Michael137 wrote:
> Some of the libc++ tests seem to be crashing on the x86_64 bot (can repro
> locally, on aarch64 too). Looks like they're segfaulting trying to access the
> member of an empty class. E.g., in
>
> ```
> Process 1370440 stopped
> * thread #1, name = 't.tmp.exe', stop reason = signal SIGSEGV: invalid
> address (fault address: 0x20)
> frame #0: 0x00005555555560c3
> t.tmp.exe`std::__1::operator==[abi:se190000](__x=0x00007fffffffdd10,
> __y=0x00007fffffffdcd0) at error_code.h:97:25
> 94 }
> 95
> 96 inline _LIBCPP_HIDE_FROM_ABI bool operator==(const error_code& __x,
> const error_condition& __y) _NOEXCEPT {
> -> 97 return __x.category().equivalent(__x.value(), __y) ||
> __y.category().equivalent(__x, __y.value());
> 98 }
> 99
> 100 #if _LIBCPP_STD_VER <= 17
> (lldb) dis -pc
> t.tmp.exe`std::__1::operator==[abi:se190000](std::__1::error_code const&,
> std::__1::error_condition const&):
> -> 0x5555555560c3 <+51>: callq *0x20(%rax)
> 0x5555555560c6 <+54>: movb %al, %cl
> 0x5555555560c8 <+56>: movb $0x1, %al
> 0x5555555560ca <+58>: testb $0x1, %cl
> (lldb)
> ```
>
> Investigating..
Reduced it to:
```
// We only crash if bar::func is virtual and Holder has a constexpr constructor
struct bar {
virtual void func() const {}
};
struct Holder {
bar m_bar;
constexpr explicit Holder() : m_bar() {}
~Holder() {}
};
const bar &foo() noexcept {
static Holder holder;
return holder.m_bar;
}
int main() {
foo().func();
return 0;
}
```
What was happening is that `Holder` was being lowered to:
```
internal global %struct.Holder undef, align 8
```
instead of:
```
internal global %struct.Holder { %struct.bar { ptr getelementptr inbounds
inrange(-16, 8) ({ [3 x ptr] }, ptr @_ZTV3bar, i32 0, i32 0, i32 2) } }, align 8
```
This was happening because `isZeroSize` considers types with virtual
bases/functions to not be zero-size (whereas the `isEmptyFieldForLayout` didn't
account for that. Fixed this in the latest commit by checking whether the
`CXXRecordDecl` is polymorphic
https://github.com/llvm/llvm-project/pull/96422
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