[lld] Incorrectly Internalizing a symbol defined through explicit template instantiation when using '-flto -fuse-ld=lld'

[pranav-159]

When creating a shared library of which there are two files with implicit and explicit template instantiations of a particular class template respectively. If the file with the implicit instantiation comes first at linking, then the member functions get internalized by lld. (If the order is reverse i.e, explicit instantiation file comes first then the member functions won't get internalized.)
Note: When using llvm-gold plugin this issue is not seen.
In the reproducer. We created a shared library as mentioned above and linked it with an executable with extern template which causes an undefined symbol error as the required functions are internalized by lld when creating the shared library, which is not expected of the linker

[pranav-159]

This issue seems to occur after refactoring the symbol resolution code in lld ELF ac2911, 9e6840

[pranav-159]

Can this issue be resolved by changing how Comdat symbols are created and resolved in lld/ELF/InputFiles.cpp, createBitcodeSymbol function as Defined rather than Undefined as shown below?

diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index 7adc35f20984..eab63ee9b7a0 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -1748,12 +1748,20 @@ createBitcodeSymbol(Symbol *&sym, const std::vector<bool> &keptComdats,
     sym = symtab.insert(saver().save(objSym.getName()));

   int c = objSym.getComdatIndex();
-  if (objSym.isUndefined() || (c != -1 && !keptComdats[c])) {
+  if (objSym.isUndefined()) {
     Undefined newSym(&f, StringRef(), binding, visibility, type);
     sym->resolve(newSym);
     sym->referenced = true;
     return;
   }
+  if ((c != -1 && !keptComdats[c])) {
+    Defined newSym(&f, StringRef(), binding, visibility, type, 0, 0, nullptr);
+    if (objSym.canBeOmittedFromSymbolTable())
+      newSym.exportDynamic = false;
+    sym->resolve(newSym);
+    sym->referenced = true;
+    return;
+  }

   if (objSym.isCommon()) {
     sym->resolve(CommonSymbol{&f, StringRef(), binding, visibility, STT_OBJECT,

[llvmbot]

@llvm/issue-subscribers-lld-elf

Author: None (pranav-159)

When creating a shared library of which there are two files with implicit and explicit template instantiations of a particular class template respectively. If the file with the implicit instantiation comes first at linking, then the member functions get internalized by lld. (If the order is reverse i.e, explicit instantiation file comes first then the member functions won't get internalized.) Note: When using llvm-gold plugin this issue is not seen. In the [reproducer](https://godbolt.org/z/7nov5Tc84). We created a shared library as mentioned above and linked it with an executable with extern template which causes an undefined symbol error as the required functions are internalized by lld when creating the shared library, which is not expected of the linker