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To: cygwin AT cygwin DOT com
Subject: Re: Request to update libc++ related packages for current
 Clang/LLVM toolchain
Date: Sun, 18 Jan 2026 00:35:37 +0000
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From: kikairoya via Cygwin <cygwin AT cygwin DOT com>
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2026年1月15日 21:53:21 (+09:00) で、Takashi Yano via Cygwin さんが書きました:


> I have just uploaded libcxx, compiler-rt, and libunwind packages.

> However, you cannot use them with -stdlib=libc++, -rtlib=compiler-rt,

> and -unwindlib=libunwind. This is because clang package has incomplete

> cygwin driver.

>

> You can link them to, for example hello.cc, with:

> clang++ -stdlib=libc++ -isystem /usr/include/c++/v1 hello.cc -lc++ $(clang --print-resource-dir)/lib/cygwin/libclang_rt.builtins-x86_64.a -lunwind

>

> The resulted a.exe will be free from libgcc_s. Please try.

>


Using compiler-rt instead of libgcc_s requires extra attention in Cygwin,
because the emulated TLS functionality implemented in compiler-rt can't
share the storage across DLL boundaries.
Particularly, LLVM itself isn't compatible with compiler-rt in an emulated
TLS environment; it exposes thread_local variables directly.


Reproducers (with 21.x packages):
---------- emutlstest.cc ----------
#include <thread>
#include <stdio.h>


extern thread_local int g_tls;
int *get_var();


#ifdef DLL
thread_local int g_tls = 3;
int *get_var() { return &g_tls; }


#endif
#ifdef EXE


void f() {
    void *exe = &g_tls;
    void *dll = get_var();
    printf("%d %p %p\n", exe == dll, exe, dll);
}
int main() {
    f();
    std::thread(f).join();
}


#endif
--------------------


libstdc++/libgcc_s case:
$ clang++ emutlstest.cc -o emudll.dll -DDLL=1 -shared && clang++ emutlstest.cc -o emuexe.exe -DEXE=1 emudll.dll && ./emuexe.exe
1 0xa000004c8 0xa000004c8
1 0xa00026958 0xa00026958


libstdc++/compiler-rt case:
$ clang++ -unwindlib=libunwind -rtlib=compiler-rt emutlstest.cc -o emudll.dll -DDLL=1 -shared && clang++ -unwindlib=libunwind -rtlib=compiler-rt emutlstest.cc -o emuexe.exe -DEXE=1 emudll.dll && ./emuexe.exe
0 0xa000004c8 0xa000004e8
0 0xa000268f8 0xa000269a8


libc++/compiler-rt case:
$ clang++ -stdlib=libc++ -unwindlib=libunwind -rtlib=compiler-rt emutlstest.cc -o emudll.dll -DDLL=1 -shared && clang++ -stdlib=libc++ -unwindlib=libunwind -rtlib=compiler-rt emutlstest.cc -o emuexe.exe -DEXE=1 emudll.dll && ./emuexe.exe
0 0xa000004c8 0xa000004e8
0 0xa00010a18 0xa00010ac8


just using libc++ case:
$ clang++ -stdlib=libc++ emutlstest.cc -o emudll.dll -DDLL=1 -shared && clang++ -stdlib=libc++ emutlstest.cc -o emuexe.exe -DEXE=1 emudll.dll && ./emuexe.exe
1 0xa000004c8 0xa000004c8
1 0xa00010a78 0xa00010a78


link to a libgcc_s-ed DLL case:
$ clang++ emutlstest.cc -o emudll.dll -DDLL=1 -shared && clang++ -stdlib=libc++ -unwindlib=libunwind -rtlib=compiler-rt emutlstest.cc -o emuexe.exe -DEXE=1 emudll.dll && ./emuexe.exe
0 0xa000004c8 0xa000004e8
0 0xa00010aa8 0xa00010be8


using a compiler-rt-ed DLL case:
$ clang++ -stdlib=libc++ -unwindlib=libunwind -rtlib=compiler-rt emutlstest.cc -o emudll.dll -DDLL=1 -shared && clang++ emutlstest.cc -o emuexe.exe -DEXE=1 emudll.dll && ./emuexe.exe
0 0xa000004c8 0xa000004e8
0 0xa00026a18 0xa00026ac8


-- 
Tomohiro Kashiwada (@kikairoya)

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