[Clang][NFC] Move FindCountedByField into FieldDecl (#104235)

FindCountedByField can be used in more places than CodeGen. Move it into
FieldDecl to avoid layering issues.

Author: bwendling

clang/include/clang/AST/Decl.h

@@ -3206,6 +3206,10 @@ class FieldDecl : public DeclaratorDecl, public Mergeable<FieldDecl> {
   /// Set the C++11 in-class initializer for this member.
   void setInClassInitializer(Expr *NewInit);
 
+  /// Find the FieldDecl specified in a FAM's "counted_by" attribute. Returns
+  /// \p nullptr if either the attribute or the field doesn't exist.
+  const FieldDecl *findCountedByField() const;
+
 private:
   void setLazyInClassInitializer(LazyDeclStmtPtr NewInit);
 

clang/lib/AST/Decl.cpp

@@ -4678,6 +4678,19 @@ void FieldDecl::printName(raw_ostream &OS, const PrintingPolicy &Policy) const {
   DeclaratorDecl::printName(OS, Policy);
 }
 
+const FieldDecl *FieldDecl::findCountedByField() const {
+  const auto *CAT = getType()->getAs<CountAttributedType>();
+  if (!CAT)
+    return nullptr;
+
+  const auto *CountDRE = cast<DeclRefExpr>(CAT->getCountExpr());
+  const auto *CountDecl = CountDRE->getDecl();
+  if (const auto *IFD = dyn_cast<IndirectFieldDecl>(CountDecl))
+    CountDecl = IFD->getAnonField();
+
+  return dyn_cast<FieldDecl>(CountDecl);
+}
+
 //===----------------------------------------------------------------------===//
 // TagDecl Implementation
 //===----------------------------------------------------------------------===//

clang/lib/CodeGen/CGBuiltin.cpp

@@ -987,7 +987,7 @@ CodeGenFunction::emitFlexibleArrayMemberSize(const Expr *E, unsigned Type,
     // attribute.
     return nullptr;
 
-  const FieldDecl *CountedByFD = FindCountedByField(FAMDecl);
+  const FieldDecl *CountedByFD = FAMDecl->findCountedByField();
   if (!CountedByFD)
     // Can't find the field referenced by the "counted_by" attribute.
     return nullptr;

clang/lib/CodeGen/CGExpr.cpp

@@ -1150,22 +1150,6 @@ llvm::Value *CodeGenFunction::EmitLoadOfCountedByField(
                                    getIntAlign(), "..counted_by.load");
 }
 
-const FieldDecl *CodeGenFunction::FindCountedByField(const FieldDecl *FD) {
-  if (!FD)
-    return nullptr;
-
-  const auto *CAT = FD->getType()->getAs<CountAttributedType>();
-  if (!CAT)
-    return nullptr;
-
-  const auto *CountDRE = cast<DeclRefExpr>(CAT->getCountExpr());
-  const auto *CountDecl = CountDRE->getDecl();
-  if (const auto *IFD = dyn_cast<IndirectFieldDecl>(CountDecl))
-    CountDecl = IFD->getAnonField();
-
-  return dyn_cast<FieldDecl>(CountDecl);
-}
-
 void CodeGenFunction::EmitBoundsCheck(const Expr *E, const Expr *Base,
                                       llvm::Value *Index, QualType IndexType,
                                       bool Accessed) {
@@ -4305,7 +4289,7 @@ LValue CodeGenFunction::EmitArraySubscriptExpr(const ArraySubscriptExpr *E,
           ME->isFlexibleArrayMemberLike(getContext(), StrictFlexArraysLevel) &&
           ME->getMemberDecl()->getType()->isCountAttributedType()) {
         const FieldDecl *FAMDecl = dyn_cast<FieldDecl>(ME->getMemberDecl());
-        if (const FieldDecl *CountFD = FindCountedByField(FAMDecl)) {
+        if (const FieldDecl *CountFD = FAMDecl->findCountedByField()) {
           if (std::optional<int64_t> Diff =
                   getOffsetDifferenceInBits(*this, CountFD, FAMDecl)) {
             CharUnits OffsetDiff = CGM.getContext().toCharUnitsFromBits(*Diff);

clang/lib/CodeGen/CodeGenFunction.h

@@ -3305,10 +3305,6 @@ class CodeGenFunction : public CodeGenTypeCache {
                                         const FieldDecl *FAMDecl,
                                         uint64_t &Offset);
 
-  /// Find the FieldDecl specified in a FAM's "counted_by" attribute. Returns
-  /// \p nullptr if either the attribute or the field doesn't exist.
-  const FieldDecl *FindCountedByField(const FieldDecl *FD);
-
   /// Build an expression accessing the "counted_by" field.
   llvm::Value *EmitLoadOfCountedByField(const Expr *Base,
                                         const FieldDecl *FAMDecl,