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c++, coroutines: Revise promise construction/destruction.

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In examining the coroutine testcases for unexpected diagnostic output
for 'Wall', I found a 'statement has no effect' warning for the promise
construction in one case.  In particular, the case is where the users
promise type has an implicit CTOR but a user-provided DTOR. Further, the
type does not actually need constructing.

In very early versions of the coroutines code we used to check
TYPE_NEEDS_CONSTRUCTING() to determine whether to attempt to build
a constructor call for the promise.  During review, it was suggested
to use type_build_ctor_call () instead.

This latter call checks the constructors in the type (both user-defined
and implicit) and returns true, amongst other cases if any of the found
CTORs are marked as deprecated.

In a number of places (for example [class.copy.ctor] / 6) the standard
says that some version of an implicit CTOR is deprecated when the user
provides a DTOR.

Thus, for this specific arrangement of promise type, type_build_ctor_call
returns true, because of (for example) a deprecated implicit copy CTOR.

We are not going to use any of the deprecated CTORs and thus will not
see warnings from this - however, since the call returned true, we have
now determined that we should attempt to build a constructor call.

Note as above, the type does not actually require construction and thus
one might expect either a NULL_TREE or error_mark_node in response to
the build_special_member_call ().  However, in practice the function
returns the original instance object instead of a call or some error.

When we add that as a statement it triggers the 'statement has no effect'
warning.

The patch here rearranges the promise construction/destruction code to
allow for the case that a DTOR is required independently of a CTOR. In
addition, we check that the return from build_special_member_call ()
has side effects before we add it as a statement.

gcc/cp/ChangeLog:

	* coroutines.cc
	(cp_coroutine_transform::build_ramp_function): Separate the
	build of promise constructor and destructor.  When evaluating
	the constructor, check that build_special_member_call returns
	an expression with side effects before adding it.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
This commit is contained in:
Iain Sandoe 2024-08-31 13:08:42 +01:00
parent 37977343ff
commit 7d14839219
1 changed files with 17 additions and 13 deletions
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30
gcc/cp/coroutines.cc
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@ -4896,16 +4896,12 @@ cp_coroutine_transform::build_ramp_function ()
tree p = build_class_member_access_expr (deref_fp, promise_m, NULL_TREE,
false, tf_warning_or_error);
tree promise_dtor = NULL_TREE;
if (type_build_ctor_call (promise_type))
{
/* Do a placement new constructor for the promise type (we never call
the new operator, just the constructor on the object in place in the
frame).
/* Construct the promise object [dcl.fct.def.coroutine] / 5.7.
First try to find a constructor with the same parameter list as the
original function (if it has params), failing that find a constructor
with no parameter list. */
First try to find a constructor with an argument list comprised of
the parameter copies. */
if (DECL_ARGUMENTS (orig_fn_decl))
{
@ -4917,19 +4913,27 @@ cp_coroutine_transform::build_ramp_function ()
else
r = NULL_TREE;
/* If that fails then the promise constructor argument list is empty. */
if (r == NULL_TREE || r == error_mark_node)
r = build_special_member_call (p, complete_ctor_identifier, NULL,
promise_type, LOOKUP_NORMAL,
tf_warning_or_error);
finish_expr_stmt (r);
/* If type_build_ctor_call() encounters deprecated implicit CTORs it will
return true, and therefore we will execute this code path. However,
we might well not actually require a CTOR and under those conditions
the build call above will not return a call expression, but the
original instance object. Do not attempt to add the statement unless
it has side-effects. */
if (r && r != error_mark_node && TREE_SIDE_EFFECTS (r))
finish_expr_stmt (r);
}
r = build_modify_expr (loc, coro_promise_live, boolean_type_node,
INIT_EXPR, loc, boolean_true_node,
boolean_type_node);
tree promise_dtor = cxx_maybe_build_cleanup (p, tf_warning_or_error);;
if (flag_exceptions && promise_dtor)
{
r = cp_build_init_expr (coro_promise_live, boolean_true_node);
finish_expr_stmt (r);
promise_dtor = cxx_maybe_build_cleanup (p, tf_warning_or_error);
}
tree get_ro