[cfe-dev] Catching Temporarily Used Types in Debug Info

[David Blaikie]

We have a few bugs like ( http://llvm.org/bugs/show_bug.cgi?id=19005 )
in debug info that all stem from the same basic problem:

A type that's not referenced by another debug entity (such as a
variable, parameter, function, etc) is not emitted in the debug info.

GCC, while not being wholely consistent at addressing this, does get
it 'more' right. I'd like Clang to be better at this as well, even if
we're not perfect.

The basic premise to implement this perfectly would be: If we're
emitting code for a function (or global variable initializer, etc) and
within that function a certain type is required to be complete, emit
the type (and include it in the "retained types list").

Does anyone have nice ideas on how we could realistically implement
that test (yeah, I'm mostly looking at Richard on this) or a rough
approximation that might get the 90% case?

And as a bonus it'd probably be good if we didn't do this for cases
where we already successfully emit the type (eg: if it's required to
be complete because there's a variable of that type we'd rather not
add that type to the retained types list needlessly... but there are
some wrinkles there)

Some examples:

Neither GCC nor Clang emit 'foo' here:

struct foo {
  foo();
  int i;
};

void func(int);

void func() {
  func(foo().i + 1);
}

int main() {
}

but GCC does emit it if the foo ctor called is not the default (
declare 'foo(int)' and use 'foo(3).i' instead of 'foo().i').

And the previously linked bug amounts to "func(((foo*)v)->i + 1);"
(for some "void* v") - in which GCC does emit 'foo' and Clang does
not.

So, yes - nothing terribly consistent. It would nice to do better
(better than we are, maybe even better than GCC, or at least more
consistent).