Allow _Alignas in compound literals (C11 DR#444)

Message ID alpine.DEB.2.20.1712071847190.13157@digraph.polyomino.org.uk
State New
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  • Allow _Alignas in compound literals (C11 DR#444)
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Commit Message

Joseph Myers Dec. 7, 2017, 6:47 p.m.
C11 DR#444 dealt with how C11 intended to allow alignment specifiers
on struct and union members, but failed to include that in the syntax.
The final resolution of that DR also allows alignment specifiers in
type names in compound literals (in order to apply an increased
alignment to the unnamed object created by the compound literal), but
not other cases of type names.

This patch implements allowing alignment specifiers in compound
literals and adds tests for the resolution of the DR (including that
they are allowed on struct and union members, which GCC already
implemented).  Because the parser has to parse the parenthesized type
name of a compound literal before it can tell that it's a compound
literal (rather than, depending on the context, a cast expression or
sizeof (type-name) or _Alignof (type-name)), this means _Alignas
specifiers are allowed syntactically in those contexts and then an
error is given once it's known to be an invalid use (whereas _Alignas
specifiers are disallowed syntactically in other contexts where type
names can occur and a compound literal is not possible).

Bootstrapped with no regressions on x86_64-pc-linux-gnu.  Applied to 
mainline.

gcc/c:
2017-12-07  Joseph Myers  <joseph@codesourcery.com>

	* c-decl.c (build_compound_literal): Add parameter alignas_align
	and set alignment of decl if nonzero.
	* c-parser.c (c_keyword_starts_typename): Allow RID_ALIGNAS.
	(c_parser_declspecs): Allow RID_ALIGNAS to follow a type, like a
	qualifier.
	(c_parser_struct_declaration): Update syntax comment.
	(c_parser_type_name): Add alignas_ok argument and pass it to
	c_parser_declspecs.
	(c_parser_cast_expression): Pass true to c_parser_type_name and
	give error if a cast used an _Alignas specifier.
	(c_parser_sizeof_expression): Pass true to c_parser_type_name and
	give error if sizeof (type-name) used an _Alignas specifier.
	(c_parser_alignof_expression): Pass true to c_parser_type_name and
	give error if _Alignof (type-name) used an _Alignas specifier.
	(c_parser_postfix_expression_after_paren_type): Check specified
	alignment for a compound literal and pass it to
	build_compound_literal.
	* c-parser.h (c_parser_type_name): Update prototype.
	* c-tree.h (build_compound_literal): Update prototype.

gcc/testsuite:
2017-12-07  Joseph Myers  <joseph@codesourcery.com>

	* gcc.dg/c11-align-7.c, gcc.dg/c11-align-8.c,
	gcc.dg/c11-align-9.c, gcc.dg/gnu11-align-1.c: New tests.
	* gcc.dg/c11-align-5.c (test): Update expected error for sizeof
	case.


-- 
Joseph S. Myers
joseph@codesourcery.com

Patch

Index: gcc/c/c-decl.c
===================================================================
--- gcc/c/c-decl.c	(revision 255464)
+++ gcc/c/c-decl.c	(working copy)
@@ -5263,10 +5263,13 @@  mark_forward_parm_decls (void)
    literal, which may be an incomplete array type completed by the
    initializer; INIT is a CONSTRUCTOR at LOC that initializes the compound
    literal.  NON_CONST is true if the initializers contain something
-   that cannot occur in a constant expression.  */
+   that cannot occur in a constant expression.  If ALIGNAS_ALIGN is nonzero,
+   it is the (valid) alignment for this compound literal, as specified
+   with _Alignas.  */
 
 tree
-build_compound_literal (location_t loc, tree type, tree init, bool non_const)
+build_compound_literal (location_t loc, tree type, tree init, bool non_const,
+			unsigned int alignas_align)
 {
   /* We do not use start_decl here because we have a type, not a declarator;
      and do not use finish_decl because the decl should be stored inside
@@ -5290,6 +5293,11 @@  tree
   DECL_IGNORED_P (decl) = 1;
   TREE_TYPE (decl) = type;
   c_apply_type_quals_to_decl (TYPE_QUALS (strip_array_types (type)), decl);
+  if (alignas_align)
+    {
+      SET_DECL_ALIGN (decl, alignas_align * BITS_PER_UNIT);
+      DECL_USER_ALIGN (decl) = 1;
+    }
   store_init_value (loc, decl, init, NULL_TREE);
 
