Re: A question about TYPE_ARG_TYPES

[Mark Mitchell <mark at codesourcery dot com>]

Kazu Hirata wrote:
> Hi Ian,
> 
>>> I keep finding places in GCC sources that check whether a member of
>>> TYPE_ARG_TYPES is 0.  For example,
>>>
>>>  for (link = TYPE_ARG_TYPES (function_or_method_type);
>>>       link && TREE_VALUE (link);
>>>       link = TREE_CHAIN (link))
>>>    gen_type_die (TREE_VALUE (link), context_die);
>>>
>>> Notice that TREE_VALUE (link) is part of the loop condition.
>>>
>>> Now, do we ever allow a NULL in TYPE_ARG_TYPES?  If so, what does that
>>> mean?  My guess is that soneone was trying to be cautious about
>>> encountering a NULL in TYPE_ARG_TYPES.  (If that's the case, we should
>>> be using gcc_assert instead.)
>>
>>
>> Just guessing here, but what happens with an old-style function
>> definition in C?
>>
>> void f();
> 
> AFAIK, that gets TYPE_ARG_TYPES (...) == NULL, so we don't even get to
> evaluate TREE_VALUE (TYPE_ARG_TYPES (...)).
> 
> On IRC, Daniel Berlin claims that there are some weird cases where
> TREE_VALUE (TYPE_ARG_TYPES (...)) is NULL.  I'll keep putting gcc_assert
> to see what happens.

That may be the difference between "void f()" (where TYPE_ARG_TYPES
might be NULL) and "void f(...)" (where TREE_VALUE (TYPE_ARG_TYPES)
would be NULL).  The latter, as Daniel says, is not valid C, but perhaps
we used to accept it.

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713