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  <jakub@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.

1 files changed, 1 insertions, 0 deletions

diff --git a/gcc/tree-core.h b/gcc/tree-core.h
index a8439f8..9fa7434 100644
--- a/gcc/tree-core.h
+++ b/gcc/tree-core.h
@@ -983,6 +983,7 @@ enum annot_expr_kind {
   annot_expr_no_vector_kind,
   annot_expr_vector_kind,
   annot_expr_parallel_kind,
+  annot_expr_maybe_infinite_kind,
   annot_expr_kind_last
 };