From 52eedfa00960f2d255ec542626e3531a65aa8bb8 Mon Sep 17 00:00:00 2001 From: Jakub Jelinek Date: Fri, 17 Nov 2023 15:43:31 +0100 Subject: c++: Implement C++ DR 2406 - [[fallthrough]] attribute and iteration statements The following patch implements CWG 2406 - [[fallthrough]] attribute and iteration statements The genericization of some loops leaves nothing at all or just a label after a body of a loop, so if the loop is later followed by case or default label in a switch, the fallthrough statement isn't diagnosed. The following patch implements it by marking the IFN_FALLTHROUGH call in such a case, such that during gimplification it can be pedantically diagnosed even if it is followed by case or default label or some normal labels followed by case/default labels. While looking into this, I've discovered other problems. expand_FALLTHROUGH_r is removing the IFN_FALLTHROUGH calls from the IL, but wasn't telling that to walk_gimple_stmt/walk_gimple_seq_mod, so the callers would then skip the next statement after it, and it would return non-NULL if the removed stmt was last in the sequence. This could lead to wi->callback_result being set even if it didn't appear at the very end of switch sequence. The patch makes use of wi->removed_stmt such that the callers properly know what happened, and use different way to handle the end of switch sequence case. That change discovered a bug in the gimple-walk handling of wi->removed_stmt. If that flag is set, the callback is telling the callers that the current statement has been removed and so the innermost walk_gimple_seq_mod shouldn't gsi_next. The problem is that wi->removed_stmt is only reset at the start of a walk_gimple_stmt, but that can be too late for some cases. If we have two nested gimple sequences, say GIMPLE_BIND as the last stmt of some gimple seq, we remove the last statement inside of that GIMPLE_BIND, set wi->removed_stmt there, don't do gsi_next correctly because already gsi_remove moved us to the next stmt, there is no next stmt, so we return back to the caller, but wi->removed_stmt is still set and so we don't do gsi_next even in the outer sequence, despite the GIMPLE_BIND (etc.) not being removed. That means we walk the GIMPLE_BIND with its whole sequence again. The patch fixes that by resetting wi->removed_stmt after we've used that flag in walk_gimple_seq_mod. Nothing really uses that flag after the outermost walk_gimple_seq_mod, it is just a private notification that the stmt callback has removed a stmt. 2023-11-17 Jakub Jelinek PR c++/107571 gcc/ * gimplify.cc (expand_FALLTHROUGH_r): Use wi->removed_stmt after gsi_remove, change the way of passing fallthrough stmt at the end of sequence to expand_FALLTHROUGH. Diagnose IFN_FALLTHROUGH with GF_CALL_NOTHROW flag. (expand_FALLTHROUGH): Change loc into array of 2 location_t elts, don't test wi.callback_result, instead check whether first elt is not UNKNOWN_LOCATION and in that case pedwarn with the second location. * gimple-walk.cc (walk_gimple_seq_mod): Clear wi->removed_stmt after the flag has been used. * internal-fn.def (FALLTHROUGH): Mention in comment the special meaning of the TREE_NOTHROW/GF_CALL_NOTHROW flag on the calls. gcc/c-family/ * c-gimplify.cc (genericize_c_loop): For C++ mark IFN_FALLTHROUGH call at the end of loop body as TREE_NOTHROW. gcc/testsuite/ * g++.dg/DRs/dr2406.C: New test. --- gcc/gimplify.cc | 33 ++++++++++++++++++++++++++------- 1 file changed, 26 insertions(+), 7 deletions(-) (limited to 'gcc/gimplify.cc') diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc index 77f07af..d52d71b 100644 --- a/gcc/gimplify.cc +++ b/gcc/gimplify.cc @@ -2791,17 +2791,33 @@ expand_FALLTHROUGH_r (gimple_stmt_iterator *gsi_p, bool *handled_ops_p, *handled_ops_p = false; break; case GIMPLE_CALL: + static_cast(wi->info)[0] = UNKNOWN_LOCATION; if (gimple_call_internal_p (stmt, IFN_FALLTHROUGH)) { + location_t loc = gimple_location (stmt); gsi_remove (gsi_p, true); + wi->removed_stmt = true; + + /* nothrow flag is added by genericize_c_loop to mark fallthrough + statement at the end of some loop's body. Those should be + always diagnosed, either because they indeed don't precede + a case label or default label, or because the next statement + is not within the same iteration statement. */ + if ((stmt->subcode & GF_CALL_NOTHROW) != 0) + { + pedwarn (loc, 0, "attribute % not preceding " + "a case label or default label"); + break; + } + if (gsi_end_p (*gsi_p)) { - *static_cast(wi->info) = gimple_location (stmt); - return integer_zero_node; + static_cast(wi->info)[0] = BUILTINS_LOCATION; + static_cast(wi->info)[1] = loc; + break; } bool found = false; - location_t loc = gimple_location (stmt); gimple_stmt_iterator gsi2 = *gsi_p; stmt = gsi_stmt (gsi2); @@ -2851,6 +2867,7 @@ expand_FALLTHROUGH_r (gimple_stmt_iterator *gsi_p, bool *handled_ops_p, } break; default: + static_cast(wi->info)[0] = UNKNOWN_LOCATION; break; } return NULL_TREE; @@ -2862,14 +2879,16 @@ static void expand_FALLTHROUGH (gimple_seq *seq_p) { struct walk_stmt_info wi; - location_t loc; + location_t loc[2]; memset (&wi, 0, sizeof (wi)); - wi.info = (void *) &loc; + loc[0] = UNKNOWN_LOCATION; + loc[1] = UNKNOWN_LOCATION; + wi.info = (void *) &loc[0]; walk_gimple_seq_mod (seq_p, expand_FALLTHROUGH_r, NULL, &wi); - if (wi.callback_result == integer_zero_node) + if (loc[0] != UNKNOWN_LOCATION) /* We've found [[fallthrough]]; at the end of a switch, which the C++ standard says is ill-formed; see [dcl.attr.fallthrough]. */ - pedwarn (loc, 0, "attribute % not preceding " + pedwarn (loc[1], 0, "attribute % not preceding " "a case label or default label"); } -- cgit v1.1