c++: Small make_tree_vector_from_ctor improvement

After committing the append_ctor_to_tree_vector patch, I've realized
that for the larger constructors make_tree_vector_from_ctor unnecessarily
wastes one GC vector; make_tree_vector () / release_tree_vector () only
caches GC vectors from 4 to 16 allocated tree elements, so in the likely
case of a rather small ctor using make_tree_vector () can be beneficial,
we can pick something from the cache and if we don't need it later,
pt.cc calls release_tree_vector on it to return it back to the cache.
But for the larger ctors, we just eat one vector from the cache, never
use it (because the vec_safe_reserve will immediately allocate a different
vector) and never return it back to the cache.

So, the following patch passes NULL for the larger vectors, which
append_ctor_to_tree_vector handles just fine now (vec_safe_reserve will
just allocate appropriately sized vector).

2025-01-23  Jakub Jelinek  <jakub@redhat.com>

	* c-common.cc (make_tree_vector_from_ctor): Only use make_tree_vector
	for ctors with <= 16 elements.

diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
index c193eb2c463a27a6bb0ed23bf9877d9ebed0d51b..49508fe9ee60f73dbfd3ab5efc42c2db9b387fbe 100644 (file)
--- a/gcc/c-family/c-common.cc
+++ b/gcc/c-family/c-common.cc
@@ -9044,7 +9044,8 @@ append_ctor_to_tree_vector (vec<tree, va_gc> *v, tree ctor)
 vec<tree, va_gc> *
 make_tree_vector_from_ctor (tree ctor)
 {
-  vec<tree,va_gc> *ret = make_tree_vector ();
+  vec<tree,va_gc> *ret
+    = CONSTRUCTOR_NELTS (ctor) <= 16 ? make_tree_vector () : NULL;
   return append_ctor_to_tree_vector (ret, ctor);
 }