Re: [PATCH] Fix PR c++/70452 (regression in C++ parsing performance)

[Patrick Palka <patrick@parcs.ath.cx>]

On Fri, 1 Apr 2016, Jason Merrill wrote:

> I like this approach a lot.  One thing, though:
> 
> On 04/01/2016 03:13 PM, Patrick Palka wrote:
> > +struct GTY((chain_next ("%h.prev"))) bpr_entry
> > +{
> > +  tree body;
> > +  tree parms;
> > +  tree res;
> > +  struct bpr_entry *prev;
> > +};
> > +
> >   /* Representation of entries in the constexpr function definition table.
> > */
> > 
> >   struct GTY((for_user)) constexpr_fundef {
> >     tree decl;
> >     tree body;
> > +  /* A chain of unused copies of this function's body awaiting reuse for
> > +     CALL_EXPR evaluation.  */
> > +  struct bpr_entry *bpr_freelist;
> >   };
> 
> The freelist should be discarded on GC.  I think the way to achieve that is to
> put all the freelists into a separate deletable hash table rather than hang
> them off the fundefs.
> 
> Jason
> 
> 

Here's a version that uses a separate deletable table to cache the
function copies.  For simplicity I used a hash_map instead of a
hash_table.  Does this look OK to commit after bootstrap + regtest?

On a related note, I noticed that the constexpr_call_table is not marked
deletable.  Marking it deletable speeds up the test case in the PR by
about 10% and saves about 10MB.  Do you think doing so is a good idea?

On another related note, I noticed that marking something as both
GTY((deletable, cache)) doesn't work as intended, because the
gt_cleare_cache functions run _after_ all deletable roots get
zeroed out.  So during GC the gt_cleare_cache function of a root
marked "deletable, cache" would always be a no-op.  Concretely I think
this means that our cv_cache and fold_cache leak memory during GC
because their underlying hash_map (allocated by operator new) is zeroed
before gc_cleare_cache could clear it.

gcc/cp/ChangeLog:

	PR c++/70452
	* constexpr.c (struct fun_copy): New struct.
	(struct fundef_copies_t): New struct.
	(fundef_copies_table): New static variable.
	(maybe_initialize_fundef_copies_table): New static function.
	(get_fun_copy): New static function.
	(save_fun_copy): New static function.
	(cxx_eval_call_expression): Use
	maybe_initialize_fundef_copies_table, get_fun_copy, and
	save_fun_copy.

gcc/testsuite/ChangeLog:

	PR c++/70452
	* g++.dg/ext/constexpr-vla4.C: New test.
---
 gcc/cp/constexpr.c                        | 96 +++++++++++++++++++++++++++++--
 gcc/testsuite/g++.dg/ext/constexpr-vla4.C | 17 ++++++
 2 files changed, 109 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/ext/constexpr-vla4.C

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index ea605dc..e1a5a4e 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -965,6 +965,76 @@ maybe_initialize_constexpr_call_table (void)
     constexpr_call_table = hash_table<constexpr_call_hasher>::create_ggc (101);
 }
 
