[Clang] NFCI: Add a new bit HasExtraBitfields to FunctionType.

The FunctionTypeExtraBitfields is currently only available when the
ExceptionSpecificationType == Dynamic, which means that there is no other
way to use or extend the FunctionTypeExtraBitfields independently of the
exception specification type.

This patch adds a new field HasExtraBitfields to specify
whether the prototype has trailing ExtraBitfields.

This patch intends to be NFC and is required for future extension
and use of the ExtraBitfields struct.

Reviewed By: aaron.ballman, erichkeane

Differential Revision: https://reviews.llvm.org/D126642

clang/include/clang/AST/Type.h

@@ -1624,6 +1624,9 @@ protected:
     /// Whether this function has extended parameter information.
     unsigned HasExtParameterInfos : 1;
 
+    /// Whether this function has extra bitfields for the prototype.
+    unsigned HasExtraBitfields : 1;
+
     /// Whether the function is variadic.
     unsigned Variadic : 1;
 
@@ -3798,13 +3801,12 @@ public:
 
   /// A simple holder for various uncommon bits which do not fit in
   /// FunctionTypeBitfields. Aligned to alignof(void *) to maintain the
-  /// alignment of subsequent objects in TrailingObjects. You must update
-  /// hasExtraBitfields in FunctionProtoType after adding extra data here.
+  /// alignment of subsequent objects in TrailingObjects.
   struct alignas(void *) FunctionTypeExtraBitfields {
     /// The number of types in the exception specification.
     /// A whole unsigned is not needed here and according to
     /// [implimits] 8 bits would be enough here.
-    unsigned NumExceptionType;
+    unsigned NumExceptionType = 0;
   };
 
 protected:
@@ -3998,6 +4000,10 @@ public:
       Result.ExceptionSpec = ESI;
       return Result;
     }
+
+    bool requiresFunctionProtoTypeExtraBitfields() const {
+      return ExceptionSpec.Type == EST_Dynamic;
+    }
   };
 
 private:
@@ -4088,16 +4094,13 @@ private:
     return getExceptionSpecSize(getExceptionSpecType(), getNumExceptions());
   }
 
-  /// Whether the trailing FunctionTypeExtraBitfields is present.
-  static bool hasExtraBitfields(ExceptionSpecificationType EST) {
-    // If the exception spec type is EST_Dynamic then we have > 0 exception
-    // types and the exact number is stored in FunctionTypeExtraBitfields.
-    return EST == EST_Dynamic;
-  }
-
   /// Whether the trailing FunctionTypeExtraBitfields is present.
   bool hasExtraBitfields() const {
-    return hasExtraBitfields(getExceptionSpecType());
+    assert((getExceptionSpecType() != EST_Dynamic ||
+            FunctionTypeBits.HasExtraBitfields) &&
+           "ExtraBitfields are required for given ExceptionSpecType");
+    return FunctionTypeBits.HasExtraBitfields;
+
   }
 
   bool hasExtQualifiers() const {

clang/lib/AST/ASTContext.cpp

@@ -4451,8 +4451,7 @@ QualType ASTContext::getFunctionTypeInternal(
       QualType, SourceLocation, FunctionType::FunctionTypeExtraBitfields,
       FunctionType::ExceptionType, Expr *, FunctionDecl *,
       FunctionProtoType::ExtParameterInfo, Qualifiers>(
-      NumArgs, EPI.Variadic,
-      FunctionProtoType::hasExtraBitfields(EPI.ExceptionSpec.Type),
+      NumArgs, EPI.Variadic, EPI.requiresFunctionProtoTypeExtraBitfields(),
       ESH.NumExceptionType, ESH.NumExprPtr, ESH.NumFunctionDeclPtr,
       EPI.ExtParameterInfos ? NumArgs : 0,
       EPI.TypeQuals.hasNonFastQualifiers() ? 1 : 0);

clang/lib/AST/Type.cpp

@@ -3213,12 +3213,15 @@ FunctionProtoType::FunctionProtoType(QualType result, ArrayRef<QualType> params,
   FunctionTypeBits.Variadic = epi.Variadic;
   FunctionTypeBits.HasTrailingReturn = epi.HasTrailingReturn;
 
-  // Fill in the extra trailing bitfields if present.
-  if (hasExtraBitfields(epi.ExceptionSpec.Type)) {
+  if (epi.requiresFunctionProtoTypeExtraBitfields()) {
+    FunctionTypeBits.HasExtraBitfields = true;
     auto &ExtraBits = *getTrailingObjects<FunctionTypeExtraBitfields>();
-    ExtraBits.NumExceptionType = epi.ExceptionSpec.Exceptions.size();
+    ExtraBits = FunctionTypeExtraBitfields();
+  } else {
+    FunctionTypeBits.HasExtraBitfields = false;
   }
 
+
   // Fill in the trailing argument array.
   auto *argSlot = getTrailingObjects<QualType>();
   for (unsigned i = 0; i != getNumParams(); ++i) {
@@ -3229,6 +3232,9 @@ FunctionProtoType::FunctionProtoType(QualType result, ArrayRef<QualType> params,
 
   // Fill in the exception type array if present.
   if (getExceptionSpecType() == EST_Dynamic) {
+    auto &ExtraBits = *getTrailingObjects<FunctionTypeExtraBitfields>();
+    ExtraBits.NumExceptionType = epi.ExceptionSpec.Exceptions.size();
+
     assert(hasExtraBitfields() && "missing trailing extra bitfields!");
     auto *exnSlot =
         reinterpret_cast<QualType *>(getTrailingObjects<ExceptionType>());