   if (TREE_CODE (type) == ARRAY_TYPE && !COMPLETE_TYPE_P (type))
Index: gcc/c/c-parser.c
===================================================================
--- gcc/c/c-parser.c	(revision 255464)
+++ gcc/c/c-parser.c	(working copy)
@@ -504,6 +504,7 @@  c_keyword_starts_typename (enum rid keyword)
     case RID_ACCUM:
     case RID_SAT:
     case RID_AUTO_TYPE:
+    case RID_ALIGNAS:
       return true;
     default:
       if (keyword >= RID_FIRST_INT_N
@@ -2594,7 +2595,8 @@  c_parser_declspecs (c_parser *parser, struct c_dec
 	 has simply forgotten a semicolon, so we exit.  */
       if ((!typespec_ok || specs->typespec_kind == ctsk_tagdef)
 	  && c_parser_next_tokens_start_typename (parser, la)
-	  && !c_parser_next_token_is_qualifier (parser))
+	  && !c_parser_next_token_is_qualifier (parser)
+	  && !c_parser_next_token_is_keyword (parser, RID_ALIGNAS))
 	break;
 
       if (c_parser_next_token_is (parser, CPP_NAME))
@@ -3225,6 +3227,7 @@  c_parser_struct_or_union_specifier (c_parser *pars
    specifier-qualifier-list:
      type-specifier specifier-qualifier-list[opt]
      type-qualifier specifier-qualifier-list[opt]
+     alignment-specifier specifier-qualifier-list[opt]
      attributes specifier-qualifier-list[opt]
 
    struct-declarator-list:
@@ -4410,7 +4413,9 @@  c_parser_attributes (c_parser *parser)
   return attrs;
 }
 
-/* Parse a type name (C90 6.5.5, C99 6.7.6, C11 6.7.7).
+/* Parse a type name (C90 6.5.5, C99 6.7.6, C11 6.7.7).  ALIGNAS_OK
+   says whether alignment specifiers are OK (only in cases that might
+   be the type name of a compound literal).
 
    type-name:
      specifier-qualifier-list abstract-declarator[opt]
@@ -4417,13 +4422,13 @@  c_parser_attributes (c_parser *parser)
 */
 
