ModuleLinker: Import linkonce even when they have no use

[mehdi_amini]

Summary

If the linker decided that LTO is supposed to provide the
"Prevailing" definition for a symbol, and ask the symbol
to be "preserved", then we should honor it and make sure
the symbol is emitted.
Here we import useless definition in some cases, but we
can rely on GlobalDCE to delete them after (unless the
linker requested us to preserve them).

A bunch of tests are failing:

Failing Tests (5):

LLVM :: Linker/comdat12.ll
LLVM :: Linker/internalize-lazy.ll
LLVM :: Linker/only-needed-named-metadata.ll
LLVM :: Linker/pr21494.ll
LLVM :: tools/lto/hide-linkonce-odr.ll

So I'm not sure at this point this is the right fix, but I'm
eager to get feedback on how to solve this.

[• rafael]

So, I assume this is because of a mixed (MachO + BC) lto, right?

If so it seems the lib/LTO version of https://llvm.org/bugs/show_bug.cgi?id=19901.

This change as is would probably be undesirable for some users. I see two ways of solving a pr19901 like problem that are a bit more light weight:

• Add a new API to lib/LTO where the linker is required to pass *all* necessary symbols is a separate list. Pass that as GlobalsToImport.
• Add a flag that say that instead of the current "if and only if" handling of GlobalsToImport, it should be just "if". That is, anything in GlobalsToImport is imported, but so is anything that would be imported by the current logic. This should not require a new lib/LTO api.

lib/Linker/LinkModules.cpp

bool ModuleLinker::linkIfNeeded(GlobalValue &GV) {
   // double frees).
   if (GV.hasAppendingLinkage() && isPerformingImport())
     return false;
 
   if (isPerformingImport()) {
     if (!doImportAsDefinition(&GV))
       return false;
   } else if (!DGV && !shouldOverrideFromSrc() &&
-             (GV.hasLocalLinkage() || GV.hasLinkOnceLinkage() ||
-              GV.hasAvailableExternallyLinkage()))
+             (GV.hasAvailableExternallyLinkage()))
     return false;
 
   if (GV.isDeclaration())
     return false;
 
   if (const Comdat *SC = GV.getComdat()) {
     bool LinkFromSrc;
     Comdat::SelectionKind SK;