Compiler frontend segfault on explicit template instantiation

[llvmbot]

Bugzilla Link	48687
Resolution	FIXED
Resolved on	Feb 12, 2021 12:58
Version	11.0
OS	Windows NT
Reporter	LLVM Bugzilla Contributor
CC	@nico,@zygoloid,@rnk

Extended Description

The following minimal example is enough to reproduce the problem on clang 11.0, using the clang-cl interface on x86_64-pc-windows-msvc, regardless the optimization level.

// A.h

template
class A
{
public:

T value;

static constexpr auto address = &A<T>::value;

};

extern template class A;

// A.cpp

#include "A.h"

template class A;

[llvmbot]

assigned to @ZequanWu

[rnk]

Zequan, can you take a look at this? I reduced it a little bit:

$ cat t.cpp
template struct A {
T value;
static constexpr auto address = &A::value;
};
extern template class A;
template class A;

$ clang -cc1 t.cpp -triple x86_64-windows-msvc -emit-llvm
clang: /usr/local/google/home/rnk/llvm-project/clang/lib/AST/MicrosoftCXXABI.cpp:170: clang::MSInheritanceModel clang::CXXRecordDecl::getMSInheritanceModel() const: Assertion `IA && "Expected MSInheritanceAttr on the CXXRecordDecl!"' failed.
...
#​13 0x0000000007e2aaad clang::CXXRecordDecl::getMSInheritanceModel() const /usr/local/google/home/rnk/llvm-project/clang/lib/AST/MicrosoftCXXABI.cpp:169:27
#​14 0x0000000005bbe487 EmitMemberDataPointer /usr/local/google/home/rnk/llvm-project/clang/lib/CodeGen/MicrosoftCXXABI.cpp:2758:35
#​15 0x0000000005bbe487 (anonymous namespace)::MicrosoftCXXABI::EmitMemberPointer(clang::APValue const&, clang::QualType) /usr/local/google/home/rnk/llvm-project/clang/lib/CodeGen/MicrosoftCXXABI.cpp:2791:9
#​16 0x00000000058affe6 clang::CodeGen::ConstantEmitter::tryEmitPrivateForMemory(clang::APValue const&, clang::QualType) /usr/local/google/home/rnk/llvm-project/clang/lib/CodeGen/CGExprConstant.cpp:1680:11
#​17 0x00000000058afd81 markIfFailed /usr/local/google/home/rnk/llvm-project/clang/lib/CodeGen/ConstantEmitter.h:162:10
#​18 0x00000000058afd81 clang::CodeGen::ConstantEmitter::tryEmitForInitializer(clang::VarDecl const&)
...

Clang is trying to emit A::address as a global variable, but we haven't assigned an inheritance model to A. That is supposed to happen whenever a member pointer type using A is required to be complete, which should have happened some time earlier, perhaps in the explicit template instantiation.

See this code here that is supposed to do that:
https://github.com/llvm/llvm-project/blob/master/clang/lib/Sema/SemaType.cpp#L8513

[ZequanWu]

I'll take a look.

[ZequanWu]

The decl of "extern template class A" gets the MSInheritanceAttr during template instantiation, but the decl of "template class A" does not because template instantiation is skipped. We might need to merge/copy MSInheritanceAttr when parsing.

[rnk]

Got it. We've had issues like this in the past, I believe we try to place the attribute on either the canonical or the most recent decl. Your direction sounds right.

[llvmbot]

Possibly related, as it involves how the MSInheritanceModel is assigned to a type:

Given two translation units as,

TU1:
struct Point { float x, y, z; };
struct C {
using ptr_to_mem = float Point::*;
ptr_to_mem k;
};

and

TU2:

struct Point; // forward declaration
struct C {
using ptr_to_mem = float Point::*;
ptr_to_mem k;
};
struct Point { float x, y, z; };

In TU1, the sizeof(C) gives 4; in TU2, the sizeof(C) gives, instead, 12!

Note that the only difference is that TU2 contains a forward-declaration of the type Point.

[rnk]

Yes, this is a known issue with the MSVC++ ABI that occurred to us as we were implementing it. :)

This is part of why the -fcomplete-member-pointers flag exists.

[nico]

did https://reviews.llvm.org/D94646 fix this?

[ZequanWu]

Yes, it's fixed.