[{"id":3685591,"web_url":"http://patchwork.ozlabs.org/comment/3685591/","msgid":"","list_archive_url":null,"date":"2026-05-04T08:48:55","subject":"Re: [PATCH v2] Add callback_only attribute","submitter":{"id":671,"url":"http://patchwork.ozlabs.org/api/people/671/","name":"Jakub Jelinek","email":"jakub@redhat.com"},"content":"On Mon, May 04, 2026 at 10:40:23AM +0200, Josef Melcr wrote:\n> gcc_checking_assert (cb_attr);\n> tree res = NULL_TREE;\n> for (; cb_attr;\n> - cb_attr = lookup_attribute (CALLBACK_ATTR_IDENT, TREE_CHAIN (cb_attr)))\n> + cb_attr = lookup_attribute (\"gnu\", CALLBACK_ATTR_IDENT, TREE_CHAIN (cb_attr)))\n\nPlease watch formatting, the above line is now definitely too long.\n> {\n> unsigned id = callback_get_fn_index (cb_attr);\n> if (id == e->callback_id)\n> @@ -229,10 +229,10 @@ handle_callback_attribute (tree *node, tree name, tree args,\n> return NULL_TREE;\n> }\n> --callback_fn_idx;\n> - if (callback_fn_idx >= decl_nargs)\n> + if (callback_fn_idx < 0 || callback_fn_idx >= decl_nargs)\n> {\n> error_at (DECL_SOURCE_LOCATION (decl),\n> -\t\t\"callback function position out of range\");\n> +\t\t\"callback function index %d is out of range\", callback_fn_idx + 1);\n\nThis one too.\n> @@ -331,8 +331,8 @@ handle_callback_attribute (tree *node, tree name, tree args,\n> \n> /* Check that the decl does not already have a callback attribute describing\n> the same argument. */\n> - it = lookup_attribute (CALLBACK_ATTR_IDENT, DECL_ATTRIBUTES (decl));\n> - for (; it; it = lookup_attribute (CALLBACK_ATTR_IDENT, TREE_CHAIN (it)))\n> + it = lookup_attribute (\"gnu\", CALLBACK_ATTR_IDENT, DECL_ATTRIBUTES (decl));\n> + for (; it; it = lookup_attribute (\"gnu\", CALLBACK_ATTR_IDENT, TREE_CHAIN (it)))\n\nAnd this one too.\n> if (callback_get_fn_index (it) == callback_fn_idx)\n> {\n> \terror_at (DECL_SOURCE_LOCATION (decl),\n> diff --git a/gcc/attr-callback.h b/gcc/attr-callback.h\n> index b2c1c3c09c5..e51c4c6c571 100644\n> --- a/gcc/attr-callback.h\n> +++ b/gcc/attr-callback.h\n> @@ -28,7 +28,7 @@ enum callback_position\n> CB_UNKNOWN_POS = 0\n> };\n> \n> -#define CALLBACK_ATTR_IDENT \" callback\"\n> +#define CALLBACK_ATTR_IDENT \"callback_only\"\n> \n> /* Returns a callback attribute with callback index FN_IDX, and ARG_COUNT\n> arguments specified by VA_ARGS. */\n> diff --git a/gcc/builtin-attrs.def b/gcc/builtin-attrs.def\n> index be80184b1a1..a395f454be1 100644\n> --- a/gcc/builtin-attrs.def\n> +++ b/gcc/builtin-attrs.def\n> @@ -130,7 +130,7 @@ DEF_ATTR_IDENT (ATTR_TM_TMPURE, \"transaction_pure\")\n> DEF_ATTR_IDENT (ATTR_RETURNS_TWICE, \"returns_twice\")\n> DEF_ATTR_IDENT (ATTR_RETURNS_NONNULL, \"returns_nonnull\")\n> DEF_ATTR_IDENT (ATTR_WARN_UNUSED_RESULT, \"warn_unused_result\")\n> -DEF_ATTR_IDENT (ATTR_CALLBACK, \" callback\")\n> +DEF_ATTR_IDENT (ATTR_CALLBACK, \"callback_only\")\n> \n> DEF_ATTR_TREE_LIST (ATTR_NOVOPS_LIST, ATTR_NOVOPS, ATTR_NULL, ATTR_NULL)\n> \n> diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc\n> index d437c55285e..2d04b4c1ad2 100644\n> --- a/gcc/c-family/c-attribs.cc\n> +++ b/gcc/c-family/c-attribs.cc\n> @@ -485,8 +485,6 @@ const struct attribute_spec c_common_gnu_attributes[] =\n> \t\t\t handle_tm_attribute, NULL },\n> { \"transaction_may_cancel_outer\", 0, 0, false, true, false, false,\n> \t\t\t handle_tm_attribute, NULL },\n> - { CALLBACK_ATTR_IDENT, 1, -1, true, false, false, false,\n> -\t\t\t handle_callback_attribute, NULL },\n> /* ??? These two attributes didn't make the transition from the\n> Intel language document to the multi-vendor language document. */\n> { \"transaction_pure\", 0, 0, false, true, false, false,\n> @@ -707,6 +705,20 @@ const struct scoped_attribute_specs c_common_format_attribute_table =\n> \"gnu\", { c_common_format_attributes }\n> };\n> \n> +/* Attribute table for the callback attribute to be used by the C frontends. We\n> + don't want to expose the attribute outside of the GNU namespace, so it has to\n\nThese two too.