 struct c_type_name *
-c_parser_type_name (c_parser *parser)
+c_parser_type_name (c_parser *parser, bool alignas_ok)
 {
   struct c_declspecs *specs = build_null_declspecs ();
   struct c_declarator *declarator;
   struct c_type_name *ret;
   bool dummy = false;
-  c_parser_declspecs (parser, specs, false, true, true, false, false,
+  c_parser_declspecs (parser, specs, false, true, true, alignas_ok, false,
 		      cla_prefer_type);
   if (!specs->declspecs_seen_p)
     {
@@ -7019,7 +7024,7 @@  c_parser_cast_expression (c_parser *parser, struct
       struct c_expr expr;
       matching_parens parens;
       parens.consume_open (parser);
-      type_name = c_parser_type_name (parser);
+      type_name = c_parser_type_name (parser, true);
       parens.skip_until_found_close (parser);
       if (type_name == NULL)
 	{
@@ -7035,6 +7040,9 @@  c_parser_cast_expression (c_parser *parser, struct
       if (c_parser_next_token_is (parser, CPP_OPEN_BRACE))
 	return c_parser_postfix_expression_after_paren_type (parser, type_name,
 							     cast_loc);
+      if (type_name->specs->alignas_p)
+	error_at (type_name->specs->locations[cdw_alignas],
+		  "alignment specified for type name in cast");
       {
 	location_t expr_loc = c_parser_peek_token (parser)->location;
 	expr = c_parser_cast_expression (parser, NULL);
@@ -7238,7 +7246,7 @@  c_parser_sizeof_expression (c_parser *parser)
       matching_parens parens;
       parens.consume_open (parser);
       expr_loc = c_parser_peek_token (parser)->location;
-      type_name = c_parser_type_name (parser);
+      type_name = c_parser_type_name (parser, true);
       parens.skip_until_found_close (parser);
       finish = parser->tokens_buf[0].location;
       if (type_name == NULL)
@@ -7260,6 +7268,9 @@  c_parser_sizeof_expression (c_parser *parser)
 	  goto sizeof_expr;
 	}
       /* sizeof ( type-name ).  */
+      if (type_name->specs->alignas_p)
+	error_at (type_name->specs->locations[cdw_alignas],
+		  "alignment specified for type name in %<sizeof%>");
       c_inhibit_evaluation_warnings--;
       in_sizeof--;
       result = c_expr_sizeof_type (expr_loc, type_name);
@@ -7321,7 +7332,7 @@  c_parser_alignof_expression (c_parser *parser)
       matching_parens parens;
       parens.consume_open (parser);
       loc = c_parser_peek_token (parser)->location;
-      type_name = c_parser_type_name (parser);
+      type_name = c_parser_type_name (parser, true);
       end_loc = c_parser_peek_token (parser)->location;
       parens.skip_until_found_close (parser);
       if (type_name == NULL)
@@ -7342,6 +7353,10 @@  c_parser_alignof_expression (c_parser *parser)
 	  goto alignof_expr;
 	}
       /* alignof ( type-name ).  */
+      if (type_name->specs->alignas_p)
+	error_at (type_name->specs->locations[cdw_alignas],
+		  "alignment specified for type name in %qE",
+		  alignof_spelling);
       c_inhibit_evaluation_warnings--;
       in_alignof--;
       ret.value = c_sizeof_or_alignof_type (loc, groktypename (type_name,
@@ -8969,7 +8984,21 @@  c_parser_postfix_expression_after_paren_type (c_pa
 	       ? CONSTRUCTOR_NON_CONST (init.value)
 	       : init.original_code == C_MAYBE_CONST_EXPR);
   non_const |= !type_expr_const;
-  expr.value = build_compound_literal (start_loc, type, init.value, non_const);
+  unsigned int alignas_align = 0;
+  if (type != error_mark_node
+      && type_name->specs->align_log != -1)
+    {
+      alignas_align = 1U << type_name->specs->align_log;
+      if (alignas_align < min_align_of_type (type))
+	{
+	  error_at (type_name->specs->locations[cdw_alignas],
+		    "%<_Alignas%> specifiers cannot reduce "
+		    "alignment of compound literal");
+	  alignas_align = 0;
+	}
+    }
+  expr.value = build_compound_literal (start_loc, type, init.value, non_const,
+				       alignas_align);
   set_c_expr_source_range (&expr, init.src_range);
   expr.original_code = ERROR_MARK;
   expr.original_type = NULL;
Index: gcc/c/c-parser.h
===================================================================
--- gcc/c/c-parser.h	(revision 255464)
+++ gcc/c/c-parser.h	(working copy)
@@ -187,6 +187,6 @@  c_parser_declarator (c_parser *parser, bool type_s
 		     bool *seen_id);
 extern void c_parser_declspecs (c_parser *, struct c_declspecs *, bool, bool,
 				bool, bool, bool, enum c_lookahead_kind);
-extern struct c_type_name *c_parser_type_name (c_parser *);
+extern struct c_type_name *c_parser_type_name (c_parser *, bool = false);
 
