Joseph Myers <[email protected]> writes:
> On Wed, 16 Aug 2023, Richard Sandiford via Gcc-patches wrote:
>
>> Would it be OK to add support for:
>>
>> [[__extension__ ...]]
>>
>> to suppress the pedwarn about using [[]] prior to C2X? Then we can
>
> That seems like a plausible feature to add.
Thanks. Of course, once I actually tried it, I hit a snag:
:: isn't a single lexing token prior to C2X, and so something like:
[[__extension__ arm::streaming]]
would not be interpreted as a scoped attribute in C11. The patch
gets around that by allowing two colons in place of :: when
__extension__ is used. I realise that's pushing the bounds of
acceptability though...
I wondered about trying to require the two colons to be immediately
adjacent. But:
(a) There didn't appear to be an existing API to check that, which seemed
like a red flag. The closest I could find was get_source_text_between.
Similarly to that, it would in principle be possible to compare
two expanded locations. But...
(b) I had a vague impression that locations were allowed to drop column
information for very large inputs (maybe I'm wrong).
(c) It wouldn't cope with token pasting.
So in the end I just used a simple two-token test, like for [[ and ]].
Bootstrapped & regression-tested on aarch64-linux-gnu.
Richard
----
[[]] attributes are a recent addition to C, but as a GNU extension,
GCC allows them to be used in C11 and earlier. Normally this use
would trigger a pedwarn (for -pedantic, -Wc11-c2x-compat, etc.).
This patch allows the pedwarn to be suppressed by starting the
attribute-list with __extension__.
Also, :: is not a single lexing token prior to C2X, so it wasn't
possible to use scoped attributes in C11, even as a GNU extension.
The patch allows two colons to be used in place of :: when
__extension__ is used. No attempt is made to check whether the
two colons are immediately adjacent.
gcc/
* doc/extend.texi: Document the C [[__extension__ ...]] construct.
gcc/c/
* c-parser.cc (c_parser_std_attribute): Conditionally allow
two colons to be used in place of ::.
(c_parser_std_attribute_list): New function, split out from...
(c_parser_std_attribute_specifier): ...here. Allow the attribute-list
to start with __extension__. When it does, also allow two colons
to be used in place of ::.
gcc/testsuite/
* gcc.dg/c2x-attr-syntax-6.c: New test.
* gcc.dg/c2x-attr-syntax-7.c: Likewise.
---
gcc/c/c-parser.cc | 68 ++++++++++++++++++------
gcc/doc/extend.texi | 27 ++++++++--
gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c | 50 +++++++++++++++++
gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c | 48 +++++++++++++++++
4 files changed, 173 insertions(+), 20 deletions(-)
create mode 100644 gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c
create mode 100644 gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c
diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
index 33fe7b115ff..82e56b28446 100644
--- a/gcc/c/c-parser.cc
+++ b/gcc/c/c-parser.cc
@@ -5390,10 +5390,18 @@ c_parser_balanced_token_sequence (c_parser *parser)
( balanced-token-sequence[opt] )
Keywords are accepted as identifiers for this purpose.