\n\n> @@ -4419,9 +4419,9 @@ cgraph_node::verify_node (void)\n> \t {\n> \t int ncallbacks = 0;\n> \t int nfound_edges = 0;\n> -\t for (tree cb = lookup_attribute (CALLBACK_ATTR_IDENT, DECL_ATTRIBUTES (\n> +\t for (tree cb = lookup_attribute (\"gnu\", CALLBACK_ATTR_IDENT, DECL_ATTRIBUTES (\n> \t\t\t\t\t\t\t e->callee->decl));\n> -\t\t cb; cb = lookup_attribute (CALLBACK_ATTR_IDENT, TREE_CHAIN (cb)),\n> +\t\t cb; cb = lookup_attribute (\"gnu\", CALLBACK_ATTR_IDENT, TREE_CHAIN (cb)),\n\nAnd here as well. Note, DECL_ATTRIBUTES ( one one line and argument on\nanother one is ugly, better\n\t for (tree cb\n\t\t = lookup_attribute (\"gnu\", CALLBACK_ATTR_IDENT,\n\t\t\t\t DECL_ATTRIBUTES (e->callee->decl));\n\t\t cb; cb = lookup_attribute (\"gnu\", CALLBACK_ATTR_IDENT,\n\t\t\t\t\t TREE_CHAIN (cb)), ncallbacks++)\n\n> +@cindex @code{callback_only} function attribute\n> +@item callback_only\n> +The @code{callback_only} attribute specifies that the annotated\n> +function may call the specified callback function. The first parameter\n> +identifies the index of the callback function, the rest of the arguments specify the\n> +indices of the arguments of the indirect call. All indices start from 1. If the function\n> +takes the implicit @code{this} pointer, it is referred to by the index 1, with\n> +the rest of the arguments starting at index 2. The index 0 marks an argument not\n> +present in the arguments of the annotated function, an argument which is\n> +modified before calling the callback function. The annotated function must pass the\n> +specified arguments in the specified order to the callback function, which must be\n> +callable with the number, order and type of the arguments. The specified pointer\n> +to the callback may not escape the translation unit of the annotated function and it\n> +may not be captured. The annotated function is required to pass the arguments\n> +through, it may not change or dereference them. The arguments also may not escape.\n> +The attribute may be used multiple times per function, though only one @code{callback_only}\n> +attribute may be used per function parameter.\n> +\n> +The attribute exposes the potentially hidden callsite in the annotated function,\n> +enabling interprocedural optimizations which may not be possible without the\n> +attribute. It is most useful for annotating functions from dynamically linked libraries,\n> +as their bodies are not available during compilation.\n> +\n> +This attribute is similar to the clang @code{callback} attribute but it is not compatible\n> +with it. The clang implementation allows identifiers as arguments, marks an unknown\n> +argument with -1 and the @code{this} pointer with the index 0.\n\nHere too, many lines.\n\nTestsuite is fine vs. line length I think, especially if it is dg- directive\ncomments that are overlong.\n\n\tJakub","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=ZdsYHehM;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; helo=vm01.sourceware.org;\n envelope-from=gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org;\n receiver=patchwork.ozlabs.org)","sourceware.org;\n\tdkim=pass (1024-bit key,\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=ZdsYHehM","sourceware.org; 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