Re: [PATCH] c++: Reuse identical ATOMIC_CONSTRs during normalization

[Patrick Palka <ppalka@redhat.com>]

On Thu, 5 Nov 2020, Jason Merrill wrote:

> On 11/3/20 3:43 PM, Patrick Palka wrote:
> > Profiling revealed that sat_hasher::equal accounts for nearly 40% of
> > compile time in some cmcstl2 tests.
> > 
> > This patch eliminates this bottleneck by caching the ATOMIC_CONSTRs
> > returned by normalize_atom.  This in turn allows us to replace the
> > expensive atomic_constraints_identical_p check in sat_hasher::equal
> > with cheap pointer equality, with no loss in cache hit rate.
> > 
> > With this patch, compile time for the cmcstl2 test
> > test/algorithm/set_symmetric_difference4.cpp drops from 19s to 11s with
> > an --enable-checking=release compiler.
> > 
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk?
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* constraint.cc (struct atom_hasher): New descriptor class for a
> > 	hash_table.  Use it to define ...
> > 	(atom_cache): ... this.
> > 	(normalize_atom): Use it to cache ATOMIC_CONSTRs when not
> > 	generating diagnostics.
> > 	(sat_hasher::hash): Use htab_hash_pointer instead of
> > 	hash_atomic_constraint.
> > 	(sat_hasher::equal): Test for pointer equality instead of
> > 	atomic_constraints_identical_p.
> > ---
> >   gcc/cp/constraint.cc | 37 ++++++++++++++++++++++++++++++++++---
> >   1 file changed, 34 insertions(+), 3 deletions(-)
> > 
> > diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> > index b6f6f0d02a5..ce720c641e8 100644
> > --- a/gcc/cp/constraint.cc
> > +++ b/gcc/cp/constraint.cc
> > @@ -710,6 +710,25 @@ normalize_concept_check (tree check, tree args,
> > norm_info info)
> >     return normalize_expression (def, subst, info);
> >   }
> >   +/* Hash functions for ATOMIC_CONSTRs.  */
> > +
> > +struct atom_hasher : default_hash_traits<tree>
> > +{
> > +  static hashval_t hash (tree atom)
> > +  {
> > +    return hash_atomic_constraint (atom);
> > +  }
> > +
> > +  static bool equal (tree atom1, tree atom2)
> > +  {
> > +    return atomic_constraints_identical_p (atom1, atom2);
> > +  }
> > +};
> 
> This is the same as constraint_hash in logic.cc; either they should be
> combined, or (probably) the hash table in logic.cc should be changed to also
> take advantage of pointer equivalence.

Ah, I forgot about the existence of this hasher.  Consider this hasher
changed to take advantage of the pointer equivalence (I'll post a
revised and tested patch later today).

> 
> > +/* Used by normalize_atom to cache ATOMIC_CONSTRs.  */
> > +
> > +static GTY((deletable)) hash_table<atom_hasher> *atom_cache;
> 
> If we're relying on pointer identity, this can't be deletable; if GC discards
> it, later normalization will generate a new equivalent ATOMIC_CONSTR, breaking
> the uniqueness assumption.

But because no ATOMIC_CONSTR is ever reachable from a GC root (since
they live only inside GC-deletable structures), there will never be two
equivalent ATOMIC_CONSTR trees live at once, which is a sufficient
enough notion of uniqueness for us, I think.

> 
> >   /* The normal form of an atom depends on the expression. The normal
> >      form of a function call to a function concept is a check constraint
> >      for that concept. The normal form of a reference to a variable
> > @@ -729,7 +748,19 @@ normalize_atom (tree t, tree args, norm_info info)
> >     /* Build a new info object for the atom.  */
> >     tree ci = build_tree_list (t, info.context);
> >   -  return build1 (ATOMIC_CONSTR, ci, map);
> > +  tree atom = build1 (ATOMIC_CONSTR, ci, map);
> > +  if (!info.generate_diagnostics ())
> > +    {
> > +      /* Cache the ATOMIC_CONSTRs that we return, so that sat_hasher::equal
> > +	 later can quickly compare two atoms using just pointer equality.  */
> > +      if (!atom_cache)
> > +	atom_cache = hash_table<atom_hasher>::create_ggc (31);
> > +      tree *slot = atom_cache->find_slot (atom, INSERT);
> > +      if (*slot)
> > +	return *slot;
> > +      *slot = atom;
> > +    }
> > +  return atom;
> >   }
> >     /* Returns the normal form of an expression. */
> > @@ -2284,13 +2315,13 @@ struct sat_hasher : ggc_ptr_hash<sat_entry>
> >   {
> >     static hashval_t hash (sat_entry *e)
> >     {
> 
> We could use a comment here about why we can just hash the pointer.

Will do.  The subsequent patch ("Use two levels of caching in
satisfy_atom") also adds

+    /* Atoms with uninstantiated mappings are built in normalize_atom.
+       Their identity is determined by their pointer value due to
+       the caching of ATOMIC_CONSTRs performed therein.  */

to sat_hasher::equal, but it could use repeating in sat_hasher::hash.

> 
> > -    hashval_t value = hash_atomic_constraint (e->constr);
> > +    hashval_t value = htab_hash_pointer (e->constr);
> >       return iterative_hash_template_arg (e->args, value);
> >     }
> >       static bool equal (sat_entry *e1, sat_entry *e2)
> >     {
> > -    if (!atomic_constraints_identical_p (e1->constr, e2->constr))
> > +    if (e1->constr != e2->constr)
> >         return false;
> >       return template_args_equal (e1->args, e2->args);
> >     }
> > 
> 
>