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1 RUN: lld-link -lldmingw %S/Inputs/inline-weak.o %S/Inputs/inline-weak2.o -out:%t.exe
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2
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3 When compiling certain forms of templated inline functions, some
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4 versions of GCC (tested with 5.4) produces a weak symbol for the function.
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5 Newer versions of GCC don't do this though.
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6
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7 The bundled object files are an example of that, they can be produced
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8 with test code like this:
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9
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10 $ cat inline-weak.h
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11 class MyClass {
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12 public:
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13 template<typename... _Args> int get(_Args&&... args) {
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14 return a;
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15 }
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16 private:
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17 int a;
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18 };
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19
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20 $ cat inline-weak.cpp
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21 #include "inline-weak.h"
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22
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23 int get(MyClass& a);
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24
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25 int main(int argc, char* argv[]) {
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26 MyClass a;
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27 int ret = a.get();
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28 ret += get(a);
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29 return ret;
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30 }
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31 extern "C" void mainCRTStartup(void) {
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32 main(0, (char**)0);
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33 }
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34 extern "C" void __main(void) {
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35 }
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36
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37 $ cat inline-weak2.cpp
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38 #include "inline-weak.h"
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39
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40 int get(MyClass& a) {
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41 return a.get();
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42 }
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43
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44 $ x86_64-w64-mingw32-g++ -std=c++11 -c inline-weak.cpp
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45 $ x86_64-w64-mingw32-g++ -std=c++11 -c inline-weak2.cpp
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46
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47 $ x86_64-w64-mingw32-nm inline-weak.o | grep MyClass3get
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48 0000000000000000 p .pdata$_ZN7MyClass3getIJEEEiDpOT_
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49 0000000000000000 t .text$_ZN7MyClass3getIJEEEiDpOT_
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50 0000000000000000 T .weak._ZN7MyClass3getIIEEEiDpOT_.main
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51 0000000000000000 r .xdata$_ZN7MyClass3getIJEEEiDpOT_
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52 w _ZN7MyClass3getIIEEEiDpOT_
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53 0000000000000000 T _ZN7MyClass3getIJEEEiDpOT_
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54
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55 $ x86_64-w64-mingw32-nm inline-weak2.o | grep MyClass3get
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56 0000000000000000 p .pdata$_ZN7MyClass3getIJEEEiDpOT_
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57 0000000000000000 t .text$_ZN7MyClass3getIJEEEiDpOT_
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58 0000000000000000 T .weak._ZN7MyClass3getIIEEEiDpOT_._Z3getR7MyClass
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59 0000000000000000 r .xdata$_ZN7MyClass3getIJEEEiDpOT_
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60 w _ZN7MyClass3getIIEEEiDpOT_
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61 0000000000000000 T _ZN7MyClass3getIJEEEiDpOT_
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62
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63 This can't be reproduced by assembling .s files with llvm-mc, since that
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64 always produces a symbol named .weak.<weaksymbol>.default, therefore
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65 the test uses prebuilt object files instead.
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66
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67 In these cases, the undefined weak symbol points to the regular symbol
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68 .weak._ZN7MyClass3getIIEEEiDpOT_.<othersymbol>, where <othersymbol>
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69 varies among the object files that emit the same function. This regular
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70 symbol points to the same location as the comdat function
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71 _ZN7MyClass3getIJEEEiDpOT_.
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72
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73 When linking, the comdat section from the second object file gets
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74 discarded, as it matches the one that already exists. This means that
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75 the uniquely named symbol .weak.<weakname>.<othername> points to a
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76 discarded section chunk.
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77
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78 Previously, this would have triggered adding an Undefined symbol for
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79 this case, which would later break linking. However, also previously,
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80 if the second object file is linked in via a static library, this
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81 leftover symbol is retained as a Lazy symbol, which would make the link
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82 succeed.
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