Re: RFC: C++ PATCH for c++/69763, making C++ alignof match C _Alignof

[Jason Merrill <jason at redhat dot com>]

On Tue, Apr 17, 2018 at 8:50 AM, Joseph Myers <joseph@codesourcery.com> wrote:
> On Tue, 10 Apr 2018, Jason Merrill wrote:
>
>> But really this is beside the point: the x86 ABI says that the
>> alignment of double is 4, so alignof(double) should be 4 regardless of
>> what GCC wants to do internally.  And I think the same is true of
>> __alignof__.
>
> __alignof__ needs to stay reflecting the preferred, standalone alignment
> of 8 bytes; changing that is ABI-incompatible; code such as that in
> stddef.h uses __attribute__((__aligned__(__alignof__(long long)))) to give
> structure members the same alignment those types would have for standalone
> objects.  (This doesn't affect the alignment of max_align_t *now* on i386,
> but only because that now includes __float128 as well.)

In that case, here's a patch following the 52023 change to the C front
end, to change only "alignof".

Tested x86_64-pc-linux-gnu, applying to trunk.

commit c42f365685b86fa7cecc2666adf93037f7bd0289
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Apr 10 11:48:51 2018 -0400

            PR c++/69560 - wrong alignof(double) on x86.
    
            CWG 1879 - Inadequate definition of alignment requirement.
            * cp-tree.h (ALIGNOF_EXPR_STD_P): New.
            * typeck.c (cxx_sizeof_or_alignof_type): Add std_alignof parm.
            (cxx_sizeof_expr, cxx_sizeof_nowarn, cxx_alignas_expr)
            (cxx_alignof_expr): Pass it.
            * parser.c (cp_parser_unary_expression): Pass it.
            * pt.c (tsubst_copy): Copy it.
            (tsubst_copy_and_build): Pass it.
            * decl.c (fold_sizeof_expr): Pass it.

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 5af4928e2fb..37770770acb 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -372,6 +372,7 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
       TEMPLATE_TYPE_PARM_FOR_CLASS (TEMPLATE_TYPE_PARM)
       DECL_NAMESPACE_INLINE_P (in NAMESPACE_DECL)
       SWITCH_STMT_ALL_CASES_P (in SWITCH_STMT)
+      ALIGNOF_EXPR_STD_P (in ALIGNOF_EXPR)
    1: IDENTIFIER_KIND_BIT_1 (in IDENTIFIER_NODE)
       TI_PENDING_TEMPLATE_FLAG.
       TEMPLATE_PARMS_FOR_INLINE.
@@ -4954,6 +4955,10 @@ more_aggr_init_expr_args_p (const aggr_init_expr_arg_iterator *iter)
 #define SIZEOF_EXPR_TYPE_P(NODE) \
   TREE_LANG_FLAG_0 (SIZEOF_EXPR_CHECK (NODE))
 
+/* True if the ALIGNOF_EXPR was spelled "alignof".  */
+#define ALIGNOF_EXPR_STD_P(NODE) \
+  TREE_LANG_FLAG_0 (ALIGNOF_EXPR_CHECK (NODE))
+
 /* An enumeration of the kind of tags that C++ accepts.  */
 enum tag_types {
   none_type = 0, /* Not a tag type.  */
@@ -7195,7 +7200,7 @@ extern int comp_cv_qualification		(const_tree, const_tree);
 extern int comp_cv_qualification		(int, int);
 extern int comp_cv_qual_signature		(tree, tree);
 extern tree cxx_sizeof_or_alignof_expr		(tree, enum tree_code, bool);
-extern tree cxx_sizeof_or_alignof_type		(tree, enum tree_code, bool);
+extern tree cxx_sizeof_or_alignof_type		(tree, enum tree_code, bool, bool);
 extern tree cxx_alignas_expr                    (tree);
 extern tree cxx_sizeof_nowarn                   (tree);
 extern tree is_bitfield_expr_with_lowered_type  (const_tree);
@@ -7292,7 +7297,7 @@ extern tree cp_build_binary_op                  (location_t,
 extern tree build_x_vec_perm_expr               (location_t,
 						 tree, tree, tree,
 						 tsubst_flags_t);
-#define cxx_sizeof(T)  cxx_sizeof_or_alignof_type (T, SIZEOF_EXPR, true)
+#define cxx_sizeof(T)  cxx_sizeof_or_alignof_type (T, SIZEOF_EXPR, false, true)
 extern tree build_simple_component_ref		(tree, tree);
 extern tree build_ptrmemfunc_access_expr	(tree, tree);
 extern tree build_address			(tree);
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index d822745b81f..55e234334ac 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -9542,10 +9542,10 @@ fold_sizeof_expr (tree t)
   tree r;
   if (SIZEOF_EXPR_TYPE_P (t))
     r = cxx_sizeof_or_alignof_type (TREE_TYPE (TREE_OPERAND (t, 0)),
-				    SIZEOF_EXPR, false);
+				    SIZEOF_EXPR, false, false);
   else if (TYPE_P (TREE_OPERAND (t, 0)))
     r = cxx_sizeof_or_alignof_type (TREE_OPERAND (t, 0), SIZEOF_EXPR,
-				    false);
+				    false, false);
   else
     r = cxx_sizeof_or_alignof_expr (TREE_OPERAND (t, 0), SIZEOF_EXPR,
 				    false);
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index bf46165f5ae..d8ce28a6d61 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -7993,19 +7993,22 @@ cp_parser_unary_expression (cp_parser *parser, cp_id_kind * pidk,
 	    location_t start_loc = token->location;
 
