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Re: [Darwin] Patch c++/15428
On Thu, 20 May 2004 14:00:32 -0700, Matt Austern <austern@apple.com> wrote:
> On May 20, 2004, at 11:57 AM, Jason Merrill wrote:
>
>> On Thu, 20 May 2004 11:38:11 -0700, Matt Austern <austern@apple.com>
>> wrote:
>>
>>>>> explicitly_instantiate_members =
>>>>> ! !TARGET_WEAK_NOT_IN_ARCHIVE_TOC
>>>>> && previous_instantiation_extern_p && ! extern_p
>>>>> && ! TYPE_FOR_JAVA (t);
>>
>>> OK, so here' s a specific code example. Suppose we have a header file
>>> x.h
>>> with the following declarations:
>>> template <typename T> struct X { void foo(); };
>>> template <typename T> void X<T>::foo() { };
>>> extern template struct X<int>;
>>>
>>> Now we include that header file into two source files, a.cc and b.cc. In
>>> a.cc we use X<int>::foo(), but we don't have an explicit instantiation.
>>> In b.cc we do have an explicit instantiation. a.o will have an
>>> unresolved reference to _ZN1XIiE3fooEv, which is expected to be defined
>>> elsewhere. b.o will have a definition. The point is that we need to
>>> make sure that the definition of _ZN1XIiE3fooEv in b.o isn't weak. So we
>>> do two things. First, we do explicit instantiations, not implicit, for
>>> member functions of explicitly instantiated class templates.
>>
>> My issue is twofold:
>>
>> 1) It seems to me that the above code would accomplish this for your
>> testcase without checking TARGET_WEAK_IN_ARCHIVE_TOC; since the previous
>> instantiation was "extern", this one will be explicit.
>
> I believe the check for the macro is necessary. We know that the
> instantiation of this class template is explicit, but we still have to
> decide whether that means that the members should be instantiated
> explicitly or implicitly.
Yes.
> On most targets we deliberately decided to do the latter.
Yes.
> On Darwin we need to do the former.
I still don't see why. You explained above why this is necessary if
there's a previous extern instantiation, but in that case we already do
explicit instantiations on all targets.
Jason