 #endif
Index: gcc/c/c-tree.h
===================================================================
--- gcc/c/c-tree.h	(revision 255464)
+++ gcc/c/c-tree.h	(working copy)
@@ -672,7 +672,8 @@  extern void set_init_index (location_t, tree, tree
 extern void set_init_label (location_t, tree, location_t, struct obstack *);
 extern void process_init_element (location_t, struct c_expr, bool,
 				  struct obstack *);
-extern tree build_compound_literal (location_t, tree, tree, bool);
+extern tree build_compound_literal (location_t, tree, tree, bool,
+				    unsigned int);
 extern void check_compound_literal_type (location_t, struct c_type_name *);
 extern tree c_start_case (location_t, location_t, tree, bool);
 extern void c_finish_case (tree, tree);
Index: gcc/testsuite/gcc.dg/c11-align-5.c
===================================================================
--- gcc/testsuite/gcc.dg/c11-align-5.c	(revision 255464)
+++ gcc/testsuite/gcc.dg/c11-align-5.c	(working copy)
@@ -19,7 +19,7 @@  test (void)
   struct s { int n; };
   __builtin_offsetof (struct s _Alignas (int), n); /* { dg-error "expected" } */
   __typeof (long double _Alignas (0)) e; /* { dg-error "expected" } */
-  sizeof (int _Alignas (int)); /* { dg-error "expected" } */
+  sizeof (int _Alignas (int)); /* { dg-error "specified for type name" } */
   _Alignas (int _Alignas (float)) int t; /* { dg-error "expected" } */
   __builtin_types_compatible_p (signed _Alignas (0), unsigned); /* { dg-error "expected" } */
   int a[_Alignas (int) 10]; /* { dg-error "expected expression before" } */
Index: gcc/testsuite/gcc.dg/c11-align-7.c
===================================================================
--- gcc/testsuite/gcc.dg/c11-align-7.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/c11-align-7.c	(working copy)
@@ -0,0 +1,20 @@ 
+/* Test C11 alignment support.  Test code valid after the resolution
+   of DR#444: alignment specifiers for struct and union members and
+   compound literals.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+#include <stddef.h>
+
+struct s
+{
+  _Alignas (_Alignof (max_align_t)) char c;
+};
+
+union u
+{
+  _Alignas (_Alignof (max_align_t)) char c;
+};
+
+char *p = &(_Alignas (_Alignof (max_align_t)) char) { 1 };
+size_t size = sizeof (_Alignas (_Alignof (max_align_t)) char) { 1 };
Index: gcc/testsuite/gcc.dg/c11-align-8.c
===================================================================
--- gcc/testsuite/gcc.dg/c11-align-8.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/c11-align-8.c	(working copy)
@@ -0,0 +1,18 @@ 
+/* Test C11 alignment support.  Test invalid use of alignment
+   specifiers in type names in cases not permitted by the resolution
+   of DR#444.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+#include <stddef.h>
+
+void
+f (void)
+{
+  _Generic (1, int: 1, _Alignas (8) long: 2); /* { dg-error "expected" } */
+  sizeof (_Alignas (8) long); /* { dg-error "specified for type name" } */
+  _Alignof (_Alignas (8) long); /* { dg-error "specified for type name" } */
+  (_Alignas (8) long) 0; /* { dg-error "specified for type name" } */
+  _Atomic (_Alignas (8) long) x; /* { dg-error "expected" } */
+  _Alignas (_Alignas (8) long) long y; /* { dg-error "expected" } */
+}
Index: gcc/testsuite/gcc.dg/c11-align-9.c
===================================================================
--- gcc/testsuite/gcc.dg/c11-align-9.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/c11-align-9.c	(working copy)
@@ -0,0 +1,9 @@ 
+/* Test C11 alignment support.  Test reducing alignment (assumes there
+   are at least some alignment constraints), case of compound literals.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+/* { dg-skip-if "no alignment constraints" { "avr-*-*" } } */
+
+#include <stddef.h>
+
+max_align_t *p = &(_Alignas (_Alignof (char)) max_align_t) { 1 }; /* { dg-error "reduce alignment" } */
Index: gcc/testsuite/gcc.dg/gnu11-align-1.c
===================================================================
--- gcc/testsuite/gcc.dg/gnu11-align-1.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/gnu11-align-1.c	(working copy)
@@ -0,0 +1,8 @@ 
+/* Test C11 alignment support.  Test code valid after the resolution
+   of DR#444: alignment specifiers for compound literals in _Alignof.  */
+/* { dg-do compile } */
+/* { dg-options "-std=gnu11" } */
+
+#include <stddef.h>
+
+size_t align = _Alignof (_Alignas (_Alignof (max_align_t)) char) { 1 };