Change DwarfCompileUnit::createGlobalVariable to getOrCreateGlobalVariable.

[friss]

Summary

This will allow to request the creation of a forward delacred variable
at is point of use (for imported declarations, this will be
DwarfDebug::constructImportedEntityDIE) rather than having to put the
forward decl in a retention list.

Note that getOrCreateGlobalVariable returns the actual definition DIE when the
routine creates a declaration and a definition DIE. If you agree this is the
right behavior, then I'll have a followup patch that registers the definition
in the DIE map instead of the declaration as it is today (this 'breaks' only
one test, where we test that the imported entity is the declaration). I'm
not sure what's best here, but it's easy enough for a consumer to follow the
DW_AT_specification link to get to the declaration, whereas it takes more
work to find the actual definition from a declaration DIE.

[dblaikie]

Looks good.

I'm on the fence about whether declaration or definition is better - but as you say, definition is somewhat more expressive/useful for some possible consumer scenarios.

[friss]

Closed by commit rL218126 (authored by @friss).

llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

for (MDNode *N : CU_Nodes->operands()) {
     for (unsigned i = 0, e = ImportedEntities.getNumElements(); i != e; ++i)
       ScopesWithImportedEntities.push_back(std::make_pair(
           DIImportedEntity(ImportedEntities.getElement(i)).getContext(),
           ImportedEntities.getElement(i)));
     std::sort(ScopesWithImportedEntities.begin(),
               ScopesWithImportedEntities.end(), less_first());
     DIArray GVs = CUNode.getGlobalVariables();
     for (unsigned i = 0, e = GVs.getNumElements(); i != e; ++i)
-      CU.createGlobalVariableDIE(DIGlobalVariable(GVs.getElement(i)));
+      CU.getOrCreateGlobalVariableDIE(DIGlobalVariable(GVs.getElement(i)));
     DIArray SPs = CUNode.getSubprograms();
     for (unsigned i = 0, e = SPs.getNumElements(); i != e; ++i)
       SPMap.insert(std::make_pair(SPs.getElement(i), &CU));
     DIArray EnumTypes = CUNode.getEnumTypes();
     for (unsigned i = 0, e = EnumTypes.getNumElements(); i != e; ++i) {
       DIType Ty(EnumTypes.getElement(i));
       // The enum types array by design contains pointers to
       // MDNodes rather than DIRefs. Unique them here.

llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.h

public:
   DwarfCompileUnit(unsigned UID, DICompileUnit Node, AsmPrinter *A,
                    DwarfDebug *DW, DwarfFile *DWU);
 
   void initStmtList(MCSymbol *DwarfLineSectionSym);
 
   /// Apply the DW_AT_stmt_list from this compile unit to the specified DIE.
   void applyStmtList(DIE &D);
 
-  /// createGlobalVariableDIE - create global variable DIE.
-  void createGlobalVariableDIE(DIGlobalVariable GV);
+  /// getOrCreateGlobalVariableDIE - get or create global variable DIE.
+  DIE *getOrCreateGlobalVariableDIE(DIGlobalVariable GV);
 
   /// addLabelAddress - Add a dwarf label attribute data and value using
   /// either DW_FORM_addr or DW_FORM_GNU_addr_index.
   void addLabelAddress(DIE &Die, dwarf::Attribute Attribute,
                        const MCSymbol *Label);
 
   /// addLocalLabelAddress - Add a dwarf label attribute data and value using
   /// DW_FORM_addr only.

llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp

static const ConstantExpr *getMergedGlobalExpr(const Value *V) {
 
   // Third operand is offset.
   if (!isa<ConstantInt>(CE->getOperand(2)))
     return nullptr;
 
   return CE;
 }
 
-/// createGlobalVariableDIE - create global variable DIE.
-void DwarfCompileUnit::createGlobalVariableDIE(DIGlobalVariable GV) {
+/// getOrCreateGlobalVariableDIE - get or create global variable DIE.
+DIE *DwarfCompileUnit::getOrCreateGlobalVariableDIE(DIGlobalVariable GV) {
   // Check for pre-existence.
-  if (getDIE(GV))
-    return;
+  if (DIE *Die = getDIE(GV))
+    return Die;
 
   assert(GV.isGlobalVariable());
 
   DIScope GVContext = GV.getContext();
   DIType GTy = DD->resolve(GV.getType());
 
   // If this is a static data member definition, some attributes belong
   // to the declaration DIE.
if (isGlobalVariable) {
     addUInt(*Loc, dwarf::DW_FORM_data1, dwarf::DW_OP_constu);
     SmallVector<Value *, 3> Idx(CE->op_begin() + 1, CE->op_end());
     addUInt(*Loc, dwarf::DW_FORM_udata,
             Asm->getDataLayout().getIndexedOffset(Ptr->getType(), Idx));
     addUInt(*Loc, dwarf::DW_FORM_data1, dwarf::DW_OP_plus);
     addBlock(*VariableDIE, dwarf::DW_AT_location, Loc);
   }
 
+  DIE *ResultDIE = VariableSpecDIE ? VariableSpecDIE : VariableDIE;
+
   if (addToAccelTable) {
-    DIE &AddrDIE = VariableSpecDIE ? *VariableSpecDIE : *VariableDIE;
-    DD->addAccelName(GV.getName(), AddrDIE);
+    DD->addAccelName(GV.getName(), *ResultDIE);
 
     // If the linkage name is different than the name, go ahead and output
     // that as well into the name table.
     if (GV.getLinkageName() != "" && GV.getName() != GV.getLinkageName())
-      DD->addAccelName(GV.getLinkageName(), AddrDIE);
+      DD->addAccelName(GV.getLinkageName(), *ResultDIE);
   }
 
-  addGlobalName(GV.getName(), VariableSpecDIE ? *VariableSpecDIE : *VariableDIE,
-                GV.getContext());
+  addGlobalName(GV.getName(), *ResultDIE, GV.getContext());
+  return ResultDIE;
 }
 
 /// constructSubrangeDIE - Construct subrange DIE from DISubrange.
 void DwarfUnit::constructSubrangeDIE(DIE &Buffer, DISubrange SR, DIE *IndexTy) {
   DIE &DW_Subrange = createAndAddDIE(dwarf::DW_TAG_subrange_type, Buffer);
   addDIEEntry(DW_Subrange, dwarf::DW_AT_type, *IndexTy);
 
   // The LowerBound value defines the lower bounds which is typically zero for