[cfe-dev] Mangling & linkage of unnamed but instantiated nested classes

[David Blaikie]

On Mon, Nov 5, 2012 at 12:29 AM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Sun, Nov 4, 2012 at 9:20 PM, David Blaikie <dblaikie at gmail.com> wrote:
>> On Wed, Oct 31, 2012 at 2:19 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>> On Wed, Oct 31, 2012 at 12:37 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>> Hi John, (& cfe-dev)
>>>>
>>>> I mentioned this in person last week & wanted to provide you with some
>>>> more details & ask for your opinion.
>>>>
>>>> Backstory:
>>>>
>>>> I originally came across this while trying to use
>>>> -Wunused-member-function & found it was flagging code like this:
>>>>
>>>> struct foo {
>>>>   struct {
>>>>     void func() { ... } // warning that this is 'unused'
>>>>   } x;
>>>> };
>>>>
>>>> This surprised me, so I looked around & found that this function has
>>>> "no linkage". This seemed strange (because I would expect to be able
>>>> to call the function from multiple TUs that included the header
>>>> defining 'foo', compare its address for equality, & such things) & it
>>>> looks like the Right Thing is for func() to have the same linkage as,
>>>> say, an inline member function of 'foo' would have.
>>>>
>>>> The standard (3.5[basic.link]) seems to "miss" this case depending on
>>>> how you read it:
>>>>
>>>> 1) arguably p2, which says that "A name is said to have linkage when
>>>> it might denote the same ... function ... as a name introduced by a
>>>> declaration in another scope: - When a name has external linkage, the
>>>> entity it denotes can be referred to by names from scopes of other
>>>> translation units or from other scopes of the same translation unit"
>>>>
>>>> 2) p5: "... a member function ... has external linkage if the name of
>>>> the class has external linkage" (with an exception only for unnamed
>>>> classes (& enumerations) defined in class-scope typedef declarations
>>>> such that the class or enumeration has the dypedef name for linkage
>>>> purposes (7.1.3)) & there are no rules that seem to govern the linkage
>>>> of this unnamed class.
>>>>     p8 "Names not covered by these rules have no linkage"
>>>
>>> [basic.link]p8: "A type is said to have linkage if and only if [...]
>>> it is an unnamed class or enumeration member of a class with linkage".
>>>
>>> I'm going to assume it's an oversight in the standard that the members
>>> of such an unnamed class don't have linkage.
>>>
>>>> The next step was also to look at the mangling compared to GCC. After
>>>> modifying the linkage of functions like this (with this change):
>>>>
>>>>
>>>> --- lib/AST/Decl.cpp
>>>> +++ lib/AST/Decl.cpp
>>>> @@ -496,8 +496,7 @@ static LinkageInfo getLVForClassMember(const
>>>> NamedDecl *D, bool OnlyTemplate) {
>>>>    if (!(isa<CXXMethodDecl>(D) ||
>>>>          isa<VarDecl>(D) ||
>>>>          isa<FieldDecl>(D) ||
>>>> -        (isa<TagDecl>(D) &&
>>>> -         (D->getDeclName() || cast<TagDecl>(D)->getTypedefNameForAnonDecl()))))
>>>> +        isa<TagDecl>(D)))
>>>>      return LinkageInfo::none();
>>>>
>>>>    LinkageInfo LV;
>>>>
>>>> (& Richard reckons we might be able to simplify that check - just to
>>>> eliminate some template cases that still seem to get filtered out by
>>>> it)
>>>
>>> This looks like a step in the right direction.
>>>
>>>> I caused one test to fail:
>>>> test/CodeGenCXX/template-anonymous-types.cpp. A modified version (to
>>>> better investigate GCC 4.7's mangling) looks like this:
>>>>
>>>> struct S {
>>>>   enum { FOO = 42 };
>>>>   enum { BAR = 42 };
>>>> };
>>>>
>>>> struct T {
>>>>   enum { FOO = 42 };
>>>>   enum { BAR = 42 };
>>>> };
>>>>
>>>> template <typename T> struct X {
>>>>   T value;
>>>>   X(T t) : value(t) {}
>>>>   int f() { return value; }
>>>> };
>>>>
>>>> template <typename T> int f(T t) {
>>>>   X<T> x(t);
>>>>   return x.f();
>>>> }
>>>>
>>>> void test() {
>>>>   (void)f(S::FOO);
>>>>   (void)f(S::BAR);
>>>>   (void)f(T::FOO);
>>>>   (void)f(T::BAR);
>>>> }
>>>>
>>>> & with my change we get the right linkage for the instantiations of
>>>> 'f' (linkonce_odr instead of internal) but the mangling is still
>>>> inconsistent with GCC at least. Clang mangles these 4 'f's as:
>>>>
>>>> _Z1fIN1S3$_0EEiT_
>>>> _Z1fIN1S3$_1EEiT_
>>>> _Z1fIN1T3$_2EEiT_
>>>> _Z1fIN1T3$_3EEiT_
>>>>
>>>> GCC 4.7 mangles them as:
>>>>
>>>> _Z1fIN1SUt_EEiT_
>>>> _Z1fIN1SUt0_EEiT_
>>>> _Z1fIN1TUt_EEiT_
>>>> _Z1fIN1TUt0_EEiT_
>>>>
>>>> I don't think we have any class-specific unnamed nested type counter
>>>> that would implement that Ut_, Ut0_, ... mangling scheme, though I can
>>>> imagine where one might be added (I'm not very familiar with IRGen
>>>> though, so I'll certainly be happy to have any pointers about how that
>>>> could/should be implemented).
>>>
>>> We have a similar scheme already implemented for lambdas; look at
>>> getLambdaManglingNumber() etc.
>>
>> Thanks for the pointer. Following the lines there I've had a first
>> blush at this (see attached).
>>
>> I've not addressed some of the issues Richard brought up but I'd be
>> happy to take the time to generalize this to handle.

