[PATCH] Reuse the saved_scope structures allocated by push_to_top_level

[Patrick Palka]

push_to_top_level gets called fairly frequently in template-heavy code
that performs a lot of instantiations, and we currently "leak" a lot of
GC memory when compiling such code since [push|pop]_to_top_level() do
not bother reusing or even freeing each saved_scope structure it
allocates.

This patch makes push_to_top_level() reuse the saved_scope structures it
allocates.  This is similar to how begin_scope() reuses the
cp_binding_level structures it allocates.

This patch reduces the maximum memory usage of the compiler by 4.5%,
from 525MB to 500MB, when compiling the Boost::Fusion test file
libs/fusion/test/compile_time/transform.cpp from the Boost 1.60 testsuite.

Bootstrapped and tested on x86_64-pc-linux-gnu, OK for
trunk or for GCC 7?

gcc/cp/ChangeLog:

        * name-lookup.c (free_saved_scope): New free list of saved_scope
        structures.
        (push_to_top_level): Attempt to reuse a saved_scope struct
        from free_saved_scope instead of allocating a new one each time.
        (pop_from_top_level_1): Chain the now-unused saved_scope structure
        onto free_saved_scope.
---
 gcc/cp/name-lookup.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/name-lookup.c b/gcc/cp/name-lookup.c
index 89d84d7..3478b6a 100644
--- a/gcc/cp/name-lookup.c
+++ b/gcc/cp/name-lookup.c
@@ -6134,6 +6134,10 @@ store_class_bindings (vec<cp_class_binding, va_gc> 
*names,
   timevar_cond_stop (TV_NAME_LOOKUP, subtime);
 }
 
+/* A chain of saved_scope structures awaiting reuse.  */
+
+static GTY((deletable)) struct saved_scope *free_saved_scope;
+
 void
 push_to_top_level (void)
 {
@@ -6144,7 +6148,21 @@ push_to_top_level (void)
   bool need_pop;
 
   bool subtime = timevar_cond_start (TV_NAME_LOOKUP);
-  s = ggc_cleared_alloc<saved_scope> ();
+
+  /* Reuse or create a new structure for this saved scope.  */
+  if (free_saved_scope != NULL)
+    {
+      s = free_saved_scope;
+      free_saved_scope = s->prev;
+
+      vec<cxx_saved_binding, va_gc> *old_bindings = s->old_bindings;
+      memset (s, 0, sizeof (*s));
+      /* Also reuse the structure's old_bindings vector.  */
+      vec_safe_truncate (old_bindings, 0);
+      s->old_bindings = old_bindings;
+    }
+  else
+    s = ggc_cleared_alloc<saved_scope> ();
 
   b = scope_chain ? current_binding_level : 0;
 
@@ -6237,6 +6255,11 @@ pop_from_top_level_1 (void)
   current_function_decl = s->function_decl;
   cp_unevaluated_operand = s->unevaluated_operand;
   c_inhibit_evaluation_warnings = s->inhibit_evaluation_warnings;
+
+  /* Make this saved_scope structure available for reuse by
+     push_to_top_level.  */
+  s->prev = free_saved_scope;
+  free_saved_scope = s;
 }
 
 /* Wrapper for pop_from_top_level_1.  */
-- 
2.8.0.rc0.11.g9bfbc33