1RUN: lld-link -lldmingw %S/Inputs/inline-weak.o %S/Inputs/inline-weak2.o -out:%t.exe 2 3When compiling certain forms of templated inline functions, some 4versions of GCC (tested with 5.4) produces a weak symbol for the function. 5Newer versions of GCC don't do this though. 6 7The bundled object files are an example of that, they can be produced 8with test code like this: 9 10$ cat inline-weak.h 11class MyClass { 12public: 13 template<typename... _Args> int get(_Args&&... args) { 14 return a; 15 } 16private: 17 int a; 18}; 19 20$ cat inline-weak.cpp 21#include "inline-weak.h" 22 23int get(MyClass& a); 24 25int main(int argc, char* argv[]) { 26 MyClass a; 27 int ret = a.get(); 28 ret += get(a); 29 return ret; 30} 31extern "C" void mainCRTStartup(void) { 32 main(0, (char**)0); 33} 34extern "C" void __main(void) { 35} 36 37$ cat inline-weak2.cpp 38#include "inline-weak.h" 39 40int get(MyClass& a) { 41 return a.get(); 42} 43 44$ x86_64-w64-mingw32-g++ -std=c++11 -c inline-weak.cpp 45$ x86_64-w64-mingw32-g++ -std=c++11 -c inline-weak2.cpp 46 47$ x86_64-w64-mingw32-nm inline-weak.o | grep MyClass3get 480000000000000000 p .pdata$_ZN7MyClass3getIJEEEiDpOT_ 490000000000000000 t .text$_ZN7MyClass3getIJEEEiDpOT_ 500000000000000000 T .weak._ZN7MyClass3getIIEEEiDpOT_.main 510000000000000000 r .xdata$_ZN7MyClass3getIJEEEiDpOT_ 52 w _ZN7MyClass3getIIEEEiDpOT_ 530000000000000000 T _ZN7MyClass3getIJEEEiDpOT_ 54 55$ x86_64-w64-mingw32-nm inline-weak2.o | grep MyClass3get 560000000000000000 p .pdata$_ZN7MyClass3getIJEEEiDpOT_ 570000000000000000 t .text$_ZN7MyClass3getIJEEEiDpOT_ 580000000000000000 T .weak._ZN7MyClass3getIIEEEiDpOT_._Z3getR7MyClass 590000000000000000 r .xdata$_ZN7MyClass3getIJEEEiDpOT_ 60 w _ZN7MyClass3getIIEEEiDpOT_ 610000000000000000 T _ZN7MyClass3getIJEEEiDpOT_ 62 63This can't be reproduced by assembling .s files with llvm-mc, since that 64always produces a symbol named .weak.<weaksymbol>.default, therefore 65the test uses prebuilt object files instead. 66 67In these cases, the undefined weak symbol points to the regular symbol 68.weak._ZN7MyClass3getIIEEEiDpOT_.<othersymbol>, where <othersymbol> 69varies among the object files that emit the same function. This regular 70symbol points to the same location as the comdat function 71_ZN7MyClass3getIJEEEiDpOT_. 72 73When linking, the comdat section from the second object file gets 74discarded, as it matches the one that already exists. This means that 75the uniquely named symbol .weak.<weakname>.<othername> points to a 76discarded section chunk. 77 78Previously, this would have triggered adding an Undefined symbol for 79this case, which would later break linking. However, also previously, 80if the second object file is linked in via a static library, this 81leftover symbol is retained as a Lazy symbol, which would make the link 82succeed. 83