Spent a little time trying to generalize this to handle the function
local case but it was taking a bit too much of my time so I figured
I'd at least get this case handled (which addresses my issue of
-Wunused-function & generally improves the world a little bit at
least).

> +  if (!Tag->getName().empty() || Tag->getTypedefNameForAnonDecl() ||
> +      !isa<CXXRecordDecl>(Tag->getParent()))
> +    return -2;
> +
> +  std::pair<llvm::DenseMap<const DeclContext *, int>::iterator, bool>
> P = UnnamedMangleContexts.insert(std::make_pair(Tag->getParent(),
> -1));
>
> getParent() doesn't return a canonical DeclContext.

I'm not entirely sure what bugs this might cause (test cases welcome)
but I believe I've addressed this in the newly attached patch.

>  Also, 80 columns.

Right, sorry, I was mostly looking to see if I was generally on the
right track. Fixed, though.

>  Also, we probably don't want to generate numbers for a decl unless
> the mangling is actually externally visible.

Done.

>
> --- include/clang/AST/Decl.h
> +++ include/clang/AST/Decl.h
> @@ -2486,6 +2486,8 @@ private:
>    /// otherwise, it is a null (TypedefNameDecl) pointer.
>    llvm::PointerUnion<TypedefNameDecl*, ExtInfo*> TypedefNameDeclOrQualifier;
>
> +  int UnnamedManglingNumber;
> +
>    bool hasExtInfo() const { return TypedefNameDeclOrQualifier.is<ExtInfo*>(); }
>    ExtInfo *getExtInfo() { return TypedefNameDeclOrQualifier.get<ExtInfo*>(); }
>    const ExtInfo *getExtInfo() const {
>
> We don't really want to add an extra member to DeclContext just to
> handle an obscure edge case.

Used a separate side table in the ASTContext to keep the numbers. (& I
changed the numbers to be simpler - counting from zero instead of -1,
then doing an offset to compute the actual mangle number. Let me know
if you'd prefer it to work the other way)

- David
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