[clang] bed5ee3 - [clang] NFC: Enhance comments in CodeGen for multiversion function support.

[Tom Honermann via cfe-commits]

Author: Tom Honermann
Date: 2022-04-05T19:50:22-04:00
New Revision: bed5ee3f4ba2406658760ec4029ed222c6213ff7

URL: https://github.com/llvm/llvm-project/commit/bed5ee3f4ba2406658760ec4029ed222c6213ff7
DIFF: https://github.com/llvm/llvm-project/commit/bed5ee3f4ba2406658760ec4029ed222c6213ff7.diff

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

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D122955

Added: 
    

Modified: 
    clang/include/clang/Basic/Attr.td
    clang/lib/CodeGen/CodeGenModule.cpp
    clang/lib/CodeGen/CodeGenModule.h

Removed: 
    


################################################################################
diff  --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index 4789493399ec2..c7141ff463b38 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -2723,8 +2723,12 @@ def TargetClones : InheritableAttr {
     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 @@ def TargetClones : InheritableAttr {
                            [](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(

diff  --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 5536626d0691a..72fea25a28cfb 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -3768,7 +3768,7 @@ llvm::Constant *CodeGenModule::GetOrCreateLLVMFunction(
     }
 
     if (FD->isMultiVersion()) {
-        UpdateMultiVersionNames(GD, FD, MangledName);
+      UpdateMultiVersionNames(GD, FD, MangledName);
       if (!IsForDefinition)
         return GetOrCreateMultiVersionResolver(GD, Ty, FD);
     }

diff  --git a/clang/lib/CodeGen/CodeGenModule.h b/clang/lib/CodeGen/CodeGenModule.h
index a95949109c371..a68c681a2dd1b 100644
--- a/clang/lib/CodeGen/CodeGenModule.h
+++ b/clang/lib/CodeGen/CodeGenModule.h
@@ -348,8 +348,9 @@ class CodeGenModule : public CodeGenTypeCache {
   /// 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 @@ class CodeGenModule : public CodeGenTypeCache {
       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 @@ class CodeGenModule : public CodeGenTypeCache {
   // 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.