c: Support C2x empty initializer braces

ISO C2x standardizes empty initializer braces {}.  Implement this
feature accordingly.  The basic case was already supported and so just
needed diagnostic adjustments.  However, the standard feature also
includes two cases that were not previously supported: empty
initializer braces for scalars, and empty initializer braces for
VLAs.  Thus, add support for those features as well, updating existing
tests that expected them to be diagnosed.

There was already some gimplifier support for converting
variable-sized initializations with empty CONSTRUCTORs to memset.
However, it didn't apply here; code earlier in gimplify_modify_expr
ended up calling gimplify_init_constructor via
gimplify_modify_expr_rhs, which ended up handling the CONSTRUCTOR in a
way that generated an ICE later.  Add a check for this case earlier in
gimplify_modify_expr to avoid that issue.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/
	* gimplify.cc (gimplify_modify_expr): Convert initialization from
	a variable-size CONSTRUCTOR to memset before call to
	gimplify_modify_expr_rhs.

gcc/c/
	* c-decl.cc (start_decl): Do not diagnose initialization of
	variable-sized objects here.
	* c-parser.cc (c_parser_braced_init): Add argument DECL.  All
	callers changed.
	(c_parser_initializer): Diagnose initialization of variable-sized
	objects other than with braced initializer.
	(c_parser_braced_init): Use pedwarn_c11 for empty initializer
	braces and update diagnostic text.  Diagnose initialization of
	variable-sized objects with nonempty braces.
	* c-typeck.cc (digest_init): Update diagnostic for initialization
	of variable-sized objects.
	(really_start_incremental_init, set_designator)
	(process_init_element): Update comments.
	(pop_init_level): Allow scalar empty initializers.

gcc/testsuite/
	* gcc.dg/c11-empty-init-1.c, gcc.dg/c11-empty-init-2.c,
	gcc.dg/c11-empty-init-3.c, gcc.dg/c2x-empty-init-1.c,
	gcc.dg/c2x-empty-init-2.c, gcc.dg/c2x-empty-init-3.c,
	gcc.dg/gnu2x-empty-init-1.c, gcc.dg/gnu2x-empty-init-2.c: New
	tests.
	* gcc.dg/torture/dfp-default-init-1.c: Also test empty
	initializers.
	* gcc.dg/init-bad-1.c, gcc.dg/noncompile/pr71583.c,
	gcc.dg/pr61096-1.c, gcc.dg/vla-init-2.c, gcc.dg/vla-init-3.c,
	gcc.target/i386/sse2-bfloat16-scalar-typecheck.c: Update expected
	diagnostics.
	* gcc.dg/ubsan/c-shift-1.c: Use nonempty initializers for VLA
	initializations expected to be diagnosed.

1 files changed, 17 insertions, 7 deletions

diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
index 759f200..1e8d9dc 100644
--- a/gcc/c/c-parser.cc
+++ b/gcc/c/c-parser.cc
@@ -1523,7 +1523,7 @@ static tree c_parser_simple_asm_expr (c_parser *);
 static tree c_parser_gnu_attributes (c_parser *);
 static struct c_expr c_parser_initializer (c_parser *, tree);
 static struct c_expr c_parser_braced_init (c_parser *, tree, bool,
-					   struct obstack *);
+					   struct obstack *, tree);
 static void c_parser_initelt (c_parser *, struct obstack *);
 static void c_parser_initval (c_parser *, struct c_expr *,
 			      struct obstack *);
@@ -5220,11 +5220,15 @@ static struct c_expr
 c_parser_initializer (c_parser *parser, tree decl)
 {
   if (c_parser_next_token_is (parser, CPP_OPEN_BRACE))
-    return c_parser_braced_init (parser, NULL_TREE, false, NULL);
+    return c_parser_braced_init (parser, NULL_TREE, false, NULL, decl);
   else
     {
       struct c_expr ret;
       location_t loc = c_parser_peek_token (parser)->location;
+      if (decl != error_mark_node && C_DECL_VARIABLE_SIZE (decl))
+	error_at (loc,
+		  "variable-sized object may not be initialized except "
+		  "with an empty initializer");
       ret = c_parser_expr_no_commas (parser, NULL);
       /* This is handled mostly by gimplify.cc, but we have to deal with
 	 not warning about int x = x; as it is a GCC extension to turn off
@@ -5251,11 +5255,12 @@ location_t last_init_list_comma;
    compound literal, and NULL_TREE for other initializers and for
    nested braced lists.  NESTED_P is true for nested braced lists,
    false for the list of a compound literal or the list that is the
-   top-level initializer in a declaration.  */
+   top-level initializer in a declaration.  DECL is the declaration for
+   the top-level initializer for a declaration, otherwise NULL_TREE.  */
 
 static struct c_expr
 c_parser_braced_init (c_parser *parser, tree type, bool nested_p,
-		      struct obstack *outer_obstack)
+		      struct obstack *outer_obstack, tree decl)
 {
   struct c_expr ret;
   struct obstack braced_init_obstack;
@@ -5273,10 +5278,15 @@ c_parser_braced_init (c_parser *parser, tree type, bool nested_p,
     really_start_incremental_init (type);
   if (c_parser_next_token_is (parser, CPP_CLOSE_BRACE))
     {
-      pedwarn (brace_loc, OPT_Wpedantic, "ISO C forbids empty initializer braces");
+      pedwarn_c11 (brace_loc, OPT_Wpedantic,
+		   "ISO C forbids empty initializer braces before C2X");
     }
   else
     {
+      if (decl && decl != error_mark_node && C_DECL_VARIABLE_SIZE (decl))
+	error_at (brace_loc,
+		  "variable-sized object may not be initialized except "
+		  "with an empty initializer");
       /* Parse a non-empty initializer list, possibly with a trailing
 	 comma.  */
       while (true)
@@ -5532,7 +5542,7 @@ c_parser_initval (c_parser *parser, struct c_expr *after,
 
   if (c_parser_next_token_is (parser, CPP_OPEN_BRACE) && !after)
     init = c_parser_braced_init (parser, NULL_TREE, true,
-				 braced_init_obstack);
+				 braced_init_obstack, NULL_TREE);
   else
     {
       init = c_parser_expr_no_commas (parser, after);
@@ -10307,7 +10317,7 @@ c_parser_postfix_expression_after_paren_type (c_parser *parser,
       error_at (type_loc, "compound literal has variable size");
       type = error_mark_node;
     }
-  init = c_parser_braced_init (parser, type, false, NULL);
+  init = c_parser_braced_init (parser, type, false, NULL, NULL_TREE);
   finish_init ();
   maybe_warn_string_init (type_loc, type, init);