 	    op = keyword == RID_ALIGNOF ? ALIGNOF_EXPR : SIZEOF_EXPR;
+	    bool std_alignof = id_equal (token->u.value, "alignof");
+
 	    /* Consume the token.  */
 	    cp_lexer_consume_token (parser->lexer);
 	    /* Parse the operand.  */
 	    operand = cp_parser_sizeof_operand (parser, keyword);
 
 	    if (TYPE_P (operand))
-	      ret = cxx_sizeof_or_alignof_type (operand, op, true);
+	      ret = cxx_sizeof_or_alignof_type (operand, op, std_alignof,
+						true);
 	    else
 	      {
 		/* ISO C++ defines alignof only with types, not with
 		   expressions. So pedwarn if alignof is used with a non-
 		   type expression. However, __alignof__ is ok.  */
-		if (id_equal (token->u.value, "alignof"))
+		if (std_alignof)
 		  pedwarn (token->location, OPT_Wpedantic,
 			   "ISO C++ does not allow %<alignof%> "
 			   "with a non-type");
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index e2a12b963ad..9fb819722ec 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -15598,7 +15598,8 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 		expanded = make_argument_pack (expanded);
 
 	      if (TYPE_P (expanded))
-		return cxx_sizeof_or_alignof_type (expanded, SIZEOF_EXPR, 
+		return cxx_sizeof_or_alignof_type (expanded, SIZEOF_EXPR,
+						   false,
 						   complain & tf_error);
 	      else
 		return cxx_sizeof_or_alignof_expr (expanded, SIZEOF_EXPR,
@@ -15636,7 +15637,10 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
       {
 	tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
 	tree op0 = tsubst_copy (TREE_OPERAND (t, 0), args, complain, in_decl);
-	return build1 (code, type, op0);
+	r = build1 (code, type, op0);
+	if (code == ALIGNOF_EXPR)
+	  ALIGNOF_EXPR_STD_P (r) = ALIGNOF_EXPR_STD_P (t);
+	return r;
       }
 
     case COMPONENT_REF:
@@ -18002,6 +18006,8 @@ tsubst_copy_and_build (tree t,
 	op1 = TREE_OPERAND (t, 0);
 	if (TREE_CODE (t) == SIZEOF_EXPR && SIZEOF_EXPR_TYPE_P (t))
 	  op1 = TREE_TYPE (op1);
+	bool std_alignof = (TREE_CODE (t) == ALIGNOF_EXPR
+			    && ALIGNOF_EXPR_STD_P (t));
         if (!args)
 	  {
 	    /* When there are no ARGS, we are trying to evaluate a
@@ -18025,7 +18031,7 @@ tsubst_copy_and_build (tree t,
 	    --c_inhibit_evaluation_warnings;
 	  }
         if (TYPE_P (op1))
-	  r = cxx_sizeof_or_alignof_type (op1, TREE_CODE (t),
+	  r = cxx_sizeof_or_alignof_type (op1, TREE_CODE (t), std_alignof,
 					  complain & tf_error);
 	else
 	  r = cxx_sizeof_or_alignof_expr (op1, TREE_CODE (t),
diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index f5081c1661c..01baf6c17a3 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -1597,10 +1597,13 @@ compparms (const_tree parms1, const_tree parms2)
 
