| Issue |
144708
|
| Summary |
CLang MSVC compatibility bug in return by register.
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
technotopia
|
Call from MSVC compiled code to CLang compiled method can result in access violation.
Condition: returned object is OK to be returned by register according to MSVC, but CLang assumes it should return a pointer on stack. Condition is described here:
https://learn.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170
POD object that fits size limitation and has no user defined constructor/destructor et.c
However, there is a problem with return of a class that has _default_ constructor and destructor:
```
//clang
class inlines_float_2
{
public:
inlines_float_2() = default;
~inlines_float_2() = default;
float val1;
float val2;
void set(float v1, float v2) { val1 = v1; val2 = v2; }
bool less() const { return val1 < val2; }
bool in(float v) const { return (val1 < v) && (val2 > v); }
};
```
```
//msvc
std::cout << "going to make 2 float struct with added default constructor/destructor. Should be bug:" << std::endl;
auto if2 = factory::make_in_f2(dummy, v);
std::cout << "got " << if2.val1 << ", " << if2.val2 << std::endl;
```
Disassembly of MSVC code, it expects return value in RAX:
<!-- Failed to upload "MSVC_CLang_rvalue_compatibility.zip" -->
Disassembly of CLang code, it expects arguments but tries to access uninitialized data.
> inlines_float_2 factory::make_in_f2(const std::vector<uint8_t>& dummy, const std::vector<float>& v)
> {
> 00007FFFF2DA1270 sub rsp,48h
> 00007FFFF2DA1274 mov qword ptr [rsp+20h],rcx
> 00007FFFF2DA1279 mov rax,rcx
> 00007FFFF2DA127C mov qword ptr [rsp+28h],rax
> 00007FFFF2DA1281 mov qword ptr [rsp+40h],rcx
> 00007FFFF2DA1286 mov qword ptr [rsp+38h],r8
> 00007FFFF2DA128B mov qword ptr [rsp+30h],rdx
---
Build options for MSVC project:
/JMC /permissive- /ifcOutput "x64\Debug\" /GS /W3 /Gy /Zc:wchar_t /I"C:\dev\MSVC_CLang_rvalue_compatibility\" /ZI /Gm- /Od /sdl /Fd"x64\Debug\vc143.pdb" /Zc:inline /D "_DEBUG" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" /errorReport:prompt /WX- /Zc:forScope /RTC1 /arch:AVX2 /Gd /MDd /std:c++20 /FC /Fa"x64\Debug\" /EHsc /nologo /Fo"x64\Debug\" /Fp"x64\Debug\MSVC_CLang_rvalue_compatibility.pch" /diagnostics:column
Build options for CLang project:
/MP /GS /W3 /Gy /Zi /Od /D "_DEBUG" /D "CLANGLIB_EXPORTS" /D "_WINDOWS" /D "_USRDLL" /D "DATA_EXPORTS" /D "_WINDLL" /D "_UNICODE" /D "UNICODE" /WX- /arch:AVX2 /Gd /MDd /std:c++20 /Fa"x64\Debug\" /EHa /nologo /Fo"x64\Debug\" /Fp"x64\Debug\clang_lib.pch" /diagnostics:column
WinSDK: 10.0.19041.0
Built under Visual Studio Version 17.2.23
---
Bottom line: I'm not sure which compiler is wrong as having default constructor and destructor is not a clear case in standard - whether it counts as POD or not.
---
Attaching a simple solution that reproduces the crash
<!-- Failed to upload "MSVC_CLang_rvalue_compatibility.zip" -->
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