C++ compiler diagnostic gone horribly wrong: error: explicit specialization in non-namespace scope

September 23, 2022 C/C++ development and debugging. , , , , , , , ,

Here is a g++ error message that took me an embarrassingly long time to figure out:

In file included from /home/llvm-project/llvm/lib/IR/Constants.cpp:15:
/home/llvm-project/llvm/lib/IR/LLVMContextImpl.h:447:11: error: explicit specialization in non-namespace scope ‘struct llvm::MDNodeKeyImpl<llvm::DIBasicType>’
 template <> struct MDNodeKeyImpl<DIStringType> {

This is the code:

template <> struct MDNodeKeyImpl<DIStringType> {
  unsigned Tag;
  MDString *Name;
  Metadata *StringLength;
  Metadata *StringLengthExp;
  Metadata *StringLocationExp;
  uint64_t SizeInBits;
  uint32_t AlignInBits;
  unsigned Encoding;

This specialization isn’t materially different than the one that preceded it:

template <> struct MDNodeKeyImpl<DIBasicType> {
  unsigned Tag;
  MDString *Name;
  MDString *PictureString;
  uint64_t SizeInBits;
  uint32_t AlignInBits;
  unsigned Encoding;
  unsigned Flags;
  Optional<DIBasicType::DecimalInfo> DecimalAttrInfo;

  MDNodeKeyImpl(unsigned Tag, MDString *Name, MDString *PictureString,
               uint64_t SizeInBits, uint32_t AlignInBits, unsigned Encoding,
                unsigned Flags,
                Optional<DIBasicType::DecimalInfo> DecimalAttrInfo)
      : Tag(Tag), Name(Name), PictureString(PictureString),
        SizeInBits(SizeInBits), AlignInBits(AlignInBits), Encoding(Encoding),
        Flags(Flags), DecimalAttrInfo(DecimalAttrInfo) {}
  MDNodeKeyImpl(const DIBasicType *N)
      : Tag(N->getTag()), Name(N->getRawName()), PictureString(N->getRawPictureString()), SizeInBits(N->getSizeInBits()),
        AlignInBits(N->getAlignInBits()), Encoding(N->getEncoding()),
        Flags(N->getFlags(), DecimalAttrInfo(N->getDecimalInfo()) {}

  bool isKeyOf(const DIBasicType *RHS) const {
    return Tag == RHS->getTag() && Name == RHS->getRawName() &&
           PictureString == RHS->getRawPictureString() &&
           SizeInBits == RHS->getSizeInBits() &&
           AlignInBits == RHS->getAlignInBits() &&
           Encoding == RHS->getEncoding() && Flags == RHS->getFlags() &&
           DecimalAttrInfo == RHS->getDecimalInfo();

  unsigned getHashValue() const {
    return hash_combine(Tag, Name, SizeInBits, AlignInBits, Encoding);

However, there is an error hiding above it on this line:

        Flags(N->getFlags(), DecimalAttrInfo(N->getDecimalInfo()) {}

i.e. a single missing brace in the initializer for the Flags member, a consequence of a cut and paste during rebase that clobbered that one character, when adding a comma after it.

It turns out that the compiler was giving me a hint that something was wrong before this in the message:

error: explicit specialization in non-namespace scope

as it states that the scope is:

‘struct llvm::MDNodeKeyImpl

which is the previous class definition. Inspection of the code made me think that the scope was ‘namespace llvm {…}’, and I’d gone looking for a rebase error that would have incorrectly terminated that llvm namespace scope. This is a classic example of not paying enough attention to what is in front of you, and going off looking based on hunches instead. I didn’t understand the compiler message, but in retrospect, non-namespace scope meant that something in that scope was incomplete. The compiler wasn’t smart enough to tell me that the previous specialization was completed due to the missing brace, but it did tell me that something was wrong in that previous specialization (which was explicitly named), and I didn’t look at that because of my “what the hell does that mean” reaction to the compilation error message.

