On Wed, Sep 27, 2017 at 07:55:20AM -0700, Nathan Sidwell wrote:
> Jakub,
> > The following patch implements P0386R1 - NSDMIs for bit-fields.
> > While working on that, I've discovered our parser mishandles attributes
> > on bitfields, already C++11 says:
> > identifier[opt] attribute-specifier-seq[opt] : constant-expression
> > in the grammar, but we actually parsed
> > identifier[opt] : constant-expression attribute-specifier-seq[opt]
>
> I'm sorry for my tardiness. It think the patch would be better broken
> apart:
> 1) fix the parsing bug you found and move to (ab)using
> DECL_BIT_FIELD_REPRESENTATIVE
>
> 2) the new c++2a feature
I don't see the first half of 1) related to second half thereof, the
DECL_BIT_FIELD_REPRESENTATIVE is unrelated to the parsing bug, and a needed
precondition of 2).
Therefore, I'm going to split the patch into 4 patches, one the
parsing bug (make sure we allow attributes where the standard allows them),
attached below, including pedwarn. Note we already error for the old
spot when using C++11 [[ ]] attributes there, like int a : 8 [[gnu::packed]];
because when parsing the width expression we see [ after 8 and so it could
be say 8[var] or similar, but error out because [[ must not appear in that
spot. But alignas there didn't result in error, neither __attribute__.
The other patches will follow, second patch will be
DECL_BIT_FIELD_REPRESENTATIVE, third one the Make-lang.in tweak and fourth
one, to be rewritten tomorrow on top of the rest will be the new c++2a
feature.
The following has been bootstrapped/regtested on x86_64-linux and i686-linux
(together with the Make-lang.in patch, but without the second and fourth
patch), ok for trunk?
As I said on IRC, I hope [[/__attribute__/alignas early is rare enough that
the tentative parsing shouldn't be a big deal, if it is, we could add some
cheaper function that allows us to skip over attributes (return a peek
offset after the attributes given a starting peek offset).
2017-09-28 Jakub Jelinek <ja...@redhat.com>
cp/
* parser.c (cp_parser_member_declaration): Parse attributes before
colon of a bitfield in addition to after colon.
testsuite/
* g++.dg/ext/bitfield7.C: New test.
* g++.dg/ext/bitfield8.C: New test.
* g++.dg/ext/bitfield9.C: New test.
--- gcc/cp/parser.c.jj 2017-09-22 20:51:48.181537880 +0200
+++ gcc/cp/parser.c 2017-09-27 17:50:15.082792676 +0200
@@ -23443,35 +23443,64 @@ cp_parser_member_declaration (cp_parser*
{
tree attributes = NULL_TREE;
tree first_attribute;
+ bool is_bitfld = false;
+ bool named_bitfld = false;
/* Peek at the next token. */
token = cp_lexer_peek_token (parser->lexer);
+ if (cp_next_tokens_can_be_attribute_p (parser)
+ || (token->type == CPP_NAME
+ && cp_nth_tokens_can_be_attribute_p (parser, 2)
+ && (named_bitfld = true)))
+ {
+ cp_parser_parse_tentatively (parser);
+ if (named_bitfld)
+ cp_lexer_consume_token (parser->lexer);
+ cp_parser_attributes_opt (parser);
+ token = cp_lexer_peek_token (parser->lexer);
+ is_bitfld = cp_lexer_next_token_is (parser->lexer, CPP_COLON);
+ cp_parser_abort_tentative_parse (parser);
+ }
+
/* Check for a bitfield declaration. */
- if (token->type == CPP_COLON
+ if (is_bitfld
+ || token->type == CPP_COLON
|| (token->type == CPP_NAME
- && cp_lexer_peek_nth_token (parser->lexer, 2)->type
