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

[Eli Friedman]

On Tue, Nov 13, 2012 at 5:19 PM, David Blaikie <dblaikie at gmail.com> wrote:
> +patch
>
> On Tue, Nov 13, 2012 at 5:19 PM, David Blaikie <dblaikie at gmail.com> wrote:
>> On Fri, Nov 9, 2012 at 5:38 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>> Comments below.
>>>
>>> On Fri, Nov 9, 2012 at 5:25 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>> On Fri, Nov 9, 2012 at 5:25 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>>> 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.
>>>
>>> It might not matter here, because all decls contained in a RecordDecl
>>> generally have the same parent... this has a bigger impact for
>>> NamespaceDecl, for example, because there are two different Decls
>>> which are semantically the same context.
>>
>> Thanks for the explanation.
>>
>>>
>>>>>>  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)
>>>>
>>>> (now with the actually up-to-date patch)
>>>
>>> @@ -2582,8 +2581,9 @@ void
>>> TagDecl::setTypedefNameForAnonDecl(TypedefNameDecl *TDD) {
>>>  void TagDecl::startDefinition() {
>>>    IsBeingDefined = true;
>>>
>>> -  if (isa<CXXRecordDecl>(this)) {
>>> -    CXXRecordDecl *D = cast<CXXRecordDecl>(this);
>>> +  getASTContext().addUnnamedTag(this);
>>>
>>> Will getTypedefNameForAnonDecl actually return the correct result
>>> here?
>>
>> Seems not - thanks for the catch.
>>
>>> (I'd like to see a test to make sure that "typedef struct {} x"
>>> doesn't cause us to increment the mangling number, assuming that's
>>> consistent with gcc.)
>>
>> Test case added & then I had to restructure the code to address this.
>> Since we don't know which way a type will be mangled until after we've
>> parsed all the declarators in the group (eg: typedef struct{} *x, *y,
>> z; - only when we see 'z' do we know that we have a linkage name for
>> the struct, if we only have x and y, then we don't have such a name &
>> we need to use the UtX_ naming) I had to inject the logic into
>> FinalizeDeclaratorGroup (or somewhere near there - perhaps there's a
>> better spot). This means we miss the original test case failure of:
>> struct foo { enum { X }; }; since there is no declarator group in tht
>> instance, just a standalone decl. So I added this to the
>> ParsedFreeStandingDeclSpec as well.
>>
>> If there's some more appropriate common point to put this logic, I'm
>> all ears - but it seems to me there's no common point in Sema for
>> these two cases so it's either duplicated or could be non-duplicated
>> if it were up in the Parser (or had a new Sema entry point to call
>> from there).
>>
>> Open to ideas - thanks again for the help/review/pointers.

That seems reasonable.

+void ASTContext::addUnnamedTag(const TagDecl *Tag) {
+  if (!Tag->getName().empty() || Tag->getTypedefNameForAnonDecl() ||
+      !isa<CXXRecordDecl>(Tag->getParent()) || Tag->getLinkage() !=
ExternalLinkage)
+    return;

Why do we need to enforce that the parent is a CXXRecordDecl? I think
you can run into the same issue in other situations, no?  (Or were you
intentionally putting that off to another patch?  I forget.)

+  std::pair<llvm::DenseMap<const DeclContext *, unsigned>::iterator,
bool> P = UnnamedMangleContexts.insert(std::make_pair(Tag->getParent(),
0));

80 columns.


I don't see any other issues.

-Eli