Re: [PATCH] Support threading of just the exit edge

[Richard Biener <rguenther@suse.de>]

On Tue, 16 Aug 2022, Aldy Hernandez wrote:

> On Tue, Aug 16, 2022 at 11:18 AM Richard Biener <rguenther@suse.de> wrote:
> >
> > On Mon, 15 Aug 2022, Aldy Hernandez wrote:
> >
> > > On Mon, Aug 15, 2022 at 9:24 PM Andrew MacLeod <amacleod@redhat.com> wrote:
> > > >
> > > > heh. or just
> > > >
> > > >
> > > > +      int_range<2> r;
> > > > +      if (!fold_range (r, const_cast <gcond *> (cond_stmt))
> > > > +      || !r.singleton_p (&val))
> > > >
> > > >
> > > > if you do not provide a range_query to any of the fold_using_range code,
> > > > it defaults to:
> > > >
> > > > fur_source::fur_source (range_query *q)
> > > > {
> > > >    if (q)
> > > >      m_query = q;
> > > >    else if (cfun)
> > > >      m_query = get_range_query (cfun);
> > > >    else
> > > >      m_query = get_global_range_query ();
> > > >    m_gori = NULL;
> > > > }
> > > >
> > >
> > > Sweet.  Even better!
> >
> > So when I do the following incremental change ontop of the posted
> > patch then I see that the path query is able to simplify more
> > "single BB paths" than the global range folding.
> >
> > diff --git a/gcc/tree-ssa-threadbackward.cc
> > b/gcc/tree-ssa-threadbackward.cc
> > index 669098e4ec3..777e778037f 100644
> > --- a/gcc/tree-ssa-threadbackward.cc
> > +++ b/gcc/tree-ssa-threadbackward.cc
> > @@ -314,6 +314,12 @@ back_threader::find_taken_edge_cond (const
> > vec<basic_block> &path,
> >  {
> >    int_range_max r;
> >
> > +  int_range<2> rf;
> > +  if (path.length () == 1)
> > +    {
> > +      fold_range (rf, cond);
> > +    }
> > +
> >    m_solver->compute_ranges (path, m_imports);
> >    m_solver->range_of_stmt (r, cond);
> >
> > @@ -325,6 +331,8 @@ back_threader::find_taken_edge_cond (const
> > vec<basic_block> &path,
> >
> >    if (r == true_range || r == false_range)
> >      {
> > +      if (path.length () == 1)
> > +       gcc_assert  (r == rf);
> >        edge e_true, e_false;
> >        basic_block bb = gimple_bb (cond);
> >        extract_true_false_edges_from_block (bb, &e_true, &e_false);
> >
> > Even doing the following (not sure what's the difference and in
> > particular expense over the path range query) results in missed
> > simplifications (checking my set of cc1files).
> >
> > diff --git a/gcc/tree-ssa-threadbackward.cc
> > b/gcc/tree-ssa-threadbackward.cc
> > index 669098e4ec3..1d43a179d08 100644
> > --- a/gcc/tree-ssa-threadbackward.cc
> > +++ b/gcc/tree-ssa-threadbackward.cc
> > @@ -99,6 +99,7 @@ private:
> >
> >    back_threader_registry m_registry;
> >    back_threader_profitability m_profit;
> > +  gimple_ranger *m_ranger;
> >    path_range_query *m_solver;
> >
> >    // Current path being analyzed.
> > @@ -146,12 +147,14 @@ back_threader::back_threader (function *fun,
> > unsigned flags, bool first)
> >    // The path solver needs EDGE_DFS_BACK in resolving mode.
> >    if (flags & BT_RESOLVE)
> >      mark_dfs_back_edges ();
> > -  m_solver = new path_range_query (flags & BT_RESOLVE);
> > +  m_ranger = new gimple_ranger;
> > +  m_solver = new path_range_query (flags & BT_RESOLVE, m_ranger);
> >  }
> 
> Passing an allocated ranger here results in less simplifications over
> letting path_range_query allocate its own?  That's not right.  Or do
> you mean that using fold_range() with the m_ranger causes ICEs with
> your patch (due to the non-null processing described below)?

Yes, I've needed a ranger to use fold_range (..., m_ranger) which
I thought might do more than not passing one.