 
 /* Process a sizeof or alignof expression where the operand is a
-   type.  */
+   type. STD_ALIGNOF indicates whether an alignof has C++11 (minimum alignment)
+   or GNU (preferred alignment) semantics; it is ignored if op is
+   SIZEOF_EXPR.  */
 
 tree
-cxx_sizeof_or_alignof_type (tree type, enum tree_code op, bool complain)
+cxx_sizeof_or_alignof_type (tree type, enum tree_code op, bool std_alignof,
+			    bool complain)
 {
   tree value;
   bool dependent_p;
@@ -1637,11 +1640,13 @@ cxx_sizeof_or_alignof_type (tree type, enum tree_code op, bool complain)
     {
       value = build_min (op, size_type_node, type);
       TREE_READONLY (value) = 1;
+      if (op == ALIGNOF_EXPR && std_alignof)
+	ALIGNOF_EXPR_STD_P (value) = true;
       return value;
     }
 
   return c_sizeof_or_alignof_type (input_location, complete_type (type),
-				   op == SIZEOF_EXPR, false,
+				   op == SIZEOF_EXPR, std_alignof,
 				   complain);
 }
 
@@ -1659,7 +1664,7 @@ cxx_sizeof_nowarn (tree type)
   else if (!COMPLETE_TYPE_P (type))
     return size_zero_node;
   else
-    return cxx_sizeof_or_alignof_type (type, SIZEOF_EXPR, false);
+    return cxx_sizeof_or_alignof_type (type, SIZEOF_EXPR, false, false);
 }
 
 /* Process a sizeof expression where the operand is an expression.  */
@@ -1725,7 +1730,7 @@ cxx_sizeof_expr (tree e, tsubst_flags_t complain)
   else
     e = TREE_TYPE (e);
 
-  return cxx_sizeof_or_alignof_type (e, SIZEOF_EXPR, complain & tf_error);
+  return cxx_sizeof_or_alignof_type (e, SIZEOF_EXPR, false, complain & tf_error);
 }
 
 /* Implement the __alignof keyword: Return the minimum required
@@ -1786,7 +1791,7 @@ cxx_alignof_expr (tree e, tsubst_flags_t complain)
       t = size_one_node;
     }
   else
-    return cxx_sizeof_or_alignof_type (TREE_TYPE (e), ALIGNOF_EXPR, 
+    return cxx_sizeof_or_alignof_type (TREE_TYPE (e), ALIGNOF_EXPR, false,
                                        complain & tf_error);
 
   return fold_convert (size_type_node, t);
@@ -1825,7 +1830,7 @@ cxx_alignas_expr (tree e)
 	   alignas(type-id ), it shall have the same effect as
 	   alignas(alignof(type-id )).  */
 
-    return cxx_sizeof_or_alignof_type (e, ALIGNOF_EXPR, false);
+    return cxx_sizeof_or_alignof_type (e, ALIGNOF_EXPR, true, false);
   
   /* If we reach this point, it means the alignas expression if of
      the form "alignas(assignment-expression)", so we should follow
diff --git a/gcc/testsuite/g++.dg/abi/align2.C b/gcc/testsuite/g++.dg/abi/align2.C
new file mode 100644
index 00000000000..c9f3a8fae24
--- /dev/null
+++ b/gcc/testsuite/g++.dg/abi/align2.C
@@ -0,0 +1,5 @@
+// PR c++/69560
+// { dg-do compile { target { ia32 && c++11 } } }
+
+#define SA(X) static_assert ((X), #X)
+SA(alignof(double) == 4);
diff --git a/libcc1/libcp1plugin.cc b/libcc1/libcp1plugin.cc
index 0137cc9e4e1..10341477c56 100644
--- a/libcc1/libcp1plugin.cc
+++ b/libcc1/libcp1plugin.cc
@@ -3046,7 +3046,8 @@ plugin_build_unary_type_expr (cc1_plugin::connection *self,
       break;
 
     default:
-      result = cxx_sizeof_or_alignof_type (type, opcode, true);
+      /* Use the C++11 alignof semantics.  */
+      result = cxx_sizeof_or_alignof_type (type, opcode, true, true);
     }
 
   if (template_dependent_p)