-*/
+
+ As an extension, we permit an attribute-specifier to be:
+
+ [ [ __extension__ attribute-list ] ]
+
+ Two colons are then accepted as a synonym for ::. No attempt is made
+ to check whether the colons are immediately adjacent. LOOSE_SCOPE_P
+ indicates whether this relaxation is in effect. */
static tree
-c_parser_std_attribute (c_parser *parser, bool for_tm)
+c_parser_std_attribute (c_parser *parser, bool for_tm,
+ bool loose_scope_p = false)
{
c_token *token = c_parser_peek_token (parser);
tree ns, name, attribute;
@@ -5406,9 +5414,18 @@ c_parser_std_attribute (c_parser *parser, bool for_tm)
}
name = canonicalize_attr_name (token->value);
c_parser_consume_token (parser);
- if (c_parser_next_token_is (parser, CPP_SCOPE))
+ if (c_parser_next_token_is (parser, CPP_SCOPE)
+ || (loose_scope_p
+ && c_parser_next_token_is (parser, CPP_COLON)
+ && c_parser_peek_token (parser)->type == CPP_COLON))
{
ns = name;
+ if (c_parser_next_token_is (parser, CPP_COLON))
+ {
+ c_parser_consume_token (parser);
+ if (!c_parser_next_token_is (parser, CPP_COLON))
+ gcc_unreachable ();
+ }
c_parser_consume_token (parser);
token = c_parser_peek_token (parser);
if (token->type != CPP_NAME && token->type != CPP_KEYWORD)
@@ -5481,19 +5498,9 @@ c_parser_std_attribute (c_parser *parser, bool for_tm)
}
static tree
-c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
+c_parser_std_attribute_list (c_parser *parser, bool for_tm,
+ bool loose_scope_p = false)
{
- location_t loc = c_parser_peek_token (parser)->location;
- if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
- return NULL_TREE;
- if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
- {
- c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
- return NULL_TREE;
- }
- if (!for_tm)
- pedwarn_c11 (loc, OPT_Wpedantic,
- "ISO C does not support %<[[]]%> attributes before C2X");
tree attributes = NULL_TREE;
while (true)
{
@@ -5505,7 +5512,7 @@ c_parser_std_attribute_specifier (c_parser *parser, bool
for_tm)
c_parser_consume_token (parser);
continue;
}
- tree attribute = c_parser_std_attribute (parser, for_tm);
+ tree attribute = c_parser_std_attribute (parser, for_tm, loose_scope_p);
if (attribute != error_mark_node)
{
TREE_CHAIN (attribute) = attributes;
@@ -5514,6 +5521,35 @@ c_parser_std_attribute_specifier (c_parser *parser, bool
for_tm)
if (c_parser_next_token_is_not (parser, CPP_COMMA))
break;
}
+ return attributes;
+}
+
+static tree
+c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
+{
+ location_t loc = c_parser_peek_token (parser)->location;
+ if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
+ return NULL_TREE;
+ if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
+ {
+ c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
+ return NULL_TREE;
+ }
+ tree attributes;
+ if (c_parser_next_token_is_keyword (parser, RID_EXTENSION))
+ {
+ auto ext = disable_extension_diagnostics ();
+ c_parser_consume_token (parser);
+ attributes = c_parser_std_attribute_list (parser, for_tm, true);
+ restore_extension_diagnostics (ext);
+ }
+ else
+ {
+ if (!for_tm)
+ pedwarn_c11 (loc, OPT_Wpedantic,
+ "ISO C does not support %<[[]]%> attributes before C2X");
+ attributes = c_parser_std_attribute_list (parser, for_tm);
+ }
c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
return nreverse (attributes);
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 73a997276cb..d764e44cbce 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -11914,10 +11914,29 @@ macros to replace them with the customary keywords.
It looks like this:
@findex __extension__
@opindex pedantic
@option{-pedantic} and other options cause warnings for many GNU C extensions.
-You can
-prevent such warnings within one expression by writing
-@code{__extension__} before the expression. @code{__extension__} has no
-effect aside from this.
+You can suppress such warnings using the keyword @code{__extension__}.
+Specifically:
+
+@itemize @bullet
+@item
+Writing @code{__extension__} before an expression prevents warnings
+about extensions within that expression.
+
+@item
+In C, writing:
+
+@smallexample
+[[__extension__ @dots{}]]
+@end smallexample
+
+suppresses warnings about using @samp{[[]]} attributes in C versions
+that predate C2X@. Since the scope token @samp{::} is not a single
+lexing token in earlier versions of C, this construct also allows two colons
+to be used in place of @code{::}. GCC does not check whether the two
+colons are immediately adjacent.
+@end itemize
+
+@code{__extension__} has no effect aside from this.
