[cfe-dev] Mangling & linkage of unnamed but instantiated nested classes
David Blaikie
dblaikie at gmail.com
Tue Nov 13 17:19:57 PST 2012
+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.
>
> - David
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