[Clang] NFC: Move Arm type attributes to separate trailing object. (#78424)

This decouples the Arm type attributes from other bits, which means
the data will only be allocated when a function uses these Arm
attributes.

The first patch adds the bit `HasArmTypeAttributes` to
`FunctionTypeBitfields`, which grows from 62 bits to 63 bits.

In the second patch, I've moved this bit (`HasArmTypeAttributes`) to
`FunctionTypeExtraBitfields`, because it looks like the bits in
`FunctionTypeBitfields` are precious and we really don't want that
struct
to grow beyond 64 bits.

I've split this out into two patches to explain the rationale, but those
can be squashed before merging.

Author: sdesmalen-arm

clang/include/clang/AST/Type.h

@@ -4029,6 +4029,22 @@ class FunctionType : public Type {
   /// because TrailingObjects cannot handle repeated types.
   struct ExceptionType { QualType Type; };
 
+  /// 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.
+  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 : 10;
+
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned HasArmTypeAttributes : 1;
+
+    FunctionTypeExtraBitfields()
+        : NumExceptionType(0), HasArmTypeAttributes(false) {}
+  };
+
   /// The AArch64 SME ACLE (Arm C/C++ Language Extensions) define a number
   /// of function type attributes that can be set on function types, including
   /// function pointers.
@@ -4042,7 +4058,8 @@ class FunctionType : public Type {
     SME_ZAMask = 0b111 << SME_ZAShift,
 
     SME_AttributeMask = 0b111'111 // We only support maximum 6 bits because of
-                                  // the bitmask in FunctionTypeExtraBitfields.
+                                  // the bitmask in FunctionTypeArmAttributes
+                                  // and ExtProtoInfo.
   };
 
   enum ArmStateValue : unsigned {
@@ -4057,20 +4074,15 @@ class FunctionType : public Type {
     return (ArmStateValue)((AttrBits & SME_ZAMask) >> SME_ZAShift);
   }
 
-  /// 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.
-  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 : 10;
-
+  /// A holder for Arm type attributes as described in the Arm C/C++
+  /// Language extensions which are not particularly common to all
+  /// types and therefore accounted separately from FunctionTypeBitfields.
+  struct alignas(void *) FunctionTypeArmAttributes {
     /// Any AArch64 SME ACLE type attributes that need to be propagated
     /// on declarations and function pointers.
     unsigned AArch64SMEAttributes : 6;
-    FunctionTypeExtraBitfields()
-        : NumExceptionType(0), AArch64SMEAttributes(SME_NormalFunction) {}
+
+    FunctionTypeArmAttributes() : AArch64SMEAttributes(SME_NormalFunction) {}
   };
 
 protected:
@@ -4169,7 +4181,8 @@ class FunctionProtoType final
       public llvm::FoldingSetNode,
       private llvm::TrailingObjects<
           FunctionProtoType, QualType, SourceLocation,
-          FunctionType::FunctionTypeExtraBitfields, FunctionType::ExceptionType,
+          FunctionType::FunctionTypeExtraBitfields,
+          FunctionType::FunctionTypeArmAttributes, FunctionType::ExceptionType,
           Expr *, FunctionDecl *, FunctionType::ExtParameterInfo, Qualifiers> {
   friend class ASTContext; // ASTContext creates these.
   friend TrailingObjects;
@@ -4276,7 +4289,11 @@ class FunctionProtoType final
 
     bool requiresFunctionProtoTypeExtraBitfields() const {
       return ExceptionSpec.Type == EST_Dynamic ||
-             AArch64SMEAttributes != SME_NormalFunction;
+             requiresFunctionProtoTypeArmAttributes();
+    }
+
+    bool requiresFunctionProtoTypeArmAttributes() const {
+      return AArch64SMEAttributes != SME_NormalFunction;
     }
 
     void setArmSMEAttribute(AArch64SMETypeAttributes Kind, bool Enable = true) {
@@ -4296,6 +4313,10 @@ class FunctionProtoType final
     return isVariadic();
   }
 