@node Incomplete Enums
@section Incomplete @code{enum} Types
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c
b/gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c
new file mode 100644
index 00000000000..01eff1f32df
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c
@@ -0,0 +1,50 @@
+/* Test C2x attribute syntax: use of __extension__ in C11 mode. */
+/* { dg-do compile } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+#define FOO ::
+#define BAR :
+
+typedef int [[__extension__ gnu :: vector_size (4)]] g1;
+typedef int [[__extension__ gnu : : vector_size (4)]] g2;
+typedef int [[__extension__ gnu FOO vector_size (4)]] g3;
+typedef int [[__extension__ gnu BAR BAR vector_size (4)]] g4;
+typedef int [[__extension__ gnu :: vector_size (sizeof (void (*)(...)))]] g5;
+typedef int [[__extension__]] g6;
+typedef int [[__extension__,]] g7;
+typedef int [[__extension__, ,,,, ,, ,]] g8;
+[[__extension__ deprecated]] int g9 ();
+[[__extension__ nodiscard]] int g10 ();
+[[__extension__ noreturn]] void g11 ();
+
+int
+cases (int x)
+{
+ switch (x)
+ {
+ case 1:
+ case 2:
+ case 4:
+ x += 1;
+ [[__extension__ fallthrough]];
+ case 19:
+ case 33:
+ x *= 2;
+ [[fallthrough]]; /* { dg-error {attributes before C2X} } */
+ case 99:
+ return x;
+
+ default:
+ return 0;
+ }
+}
+
+typedef int [[__extension__ vector_size (4)]] b1; /* { dg-error {'vector_size'
attribute ignored} } */
+typedef int [[__extension__ __extension__]] b2; /* { dg-error {'extension'
attribute ignored} } */
+typedef int [[__extension__ unknown_attribute]] b3; /* { dg-error
{'unknown_attribute' attribute ignored} } */
+typedef int [[gnu::vector_size(4)]] b4; /* { dg-error {expected '\]' before
':'} } */
+/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
+/* { dg-error {attributes before C2X} "" { target *-*-* } .-2 } */
+typedef int [[gnu : : vector_size(4)]] b5; /* { dg-error {expected '\]' before
':'} } */
+/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
+/* { dg-error {attributes before C2X} "" { target *-*-* } .-2 } */
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c
b/gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c
new file mode 100644
index 00000000000..cfa9b00cfc9
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c
@@ -0,0 +1,48 @@
+/* Test C2x attribute syntax: use of __extension__ in C11 mode. */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors -Wc11-c2x-compat" } */
+
+#define FOO ::
+#define BAR :
+
+typedef int [[__extension__ gnu :: vector_size (4)]] g1;
+typedef int [[__extension__ gnu : : vector_size (4)]] g2;
+typedef int [[__extension__ gnu FOO vector_size (4)]] g3;
+typedef int [[__extension__ gnu BAR BAR vector_size (4)]] g4;
+typedef int [[__extension__ gnu :: vector_size (sizeof (void (*)(...)))]] g5;
+typedef int [[__extension__]] g6;
+typedef int [[__extension__,]] g7;
+typedef int [[__extension__, ,,,, ,, ,]] g8;
+[[__extension__ deprecated]] int g9 ();
+[[__extension__ nodiscard]] int g10 ();
+[[__extension__ noreturn]] void g11 ();
+
+int
+cases (int x)
+{
+ switch (x)
+ {
+ case 1:
+ case 2:
+ case 4:
+ x += 1;
+ [[__extension__ fallthrough]];
+ case 19:
+ case 33:
+ x *= 2;
+ [[fallthrough]]; /* { dg-warning {attributes before C2X} } */
+ case 99:
+ return x;
+
+ default:
+ return 0;
+ }
+}
+
+typedef int [[__extension__ vector_size (4)]] b1; /* { dg-error {'vector_size'
attribute ignored} } */
+typedef int [[__extension__ __extension__]] b2; /* { dg-error {'extension'
attribute ignored} } */
+typedef int [[__extension__ unknown_attribute]] b3; /* { dg-error
{'unknown_attribute' attribute ignored} } */
+typedef int [[gnu::vector_size(4)]] b4; /* { dg-warning {attributes before
C2X} } */
+typedef int [[gnu : : vector_size(4)]] b5; /* { dg-error {expected '\]' before
':'} } */
+/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
+/* { dg-warning {attributes before C2X} "" { target *-*-* } .-2 } */
--
2.25.1
