Move isDSOLocal check and add a comment

[sfertile]

This LGTM.

lib/Target/TargetMachine.cpp

117

minor nit: it might not be the front-end that set the value, the lto backends could refine a GV to dso_local using the symbol resolutions the linker provides.

[• rafael]

r316920

lib/Target/TargetMachine.cpp

static TLSModel::Model getSelectedTLSModel(const GlobalValue *GV) {
   case GlobalVariable::LocalExecTLSModel:
     return TLSModel::LocalExec;
   }
   llvm_unreachable("invalid TLS model");
 }
 
 bool TargetMachine::shouldAssumeDSOLocal(const Module &M,
                                          const GlobalValue *GV) const {
+  // If the front end requested that this GV be treated as dso local, obey.
+  if (GV && GV->isDSOLocal())
+    return true;
+
+  // According to the llvm language reference, we should be able to just return
+  // false in here if we have a GV, as we know it is dso_preemptable.
+  // At this point in time, the frontends have not been transitioned to always
+  // produce a dso_local when it is possible to do so. As a result we still have
+  // some pre-dso_local logic in here to improve the quality of the generated
+  // code:
+
   Reloc::Model RM = getRelocationModel();
   const Triple &TT = getTargetTriple();
 
   // DLLImport explicitly marks the GV as external.
   if (GV && GV->hasDLLImportStorageClass())
     return false;
 
   // Every other GV is local on COFF.
   // Make an exception for windows OS in the triple: Some firmwares builds use
   // *-win32-macho triples. This (accidentally?) produced windows relocations
   // without GOT tables in older clang versions; Keep this behaviour.
   if (TT.isOSBinFormatCOFF() || (TT.isOSWindows() && TT.isOSBinFormatMachO()))
     return true;
 
   // Most PIC code sequences that assume that a symbol is local cannot
   // produce a 0 if it turns out the symbol is undefined. While this
   // is ABI and relocation depended, it seems worth it to handle it
   // here.
   // FIXME: this is probably not ELF specific.
   if (GV && isPositionIndependent() && TT.isOSBinFormatELF() &&
       GV->hasExternalWeakLinkage())
     return false;
 
-  if (GV && (GV->hasLocalLinkage() || !GV->hasDefaultVisibility() ||
-              GV->isDSOLocal()))
+  if (GV && (GV->hasLocalLinkage() || !GV->hasDefaultVisibility()))
     return true;
 
   if (TT.isOSBinFormatMachO()) {
     if (RM == Reloc::Static)
       return true;
     return GV && GV->isStrongDefinitionForLinker();
   }