On Thu, Jan 13, 2022 at 10:54:15AM +0100, Richard Biener wrote:
> > The following patch avoids emitting a parameter copy statement when inlining
> > if the parameter has empty type. E.g. the gimplifier does something similar
> > (except that it needs to evaluate side-effects if any, which isn't the case
> > here):
> > /* For empty types only gimplify the left hand side and right hand
> > side as statements and throw away the assignment. Do this after
> > gimplify_modify_expr_rhs so we handle TARGET_EXPRs of addressable
> > types properly. */
> > if (is_empty_type (TREE_TYPE (*from_p))
> > && !want_value
> > /* Don't do this for calls that return addressable types, expand_call
> > relies on those having a lhs. */
> > && !(TREE_ADDRESSABLE (TREE_TYPE (*from_p))
> > && TREE_CODE (*from_p) == CALL_EXPR))
> > {
> > gimplify_stmt (from_p, pre_p);
> > gimplify_stmt (to_p, pre_p);
> > *expr_p = NULL_TREE;
> > return GS_ALL_DONE;
> > }
> > Unfortunately, this patch doesn't cure the uninit warnings in that PR,
> > but I think is desirable anyway.
> >
> > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> Hmm, but not emitting the initialization might cause even more such
> warnings for the case where the passed in argument _is_ initialized
> (or not visible as not, like when being a function parameter itself)?
Most of the time it won't be initialized either, but sure, there
can be some cases like when a larger struct is initialized with memset
and then we pass a field from that as an argument.
> Otherwise sure, it's the same what the gimplifier does.
>
> I wonder if instead uninit warning should simply ignore uses of
> "empty" typed variables?
Apparently it does already:
/* Avoid warning about empty types such as structs with no members.
The first_field() test is important for C++ where the predicate
alone isn't always sufficient. */
tree rhstype = TREE_TYPE (rhs);
if (POINTER_TYPE_P (rhstype))
rhstype = TREE_TYPE (rhstype);
if (is_empty_type (rhstype))
return NULL_TREE;
Though, the above
if (POINTER_TYPE_P (rhstype))
rhstype = TREE_TYPE (rhstype);
is just extremely suspicious, either we care about what type rhs has,
or it is dereferenced and it must be a pointer type and we care about
what it points to, but the simple fact whether rhs has a pointer type
or some other type shouldn't change what we test is_empty_type on.
When I was briefly looking at the assignment on which it actually warned,
it actually looked not empty type related.
Jakub
