Re: [C++ PATCH] Fix constexpr switch handling (PR c++/77467)

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Sep 16, 2016 at 03:51:11PM -0400, Jason Merrill wrote:
> On Mon, Sep 5, 2016 at 1:11 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> > +  /* If body is a statement other than STATEMENT_LIST or BIND_EXPR,
> > +     it should be skipped.  E.g. switch (a) b = a;  */
> > +  if (TREE_CODE (body) == STATEMENT_LIST
> > +      || TREE_CODE (body) == BIND_EXPR)
> 
> I'm nervous about this optimization for useless code breaking other
> things that might (one day) wrap a case label; I think I'd prefer to
> drop the condition.

By droping the condition you mean unconditionally call
  cxx_eval_constant_expression (ctx, body, false,
			        non_constant_p, overflow_p, jump_target);
?  That is known not to work, that breaks the
+constexpr int
+bar (int x)
+{
+  int a = x;
+  switch (x)
+    a = x + 1;
+  return a;
+}
handling in the testcase, where body is the MODIFY_EXPR which doesn't have
the label and thus needs to be skipped.  The problem is that all the logic for
skipping statements until the label is found is in cxx_eval_statement_list
only.  For STATEMENT_LIST that is called by cxx_eval_constant_expression,
for BIND_EXPR if we are lucky enough that BIND_EXPR_BODY is a STATEMENT_LIST
too (otherwise I assume even my patch doesn't fix it, it would need to
verify that).  If body is some other statement, then it really should be
skipped, but it isn't, because cxx_eval_constant_expression ignores it.

I wonder if we e.g. cxx_eval_constant_expression couldn't early in the
function for if (*jump_target) return immediately unless code is something
like STATEMENT_LIST or BIND_EXPR with BIND_EXPR_BODY being STATEMENT_LIST,
or perhaps in the future other construct containing other stmts.
I've beeing thinking about TRY block, but at least on the testcases I've
tried it has been rejected in constexpr functions, I think one can't branch
into statement expressions, so that should be fine, OpenMP/OpenACC
constructs are hopefully also rejected in constexpr, what else?

	Jakub