In this case, I was building on RHEL8.3, which uses an ancient GCC toolchain. I wonder if newer versions of g++ fare better (i.e.: a message like “possibly unterminated brace on line …” would have been much nicer)? I wasn’t able to try with clang++ as I was building llvm+clang+lldb (V14), and had uninstalled all of the llvm related toolchain to avoid interference.

LLVM IR Null pointer constants and function pointers. A wild goose chase after a bad assumption.

March 30, 2017 clang/llvm , , , , , ,

With ELLCC, you can easily check out the LLVM IR for code like:

typedef void ( *f )( void );
void foo( void );

f bar() {
    return (f)foo;

That code is:

define nonnull void ()* @bar() local_unnamed_addr {
  ret void ()* @foo

declare void @foo()

I was trying to use @foo in a “struct” object, and was getting an error attempting this:

 llvm::CompositeType*, llvm::Value::ValueTy, llvm::ArrayRef<llvm::Constant*>):
 Assertion `V[I]->getType() == T->getTypeAtIndex(I) &&
 "Initializer for composite element doesn't match!"' failed.

After adding:


where it shows the whole function body of foo(), I thought that’s where the error was coming from, and that I needed some other method to obtain just “@foo”, a global variable reference to the function, and not the function body itself.

The actual story is much simpler. Here the LLVM code to generate the IR for a foo() with this interface:

// void foo(){ }
auto vt = m_builder.getVoidTy();
auto voidFuncVoidType = FunctionType::get( vt, false /* varargs */ );

Function *fooFunc = Function::Create(
    voidFuncVoidType, Function::InternalLinkage, "foo",
    m_module );
BasicBlock *fooBB =
    BasicBlock::Create( m_context, "", fooFunc );
m_builder.SetInsertPoint( fooBB );

My clue that the error is something else is that I am able to build a function that returns a foo function pointer:

// void(*)() bar() { return foo ; }
auto fpRetFuncType = FunctionType::get( voidFuncVoidType->getPointerTo(), false /* varargs */ );

Function *barFunc = Function::Create(
    fpRetFuncType, Function::ExternalLinkage, "bar",
    m_module );
BasicBlock *barBB =
    BasicBlock::Create( m_context, "", barFunc );
m_builder.SetInsertPoint( barBB );
m_builder.CreateRet( fooFunc );

The module at this point looks like:

define internal void @foo() {
   ret void

define void ()* @bar() {
   ret void ()* @foo

So why can I used fooFunc in a return statement, but don’t appear to be able to use it in a structure object? Here’s the code that created that structure type

// struct { int, void (*)(), char * }
auto i8t = m_builder.getInt8Ty();
auto i32t = m_builder.getInt32Ty();
std::vector<Type *> consStructMembers{
    i32t, voidFuncVoidType->getPointerTo(), i8t->getPointerTo()};
auto consStructType =
    StructType::create( m_context, consStructMembers, "" );

and my attempt to populate an object of this type:

// %struct { int, void (*)(), char * } = { 65535, foo, null };
auto consPriority = ConstantInt::get( i32t, 65535 );
auto consDataZero = ConstantInt::get( i8t->getPointerTo(), 0 );

std::vector<Constant *> v{consPriority, fooFunc, consDataZero};
Constant *g = ConstantStruct::get( consStructType, v );

The actual error was in the third struct member initialization, and had nothing to do with the function pointer value. In retrospect, this makes sense since llvm::Function is derived from llvm::Constant, so there shouldn’t logically be a mismatch there.

What actually fixed the error was simply:

auto consDataZero = ConstantPointerNull::get( i8t->getPointerTo() );

It appears that the numeric zero value isn’t the same thing as an LLVM ‘null’. With that corrected, my variable declaration is:

%"type 0x10ea0c0" { i32 65535, void ()* @foo, i8* null }

… so I should now be able to proceed with the actual task at hand.