- == CPP_COLON))
+ && cp_lexer_nth_token_is (parser->lexer, 2, CPP_COLON)
+ && (named_bitfld = true)))
{
tree identifier;
tree width;
+ tree late_attributes = NULL_TREE;
- /* Get the name of the bitfield. Note that we cannot just
- check TOKEN here because it may have been invalidated by
- the call to cp_lexer_peek_nth_token above. */
- if (cp_lexer_peek_token (parser->lexer)->type != CPP_COLON)
+ if (named_bitfld)
identifier = cp_parser_identifier (parser);
else
identifier = NULL_TREE;
+ /* Look for attributes that apply to the bitfield. */
+ attributes = cp_parser_attributes_opt (parser);
+
/* Consume the `:' token. */
cp_lexer_consume_token (parser->lexer);
+
/* Get the width of the bitfield. */
- width
- = cp_parser_constant_expression (parser);
+ width = cp_parser_constant_expression (parser);
+
+ /* Look for attributes that apply to the bitfield after
+ the `:' token and width. This is where GCC used to
+ parse attributes in the past, pedwarn if there is
+ a std attribute. */
+ if (cp_next_tokens_can_be_std_attribute_p (parser))
+ pedwarn (input_location, OPT_Wpedantic,
+ "ISO C++ allows bit-field attributes only before "
+ "the %<:%> token");
+
+ late_attributes = cp_parser_attributes_opt (parser);
+
+ attributes = chainon (attributes, late_attributes);
- /* Look for attributes that apply to the bitfield. */
- attributes = cp_parser_attributes_opt (parser);
/* Remember which attributes are prefix attributes and
which are not. */
first_attribute = attributes;
--- gcc/testsuite/g++.dg/ext/bitfield7.C.jj 2017-09-27 17:37:11.118154267
+0200
+++ gcc/testsuite/g++.dg/ext/bitfield7.C 2017-09-27 17:37:11.118154267
+0200
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-options "-Wno-packed-bitfield-compat -mno-ms-bitfields" { target {
i?86-*-mingw* x86_64-*-mingw* } } } */
+
+struct t /* { dg-message "note: offset of packed bit-field 't::b' has changed
in GCC 4.4" "" { target pcc_bitfield_type_matters } } */
+{
+ char a:4;
+ char b __attribute__ ((packed)) : 8;
+ char c:4;
+};
+
+int assrt[sizeof (struct t) == 2 ? 1 : -1];
--- gcc/testsuite/g++.dg/ext/bitfield8.C.jj 2017-09-27 17:37:11.118154267
+0200
+++ gcc/testsuite/g++.dg/ext/bitfield8.C 2017-09-27 17:37:11.118154267
+0200
@@ -0,0 +1,12 @@
+/* { dg-do compile { target c++11 } } */
+/* { dg-options "" } */
+/* { dg-options "-Wno-packed-bitfield-compat -mno-ms-bitfields" { target {
i?86-*-mingw* x86_64-*-mingw* } } } */
+
+struct t /* { dg-message "note: offset of packed bit-field 't::b' has changed
in GCC 4.4" "" { target pcc_bitfield_type_matters } } */
+{
+ char a:4;
+ char b [[gnu::packed]] : 8;
+ char c:4;
+};
+
+int assrt[sizeof (struct t) == 2 ? 1 : -1];
--- gcc/testsuite/g++.dg/ext/bitfield9.C.jj 2017-09-27 18:00:54.903153797
+0200
+++ gcc/testsuite/g++.dg/ext/bitfield9.C 2017-09-27 18:20:24.823186005
+0200
@@ -0,0 +1,10 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-pedantic" }
+
+struct S
+{
+ char a:4;
+ char b:8 alignas(int); // { dg-warning "ISO C\\+\\+ allows bit-field
attributes only before the ':' token" }
+ char c:8 [[gnu::aligned(8)]]; // { dg-warning "ISO C\\+\\+ allows
bit-field attributes only before the ':' token" }
+ // { dg-error "two consecutive '\\\[' shall
only introduce an attribute before '\\\[' token" "" { target *-*-* } .-1 }
+};
Jakub
