind)
> > {
> > - do_rpo_vn_1 (cfun, NULL, NULL, true, false, kind);
> > + do_rpo_vn_1 (cfun, NULL, NULL, true, false, kind, NULL);
> >
> > /* ??? Prune requirement of these. */
> > constant_to_value_id = new hash_table<vn_constant_hasher> (23);
> > @@ -7580,7 +7612,8 @@ insert_related_predicates_on_edge (enum tree_code
> > code, tree *ops, edge pred_e)
> > static unsigned
> > process_bb (rpo_elim &avail, basic_block bb,
> > bool bb_visited, bool iterate_phis, bool iterate, bool eliminate,
> > - bool do_region, bitmap exit_bbs, bool skip_phis)
> > + bool do_region, bitmap exit_bbs, bool skip_phis,
> > + gimple_ranger *ranger)
> > {
> > unsigned todo = 0;
> > edge_iterator ei;
> > @@ -7766,6 +7799,20 @@ process_bb (rpo_elim &avail, basic_block bb,
> > }
> > }
> > }
> > + if (!val && ranger)
> > + {
> > + int_range_max r;
> > + fold_using_range f;
> > + // ??? We get here with not valueized operands but
> > + // the fur_source only is involved for operands of
> > + // this very stmt, not when ranger prefills its cache
> > + // walking use->def edges.
> > + vn_fur src (last, &ranger->gori (), ranger);
> > + tree tem;
> > + if (f.fold_stmt (r, last, src)
> > + && r.singleton_p (&tem))
> > + val = tem;
> > + }
> > if (val)
> > e = find_taken_edge (bb, val);
> > if (! e)
> > @@ -7997,7 +8044,8 @@ do_unwind (unwind_state *to, rpo_elim &avail)
> >
> > static unsigned
> > do_rpo_vn_1 (function *fn, edge entry, bitmap exit_bbs,
> > - bool iterate, bool eliminate, vn_lookup_kind kind)
> > + bool iterate, bool eliminate, vn_lookup_kind kind,
> > + gimple_ranger *ranger)
> > {
> > unsigned todo = 0;
> > default_vn_walk_kind = kind;
> > @@ -8213,7 +8261,8 @@ do_rpo_vn_1 (function *fn, edge entry, bitmap
> > exit_bbs,
> > todo |= process_bb (avail, bb,
> > rpo_state[idx].visited != 0,
> > rpo_state[idx].iterate,
> > - iterate, eliminate, do_region, exit_bbs, false);
> > + iterate, eliminate, do_region, exit_bbs, false,
> > + ranger);
> > rpo_state[idx].visited++;
> >
> > /* Verify if changed values flow over executable outgoing backedges
> > @@ -8326,7 +8375,8 @@ do_rpo_vn_1 (function *fn, edge entry, bitmap
> > exit_bbs,
> > nblk++;
> > todo |= process_bb (avail, bb, false, false, false, eliminate,
> > do_region, exit_bbs,
> > - skip_entry_phis && bb == entry->dest);
> > + skip_entry_phis && bb == entry->dest,
> > + ranger);
> > rpo_state[idx].visited++;
> >
> > FOR_EACH_EDGE (e, ei, bb->succs)
> > @@ -8419,14 +8469,17 @@ do_rpo_vn_1 (function *fn, edge entry, bitmap
> > exit_bbs,
> > If ITERATE is true then treat backedges optimistically as not
> > executed and iterate. If ELIMINATE is true then perform
> > elimination, otherwise leave that to the caller.
> > - KIND specifies the amount of work done for handling memory operations.
> > */
> > + KIND specifies the amount of work done for handling memory operations.
> > + When RANGER is specified use that to simplify conditionals. */
> >
> > unsigned
> > do_rpo_vn (function *fn, edge entry, bitmap exit_bbs,
> > - bool iterate, bool eliminate, vn_lookup_kind kind)
> > + bool iterate, bool eliminate, vn_lookup_kind kind,
> > + gimple_ranger *ranger)
> > {
> > auto_timevar tv (TV_TREE_RPO_VN);
> > - unsigned todo = do_rpo_vn_1 (fn, entry, exit_bbs, iterate, eliminate,
> > kind);
> > + unsigned todo = do_rpo_vn_1 (fn, entry, exit_bbs, iterate, eliminate,
> > kind,
> > + ranger);
> > free_rpo_vn ();
> > return todo;
> > }
> > @@ -8482,7 +8535,9 @@ pass_fre::execute (function *fun)
> > if (iterate_p)
> > loop_optimizer_init (AVOID_CFG_MODIFICATIONS);
> > - todo = do_rpo_vn_1 (fun, NULL, NULL, iterate_p, true, VN_WALKREWRITE);
> > + gimple_ranger ranger;
> > + todo = do_rpo_vn_1 (fun, NULL, NULL, iterate_p, true, VN_WALKREWRITE,
> > + &ranger); //!iterate_p ? &ranger : NULL);
> > free_rpo_vn ();
> >
> > if (iterate_p)
> > diff --git a/gcc/tree-ssa-sccvn.h b/gcc/tree-ssa-sccvn.h
> > index abcf7e666c2..b9c4ac787b4 100644
> > --- a/gcc/tree-ssa-sccvn.h
> > +++ b/gcc/tree-ssa-sccvn.h
> > @@ -298,7 +298,8 @@ tree vn_nary_simplify (vn_nary_op_t);
> > unsigned do_rpo_vn (function *, edge, bitmap,
> > /* iterate */ bool = false,
> > /* eliminate */ bool = true,
> > - vn_lookup_kind = VN_WALKREWRITE);
> > + vn_lookup_kind = VN_WALKREWRITE,
> > + class gimple_ranger * = NULL);
> >
> > /* Private interface for PRE. */
> > void run_rpo_vn (vn_lookup_kind);
>
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)
