[PATCH] D122955: [clang] NFC: Enhance comments in CodeGen for multiversion function support.

Tom Honermann via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Tue Apr 5 11:41:04 PDT 2022

tahonermann updated this revision to Diff 420586.
tahonermann added a comment.

Reworded a comment to address code review commentary.

  rG LLVM Github Monorepo




Index: clang/lib/CodeGen/CodeGenModule.h
--- clang/lib/CodeGen/CodeGenModule.h
+++ clang/lib/CodeGen/CodeGenModule.h
@@ -348,8 +348,9 @@
   /// 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;
@@ -1466,9 +1467,20 @@
       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);
@@ -1561,6 +1573,7 @@
   // 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.
Index: clang/lib/CodeGen/CodeGenModule.cpp
--- clang/lib/CodeGen/CodeGenModule.cpp
+++ clang/lib/CodeGen/CodeGenModule.cpp
@@ -3768,7 +3768,7 @@
     if (FD->isMultiVersion()) {
-        UpdateMultiVersionNames(GD, FD, MangledName);
+      UpdateMultiVersionNames(GD, FD, MangledName);
       if (!IsForDefinition)
         return GetOrCreateMultiVersionResolver(GD, Ty, FD);
Index: clang/include/clang/Basic/Attr.td
--- clang/include/clang/Basic/Attr.td
+++ clang/include/clang/Basic/Attr.td
@@ -2723,8 +2723,12 @@
     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(),
@@ -2734,9 +2738,10 @@
                            [](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(

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