On Mon, Jul 20, 2026 at 05:28:42PM +0200, Josef Melcr wrote:
> gcc/ChangeLog:
>
> * attr-callback.cc (callback_edge_callee_has_attr): Add gnu
> namespace to lookup.
> (callback_fetch_attr_by_edge): Likewise.
> (handle_callback_attribute): Warn when the attribute is used
> without the gnu namespace, fix bounds check.
> * attr-callback.h (CALLBACK_ATTR_IDENT): Change name to 'callback_only'.
> * builtin-attrs.def (ATTR_CALLBACK): Likewise.
> * cgraph.cc (cgraph_edge::redirect_call_stmt_to_callee): Add gnu
> namespace to lookup.
> (cgraph_node::verify_node): Likewise.
> * doc/extend.texi: Add gnu::callback_only docs.
> * ipa-cp.cc (purge_useless_callback_edges): Add gnu namespace to
> lookup.
> * ipa-prop.cc (ipa_compute_jump_functions_for_edge): Likewise.
> * tree-core.h: Fix comment describing the ECF_CB flag.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/attr-callback.c: New test.
> * gcc.dg/ipa/ipcp-cb-noprop.c: New test.
> * gcc.dg/ipa/ipcp-cb2.c: New test.
>
> Signed-off-by: Josef Melcr <[email protected]>
> --- a/gcc/attr-callback.cc
> +++ b/gcc/attr-callback.cc
> @@ -22,6 +22,7 @@
> #include "system.h"
> #include "coretypes.h"
> #include "backend.h"
> +#include "tree-core.h"
Why?
tree.h already includes tree-core.h.
> #include "tree.h"
> #include "gimple.h"
> #include "alloc-pool.h"
> @@ -87,7 +88,7 @@ callback_special_case_attr (tree decl)
> bool
> callback_edge_callee_has_attr (cgraph_edge *e)
> {
> - return lookup_attribute (CALLBACK_ATTR_IDENT,
> + return lookup_attribute ("gnu", CALLBACK_ATTR_IDENT,
My preference would be to s/CALLBACK_ATTR_IDENT/"callback_only"/g now,
we don't obfuscate through macros other attributes either.
> @@ -190,8 +191,23 @@ get_nth_list_elem (tree list, unsigned idx)
> struct attribute_spec.handler. */
> tree
> handle_callback_attribute (tree *node, tree name, tree args,
> - int ARG_UNUSED (flags), bool *no_add_attrs)
> + int flags, bool *no_add_attrs)
> {
> + bool in_gnu_namespace = flags & (ATTR_FLAG_CXX11 | ATTR_FLAG_BUILT_IN);
> +
> + /* Reject the spelling __attribute__((callback_only (...))). We want the
> user
> + to explicitly specify the gnu namespace to avoid clashes with clang's
> + implementation. The namespace cannot be specified for builtins, ignore
> + that case. */
> + if (!in_gnu_namespace)
> + {
> + warning (OPT_Wattributes,
> + "%<__attribute__((callback_only (...)))%> is not supported; use "
> + "%<[[gnu::callback_only (...)]]%> instead");
Unsure if this is the best recommendation for C++98 or C89 to C17.
For C89 to C17 to avoid warnings with -pedantic or errors with
-pedantic-errors, one can use [[__extension__ gnu::callback_only (...)]]
instead. That doesn't work for C++98 though, in that case one needs
to use __extension__ [[gnu::callback_only (...)]] (but it only works like
that if the attribute is at the start of the function declaration, if
it is specified in other locations, __extension__ needs to be separated
from the attribute.
Any reason why the handle_callback_attribute function is in the middle-end
rather than say c-family/c-attribs.cc ? In there you could
emit different warning wording depending on c_dialect_cxx () and
for e.g. C++ depending on cxx_dialect == cxx98 or in C on !flag_isoc23.
Otherwise LGTM.
Jakub