+/* The representation of a single node in the per-function freelist maintained
+   by FUNDEF_COPIES_TABLE.  */
+
+struct fun_copy
+{
+  tree body;
+  tree parms;
+  tree res;
+  fun_copy *prev;
+};
+
+/* During constexpr CALL_EXPR evaluation, to avoid issues with sharing when
+   a function happens to get called recursively, we unshare the callee
+   function's body and evaluate this unshared copy instead of evaluating the
+   original body.
+
+   FUNDEF_COPIES_TABLE is a per-function freelist of these unshared function
+   copies.  The underlying data structure of FUNDEF_COPIES_TABLE is a hash_map
+   that's keyed off of the original FUNCTION_DECL and whose value is the chain
+   of this function's unused copies awaiting reuse.  */
+
+struct fundef_copies_table_t
+{
+  hash_map<tree, fun_copy *> *map;
+};
+
+static GTY((deletable)) fundef_copies_table_t fundef_copies_table;
+
+/* Initialize FUNDEF_COPIES_TABLE if it's not initialized.  */
+
+static void
+maybe_initialize_fundef_copies_table ()
+{
+  if (fundef_copies_table.map == NULL)
+    fundef_copies_table.map = hash_map<tree, fun_copy *>::create_ggc (101);
+}
+
+/* Reuse or create a new unshared copy of the function FUN.
+   Return this copy.  */
+
+static fun_copy *
+get_fun_copy (tree fun)
+{
+  fun_copy *copy;
+  fun_copy **slot = &fundef_copies_table.map->get_or_insert (fun, NULL);
+  if (*slot == NULL)
+    {
+      copy = ggc_alloc<fun_copy> ();
+      copy->body = copy_fn (fun, copy->parms, copy->res);
+      copy->prev = NULL;
+    }
+  else
+    {
+      copy = *slot;
+      *slot = (*slot)->prev;
+    }
+
+  return copy;
+}
+
+/* Save the copy COPY of function FUN for later reuse by get_fun_copy().  */
+
+static void
+save_fun_copy (tree fun, fun_copy *copy)
+{
+  fun_copy **slot = &fundef_copies_table.map->get_or_insert (fun, NULL);
+  copy->prev = *slot;
+  *slot = copy;
+}
+
 /* We have an expression tree T that represents a call, either CALL_EXPR
    or AGGR_INIT_EXPR.  If the call is lexically to a named function,
    retrun the _DECL for that function.  */
@@ -1377,10 +1447,14 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
       if (!result || result == error_mark_node)
 	{
 	  gcc_assert (DECL_SAVED_TREE (fun));
-	  tree parms, res;
+	  tree body, parms, res;
 
-	  /* Unshare the whole function body.  */
-	  tree body = copy_fn (fun, parms, res);
+	  /* Reuse or create a new unshared copy of this function's body.  */
+	  maybe_initialize_fundef_copies_table ();
+	  fun_copy *copy = get_fun_copy (fun);
+	  body = copy->body;
+	  parms = copy->parms;
+	  res = copy->res;
 
 	  /* Associate the bindings with the remapped parms.  */
 	  tree bound = new_call.bindings;
@@ -1409,8 +1483,14 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 	  else
 	    ctx->values->put (res, NULL_TREE);
 
+	  /* Track the callee's evaluated SAVE_EXPRs so that we can forget
+	     their values after the call.  */
+	  constexpr_ctx ctx_with_save_exprs = *ctx;
+	  hash_set<tree> save_exprs;
+	  ctx_with_save_exprs.save_exprs = &save_exprs;
+
 	  tree jump_target = NULL_TREE;
-	  cxx_eval_constant_expression (ctx, body,
+	  cxx_eval_constant_expression (&ctx_with_save_exprs, body,
 					lval, non_constant_p, overflow_p,
 					&jump_target);
 
@@ -1435,6 +1515,11 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 		}
 	    }
 
+	  /* Forget the saved values of the callee's SAVE_EXPRs.  */
+	  for (hash_set<tree>::iterator iter = save_exprs.begin();
+	       iter != save_exprs.end(); ++iter)
+	    ctx_with_save_exprs.values->remove (*iter);
+
 	  /* Remove the parms/result from the values map.  Is it worth
 	     bothering to do this when the map itself is only live for
 	     one constexpr evaluation?  If so, maybe also clear out
@@ -1444,6 +1529,9 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 	    ctx->values->remove (slot);
 	  for (tree parm = parms; parm; parm = TREE_CHAIN (parm))
 	    ctx->values->remove (parm);
+
+	  /* Make the unshared function copy we used available for re-use.  */
+	  save_fun_copy (fun, copy);
 	}
 
       if (result == error_mark_node)
diff --git a/gcc/testsuite/g++.dg/ext/constexpr-vla4.C b/gcc/testsuite/g++.dg/ext/constexpr-vla4.C
new file mode 100644
index 0000000..428a8fd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/constexpr-vla4.C
@@ -0,0 +1,17 @@
+// PR c++/70452
+// { dg-do compile { target c++14 } }
+
+constexpr int
+foo (int n, bool p)
+{
+  __extension__ int a [n] = { 0 };
+  if (n == 3)
+    foo (n - 2, false);
+  if (n == 3)
+    foo(n - 1, true);
+  if (p)
+    return a[1];
+  return 0;
+}
+
+constexpr int i2 = foo (3, false); // { dg-bogus "array subscript out of bound" }
-- 
2.8.0.rc3.27.gade0865