+  unsigned numTrailingObjects(OverloadToken<FunctionTypeArmAttributes>) const {
+    return hasArmTypeAttributes();
+  }
+
   unsigned numTrailingObjects(OverloadToken<FunctionTypeExtraBitfields>) const {
     return hasExtraBitfields();
   }
@@ -4384,6 +4405,12 @@ class FunctionProtoType final
 
   }
 
+  bool hasArmTypeAttributes() const {
+    return FunctionTypeBits.HasExtraBitfields &&
+           getTrailingObjects<FunctionTypeExtraBitfields>()
+               ->HasArmTypeAttributes;
+  }
+
   bool hasExtQualifiers() const {
     return FunctionTypeBits.HasExtQuals;
   }
@@ -4595,9 +4622,9 @@ class FunctionProtoType final
   /// Return a bitmask describing the SME attributes on the function type, see
   /// AArch64SMETypeAttributes for their values.
   unsigned getAArch64SMEAttributes() const {
-    if (!hasExtraBitfields())
+    if (!hasArmTypeAttributes())
       return SME_NormalFunction;
-    return getTrailingObjects<FunctionTypeExtraBitfields>()
+    return getTrailingObjects<FunctionTypeArmAttributes>()
         ->AArch64SMEAttributes;
   }
 

clang/lib/AST/ASTContext.cpp

@@ -4484,10 +4484,11 @@ QualType ASTContext::getFunctionTypeInternal(
       EPI.ExceptionSpec.Type, EPI.ExceptionSpec.Exceptions.size());
   size_t Size = FunctionProtoType::totalSizeToAlloc<
       QualType, SourceLocation, FunctionType::FunctionTypeExtraBitfields,
-      FunctionType::ExceptionType, Expr *, FunctionDecl *,
-      FunctionProtoType::ExtParameterInfo, Qualifiers>(
+      FunctionType::FunctionTypeArmAttributes, FunctionType::ExceptionType,
+      Expr *, FunctionDecl *, FunctionProtoType::ExtParameterInfo, Qualifiers>(
       NumArgs, EPI.Variadic, EPI.requiresFunctionProtoTypeExtraBitfields(),
-      ESH.NumExceptionType, ESH.NumExprPtr, ESH.NumFunctionDeclPtr,
+      EPI.requiresFunctionProtoTypeArmAttributes(), ESH.NumExceptionType,
+      ESH.NumExprPtr, ESH.NumFunctionDeclPtr,
       EPI.ExtParameterInfos ? NumArgs : 0,
       EPI.TypeQuals.hasNonFastQualifiers() ? 1 : 0);
 

clang/lib/AST/Type.cpp

@@ -3459,6 +3459,14 @@ FunctionProtoType::FunctionProtoType(QualType result, ArrayRef<QualType> params,
     FunctionTypeBits.HasExtraBitfields = false;
   }
 
+  if (epi.requiresFunctionProtoTypeArmAttributes()) {
+    auto &ArmTypeAttrs = *getTrailingObjects<FunctionTypeArmAttributes>();
+    ArmTypeAttrs = FunctionTypeArmAttributes();
+
+    // Also set the bit in FunctionTypeExtraBitfields
+    auto &ExtraBits = *getTrailingObjects<FunctionTypeExtraBitfields>();
+    ExtraBits.HasArmTypeAttributes = true;
+  }
 
   // Fill in the trailing argument array.
   auto *argSlot = getTrailingObjects<QualType>();
@@ -3470,10 +3478,10 @@ FunctionProtoType::FunctionProtoType(QualType result, ArrayRef<QualType> params,
 
   // Propagate the SME ACLE attributes.
   if (epi.AArch64SMEAttributes != SME_NormalFunction) {
-    auto &ExtraBits = *getTrailingObjects<FunctionTypeExtraBitfields>();
+    auto &ArmTypeAttrs = *getTrailingObjects<FunctionTypeArmAttributes>();
     assert(epi.AArch64SMEAttributes <= SME_AttributeMask &&
            "Not enough bits to encode SME attributes");
-    ExtraBits.AArch64SMEAttributes = epi.AArch64SMEAttributes;
+    ArmTypeAttrs.AArch64SMEAttributes = epi.AArch64SMEAttributes;
   }
 
   // Fill in the exception type array if present.