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, 7 insertions, 3 deletions

diff --git a/gcc/tree-cfg.cc b/gcc/tree-cfg.cc
index bdffc3b..96686db 100644
--- a/gcc/tree-cfg.cc
+++ b/gcc/tree-cfg.cc
@@ -297,6 +297,9 @@ replace_loop_annotate_in_block (basic_block bb, class loop *loop)
 	  loop->can_be_parallel = true;
 	  loop->safelen = INT_MAX;
 	  break;
+	case annot_expr_maybe_infinite_kind:
+	  loop->finite_p = false;
+	  break;
 	default:
 	  gcc_unreachable ();
 	}
@@ -320,12 +323,12 @@ replace_loop_annotate (void)
 
   for (auto loop : loops_list (cfun, 0))
     {
+      /* Push the global flag_finite_loops state down to individual loops.  */
+      loop->finite_p = flag_finite_loops;
+
       /* Check all exit source blocks for annotations.  */
       for (auto e : get_loop_exit_edges (loop))
 	replace_loop_annotate_in_block (e->src, loop);
-
-      /* Push the global flag_finite_loops state down to individual loops.  */
-      loop->finite_p = flag_finite_loops;
     }
 
   /* Remove IFN_ANNOTATE.  Safeguard for the case loop->latch == NULL.  */
@@ -347,6 +350,7 @@ replace_loop_annotate (void)
 	    case annot_expr_no_vector_kind:
 	    case annot_expr_vector_kind:
 	    case annot_expr_parallel_kind:
+	    case annot_expr_maybe_infinite_kind:
 	      break;
 	    default:
 	      gcc_unreachable ();