[clang] NFC: Enhance comments in CodeGen for multiversion function support.

[tahonermann]

clang/lib/CodeGen/CodeGenModule.cpp

3771

This is a drive-by indentation fix.

[erichkeane]

clang/include/clang/Basic/Attr.td

2730

this comma doesn't read right to me, I don't think it is required.

[tahonermann]

Reworded a comment to address code review commentary.

[erichkeane]

Still LGTM.

clang/include/clang/Basic/Attr.td

def TargetClones : InheritableAttr {
   let Spellings = [GCC<"target_clones">];
   let Args = [VariadicStringArgument<"featuresStrs">];
   let Documentation = [TargetClonesDocs];
   let Subjects = SubjectList<[Function], ErrorDiag>;
   let AdditionalMembers = [{
     StringRef getFeatureStr(unsigned Index) const {
       return *(featuresStrs_begin() + Index);
     }
-    // 'default' is always moved to the end, so it isn't considered
-    // when mangling the index.
+    // Given an index into the 'featuresStrs' sequence, compute a unique
+    // ID to be used with function name mangling for the associated variant.
+    // This mapping is necessary due to a requirement that the mangling ID
+    // used for the "default" variant be the largest mangling ID in the
+    // variant set. Duplicate variants present in 'featuresStrs' are also
+    // assigned their own unique ID (the mapping is bijective).
     unsigned getMangledIndex(unsigned Index) const {
       if (getFeatureStr(Index) == "default")
         return std::count_if(featuresStrs_begin(), featuresStrs_end(),
                               [](StringRef S) { return S != "default"; });
 
       return std::count_if(featuresStrs_begin(), featuresStrs_begin() + Index,
                            [](StringRef S) { return S != "default"; });
     }
 
-    // True if this is the first of this version to appear in the config string.
-    // This is used to make sure we don't try to emit this function multiple
-    // times.
+    // Given an index into the 'featuresStrs' sequence, determine if the
+    // index corresponds to the first instance of the named variant. This
+    // is used to skip over duplicate variant instances when iterating over
+    // 'featuresStrs'.
     bool isFirstOfVersion(unsigned Index) const {
       StringRef FeatureStr(getFeatureStr(Index));
       return 0 == std::count_if(
                       featuresStrs_begin(), featuresStrs_begin() + Index,
                       [FeatureStr](StringRef S) { return S == FeatureStr; });
     }
   }];
 }

clang/lib/CodeGen/CodeGenModule.h

private:
   void addDeferredDeclToEmit(GlobalDecl GD) {
     DeferredDeclsToEmit.emplace_back(GD);
   }
 
   /// List of alias we have emitted. Used to make sure that what they point to
   /// is defined once we get to the end of the of the translation unit.
   std::vector<GlobalDecl> Aliases;
 
-  /// List of multiversion functions that have to be emitted.  Used to make sure
-  /// we properly emit the iFunc.
+  /// List of multiversion functions to be emitted. This list is processed in
+  /// conjunction with other deferred symbols and is used to ensure that
+  /// multiversion function resolvers and ifuncs are defined and emitted.
   std::vector<GlobalDecl> MultiVersionFuncs;
 
   typedef llvm::StringMap<llvm::TrackingVH<llvm::Constant> > ReplacementsTy;
   ReplacementsTy Replacements;
 
   /// List of global values to be replaced with something else. Used when we
   /// want to replace a GlobalValue but can't identify it by its mangled name
   /// anymore (because the name is already taken).

 
 private:
   llvm::Constant *GetOrCreateLLVMFunction(
       StringRef MangledName, llvm::Type *Ty, GlobalDecl D, bool ForVTable,
       bool DontDefer = false, bool IsThunk = false,
       llvm::AttributeList ExtraAttrs = llvm::AttributeList(),
       ForDefinition_t IsForDefinition = NotForDefinition);
 
+  // References to multiversion functions are resolved through an implicitly
+  // defined resolver function. This function is responsible for creating
+  // the resolver symbol for the provided declaration. The value returned
+  // will be for an ifunc (llvm::GlobalIFunc) if the current target supports
+  // that feature and for a regular function (llvm::GlobalValue) otherwise.
   llvm::Constant *GetOrCreateMultiVersionResolver(GlobalDecl GD,
                                                   llvm::Type *DeclTy,
                                                   const FunctionDecl *FD);
+
+  // In scenarios where a function is not known to be a multiversion function
+  // until a later declaration, it is sometimes necessary to change the
+  // previously created mangled name to align with requirements of whatever
+  // multiversion function kind the function is now known to be. This function
+  // is responsible for performing such mangled name updates.
   void UpdateMultiVersionNames(GlobalDecl GD, const FunctionDecl *FD,
                                StringRef &CurName);
 
   llvm::Constant *
   GetOrCreateLLVMGlobal(StringRef MangledName, llvm::Type *Ty, LangAS AddrSpace,
                         const VarDecl *D,
                         ForDefinition_t IsForDefinition = NotForDefinition);
 
private:
   /// __cxa_atexit, if it is available, or atexit otherwise.
   void registerGlobalDtorsWithAtExit();
 
   // When using sinit and sterm functions, unregister
   // __attribute__((destructor)) annotated functions which were previously
   // registered by the atexit subroutine using unatexit.
   void unregisterGlobalDtorsWithUnAtExit();
 
+  /// Emit deferred multiversion function resolvers and associated variants.
   void emitMultiVersionFunctions();
 
   /// Emit any vtables which we deferred and still have a use for.
   void EmitDeferredVTables();
 
   /// Emit a dummy function that reference a CoreFoundation symbol when
   /// @available is used on Darwin.
   void emitAtAvailableLinkGuard();

clang/lib/CodeGen/CodeGenModule.cpp

if (getLangOpts().OpenMPIsDevice && OpenMPRuntime &&
           GDDef = GlobalDecl(DD, GD.getDtorType());
         else
           GDDef = GlobalDecl(FDDef);
         EmitGlobal(GDDef);
       }
     }
 
     if (FD->isMultiVersion()) {
-        UpdateMultiVersionNames(GD, FD, MangledName);
       if (!IsForDefinition)
         return GetOrCreateMultiVersionResolver(GD, Ty, FD);
     }
   }
 
   // Lookup the entry, lazily creating it if necessary.
   llvm::GlobalValue *Entry = GetGlobalValue(MangledName);
   if (Entry) {