https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324
--- Comment #21 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Andi Kleen <a...@gcc.gnu.org>: https://gcc.gnu.org/g:59dd1d7ab21ad9a7ebf641ec9aeea609c003ad2f commit r15-2169-g59dd1d7ab21ad9a7ebf641ec9aeea609c003ad2f Author: Andi Kleen <a...@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.
