[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

--- Comment #24 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Andi Kleen <ak@gcc.gnu.org>:

https://gcc.gnu.org/g:2bd8177256b6d87f6e75819218cf22c2c0bfc1ac

commit r15-2231-g2bd8177256b6d87f6e75819218cf22c2c0bfc1ac
Author: Andi Kleen <ak@linux.intel.com>
Date:   Tue Jan 23 23:44:48 2024 -0800

    C++: Support clang compatible [[musttail]] (PR83324)

    This patch implements a clang compatible [[musttail]] attribute for
    returns.

    musttail is useful as an alternative to computed goto for interpreters.
    With computed goto the interpreter function usually ends up very big
    which causes problems with register allocation and other per function
    optimizations not scaling. With musttail the interpreter can be instead
    written as a sequence of smaller functions that call each other. To
    avoid unbounded stack growth this requires forcing a sibling call, which
    this attribute does. It guarantees an error if the call cannot be tail
    called which allows the programmer to fix it instead of risking a stack
    overflow. Unlike computed goto it is also type-safe.

    It turns out that David Malcolm had already implemented middle/backend
    support for a musttail attribute back in 2016, but it wasn't exposed
    to any frontend other than a special plugin.

    This patch adds a [[gnu::musttail]] attribute for C++ that can be added
    to return statements. The return statement must be a direct call
    (it does not follow dependencies), which is similar to what clang
    implements. It then uses the existing must tail infrastructure.

    For compatibility it also detects clang::musttail

    Passes bootstrap and full test

    gcc/c-family/ChangeLog:

            * c-attribs.cc (set_musttail_on_return): New function.
            * c-common.h (set_musttail_on_return): Declare new function.

    gcc/cp/ChangeLog:

            PR c/83324
            * cp-tree.h (AGGR_INIT_EXPR_MUST_TAIL): Add.
            * parser.cc (cp_parser_statement): Handle musttail.
            (cp_parser_jump_statement): Dito.
            * pt.cc (tsubst_expr): Copy CALL_EXPR_MUST_TAIL_CALL.
            * semantics.cc (simplify_aggr_init_expr): Handle musttail.

--- Comment #25 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Andi Kleen <ak@gcc.gnu.org>:

https://gcc.gnu.org/g:78bbdbd5352df527feccf0a8c2f862f25a2e88b4

commit r15-2232-g78bbdbd5352df527feccf0a8c2f862f25a2e88b4
Author: Andi Kleen <ak@linux.intel.com>
Date:   Wed Jan 24 07:44:23 2024 -0800

    C: Implement musttail attribute for returns

    Implement a C23 clang compatible musttail attribute similar to the earlier
    C++ implementation in the C parser.

    gcc/c/ChangeLog:

            PR c/83324
            * c-parser.cc (struct attr_state): Define with musttail_p.
            (c_parser_statement_after_labels): Handle [[musttail]].
            (c_parser_std_attribute): Dito.
            (c_parser_handle_musttail): Dito.
            (c_parser_compound_statement_nostart): Dito.
            (c_parser_all_labels): Dito.
            (c_parser_statement): Dito.
            * c-tree.h (c_finish_return): Add musttail_p flag.
            * c-typeck.cc (c_finish_return): Handle musttail_p flag.