On 8/28/23 06:34, Richard Biener wrote:
On Fri, 25 Aug 2023, Jakub Jelinek wrote:
Hi!
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.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
The gimple-walk.cc/gimplify.cc changes are OK, I don't understand
the c-gimplify.cc one.
Seems like it would be good to document this non-obvious meaning of
*_NOTHROW for ICF_FALLTHROUGH. Maybe at its entry in internal-fn.def?
OK with that change.
Jason
