https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114462
--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:4be1cc5f50578fafcdcbd09160235066d76a3f86 commit r14-9887-g4be1cc5f50578fafcdcbd09160235066d76a3f86 Author: Jakub Jelinek <ja...@redhat.com> Date: Wed Apr 10 10:08:12 2024 +0200 c++: Implement C++26 P2809R3 - Trivial infinite loops are not Undefined Behavior The following patch attempts to implement P2809R3, which has been voted in as a DR. The middle-end has its behavior documented: '-ffinite-loops' Assume that a loop with an exit will eventually take the exit and not loop indefinitely. This allows the compiler to remove loops that otherwise have no side-effects, not considering eventual endless looping as such. This option is enabled by default at '-O2' for C++ with -std=c++11 or higher. So, the following patch attempts to detect trivial infinite loops by detecting trivially empty loops, if their condition is not INTEGER_CST (that case is handled by the middle-end right already) trying to constant evaluate with mce=true their condition and if it evaluates to true (and -ffinite-loops and not processing_template_decl) wraps the condition into an ANNOTATE_EXPR which tells the middle-end that the loop shouldn't be loop->finite_p despite -ffinite-loops). Furthermore, the patch adds -Wtautological-compare warnings for loop conditions containing std::is_constant_evaluated(), either if those always evaluate to true, or always evaluate to false, or will evaluate to true just when checking if it is trivial infinite loop (and if in non-constexpr function also say that it will evaluate to false otherwise). The user is doing something weird in all those cases. 2024-04-10 Jakub Jelinek <ja...@redhat.com> PR c++/114462 gcc/ * tree-core.h (enum annot_expr_kind): Add annot_expr_maybe_infinite_kind enumerator. * gimplify.cc (gimple_boolify): Handle annot_expr_maybe_infinite_kind. * tree-cfg.cc (replace_loop_annotate_in_block): Likewise. (replace_loop_annotate): Likewise. Move loop->finite_p initialization before the replace_loop_annotate_in_block calls. * tree-pretty-print.cc (dump_generic_node): Handle annot_expr_maybe_infinite_kind. gcc/cp/ * semantics.cc: Implement C++26 P2809R3 - Trivial infinite loops are not Undefined Behavior. (maybe_warn_for_constant_evaluated): Add trivial_infinite argument and emit special diagnostics for that case. (finish_if_stmt_cond): Adjust caller. (finish_loop_cond): New function. (finish_while_stmt): Use it. (finish_do_stmt): Likewise. (finish_for_stmt): Likewise. gcc/testsuite/ * g++.dg/cpp26/trivial-infinite-loop1.C: New test. * g++.dg/cpp26/trivial-infinite-loop2.C: New test. * g++.dg/cpp26/trivial-infinite-loop3.C: New test.
