Do not put incomplete CONSTRUCTORs into static memory

The CONSTRUCTOR_NO_CLEARING flag was invented to avoid generating a memset
for  CONSTRUCTORS that lack elements, but it turns out that the gimplifier
can generate a memcpy for them instead, which is worse performance-wise,
so this prevents it from doing that for them.

	* gimplify.c (gimplify_init_constructor): Do not put the constructor
	into static memory if it is not complete.

gcc/ChangeLog

@@ -1,3 +1,8 @@
+2020-05-05  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* gimplify.c (gimplify_init_constructor): Do not put the constructor
+	into static memory if it is not complete.
+
 2020-05-05  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/94949

gcc/gimplify.c

@@ -4995,8 +4995,8 @@ gimplify_init_constructor (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
 	   all-zero initializers (which aren't big enough to merit
 	   clearing), and don't try to make bitwise copies of
 	   TREE_ADDRESSABLE types.  */
-
 	if (valid_const_initializer
+	    && complete_p
 	    && !(cleared || num_nonzero_elements == 0)
 	    && !TREE_ADDRESSABLE (type))
 	  {

gcc/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-05-05  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* gnat.dg/aggr29.adb: New test.
+
 2020-05-05  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/94949

gcc/testsuite/gnat.dg/aggr29.adb

@@ -0,0 +1,12 @@
+-- { dg-do compile }
+-- { dg-options "-fdump-tree-gimple" }
+
+with Ada.Streams; use Ada.Streams;
+
+procedure Aggr29 is
+  A : aliased Stream_Element_Array := (1 .. 512 => <>);
+begin
+  null;
+end;
+
+-- { dg-final { scan-tree-dump-not "a___UNC = \\*" "gimple" } }