> >
> >  back_threader::~back_threader ()
> >  {
> >    delete m_solver;
> > +  delete m_ranger;
> >
> >    loop_optimizer_finalize ();
> >  }
> > @@ -314,6 +317,12 @@ back_threader::find_taken_edge_cond (const
> > vec<basic_block> &path,
> >  {
> >    int_range_max r;
> >
> > +  int_range<2> rf;
> > +  if (path.length () == 1)
> > +    {
> > +      fold_range (rf, cond, m_ranger);
> > +    }
> > +
> >    m_solver->compute_ranges (path, m_imports);
> >    m_solver->range_of_stmt (r, cond);
> >
> > @@ -325,6 +334,8 @@ back_threader::find_taken_edge_cond (const
> > vec<basic_block> &path,
> >
> >    if (r == true_range || r == false_range)
> >      {
> > +      if (path.length () == 1)
> > +       gcc_assert  (r == rf);
> >        edge e_true, e_false;
> >        basic_block bb = gimple_bb (cond);
> >        extract_true_false_edges_from_block (bb, &e_true, &e_false);
> >
> > one example is
> >
> > <bb 176> [local count: 14414059]:
> > _128 = node_177(D)->typed.type;
> > pretmp_413 = MEM[(const union tree_node *)_128].base.code;
> > _431 = pretmp_413 + 65519;
> > if (_128 == 0B)
> >   goto <bb 199>; [18.09%]
> > else
> >   goto <bb 177>; [81.91%]
> >
> > where m_imports for the path is just _128 and the range computed is
> > false while the ranger query returns VARYING.  But
> > path_range_query::range_defined_in_block does
> >
> >   if (bb && POINTER_TYPE_P (TREE_TYPE (name)))
> >     m_ranger->m_cache.m_exit.maybe_adjust_range (r, name, bb);
> >
> > which adjusts the range to ~[0, 0], probably because of the
> > dereference in the following stmt.
> >
> > Why does fold_range not do this when folding the exit test?  Is there
> > a way to make it do so?  It looks like the only routine using this
> > in gimple-range.cc is range_on_edge and there it's used for e->src
> > after calling range_on_exit for e->src (why's it not done in
> > range_on_exit?).  A testcase for this is
> 
> Andrew's gonna have to answer this one, because I'm just a user of the
> infer_range infrastructure.  But yes, you're right... fold_range
> doesn't seem to take into account side-effects such as non-null.

OK, let's see.  I can probably use the path query as well - I'll
see to produce a prototype of doing those simplifications early,
avoiding threadings there and through unreachable parts of the CFG.

Richard.

> Aldy
> 
> >
> > int foo (int **p, int i)
> > {
> >   int *q = *p;
> >   int res = *q + i;
> >   if (q)
> >     return res;
> >   return -1;
> > }
> >
> > which we "thread" with the path and with the above ICEs because
> > fold_range doesn't get that if (q) is always true.  Without the
> > patch ethread doesn't want to duplicate the block (it's too large)
> > but threadfull will if you disable evrp (if you remove the increment
> > by 'i' it again won't since nothing interesting prevails and it
> > won't go to BB 0 and fails to pick up a thread of length > 1):
> >
> > Checking profitability of path (backwards):  bb:2 (6 insns) bb:0
> >   Control statement insns: 2
> >   Overall: 4 insns
> >   [1] Registering jump thread: (0, 2) incoming edge;  (2, 3) nocopy;
> > path: 0->2->3 SUCCESS
> > Removing basic block 2
> > ;; basic block 2, loop depth 0
> > ;;  pred:
> > _1 = *p_6(D);
> > _2 = (long unsigned int) n_7(D);
> > _3 = _2 * 4;
> > q_8 = _1 + _3;
> > res_9 = *q_8;
> > if (q_8 != 0B)
> >   goto <bb 3>; [98.33%]
> > else
> >   goto <bb 4>; [1.67%]
> > ;;  succ:       3
> > ;;              4
> >
> > ... (it copied BB 2) ...
> >
> > int foo (int * * p, int n)
> > {
> >   int res;
> >   int * q;
> >   int * _10;
> >   long unsigned int _11;
> >   long unsigned int _12;
> >
> >   <bb 2> [local count: 1073741824]:
> >   _10 = *p_6(D);
> >   _11 = (long unsigned int) n_7(D);
> >   _12 = _11 * 4;
> >   q_13 = _10 + _12;
> >   res_14 = *q_13;
> >   return res_14;
> >
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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