| Issue |
161039
|
| Summary |
LTO breaks importing TLS from shared libraries on MinGW, making libstdc++ unusable
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
HolyBlackCat
|
Consider those files:
```cpp
// 1.cpp
extern thread_local int x;
int main() {return x;}
```
```cpp
// 2.cpp
thread_local int x;
```
If you do following on MinGW (e.g. in MSYS2):
```cpp
clang++ -fuse-ld=lld 2.cpp -shared -o lib2.dll
clang++ -fuse-ld=lld 1.cpp -l2 -L. -flto
```
Then you get one of the following two errors. If `-femulated-tls` is used (as is necessary for libstdc++, default in MSYS2 MINGW64 and MSYS2 CLANG64):
```console
ld.lld: error: undefined symbol: x
>>> referenced by 1.cpp
>>> /tmp/1-061a94.o
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
```
Or if `-fno-emulated-tls` is used: (default in MSYS2 CLANG64)
```console
ld.lld: error: undefined symbol: x
>>> referenced by 1.cpp
>>> /tmp/1-061a94.o
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
```
This bug makes libstdc++ unusable with LTO on MinGW, because it uses `thread_local` variables for `std::once_flag` (if you use this feature, that is):
```cpp
// 3.cpp
#include <mutex>
int main() {std::once_flag f; std::call_once(f, []{});}
```
Compiling with `clang++ -fuse-ld=lld 3.cpp -flto` I get:
```console
ld.lld: error: undefined symbol: std::__once_callable
>>> referenced by 3.cpp
>>> /tmp/3-c926bc.o
ld.lld: error: undefined symbol: std::__once_call
>>> referenced by 3.cpp
>>> /tmp/3-c926bc.o
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
```
![]()
_______________________________________________
llvm-bugs mailing list
[email protected]
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
