[Clang] Improve testing for the flexible array member (#89462)

Testing for the name of the flexible array member isn't as robust as
testing the FieldDecl pointers.

Author: bwendling

clang/lib/CodeGen/CGBuiltin.cpp

@@ -822,8 +822,9 @@ CodeGenFunction::evaluateOrEmitBuiltinObjectSize(const Expr *E, unsigned Type,
   return ConstantInt::get(ResType, ObjectSize, /*isSigned=*/true);
 }
 
-const FieldDecl *CodeGenFunction::FindFlexibleArrayMemberField(
-    ASTContext &Ctx, const RecordDecl *RD, StringRef Name, uint64_t &Offset) {
+const FieldDecl *CodeGenFunction::FindFlexibleArrayMemberFieldAndOffset(
+    ASTContext &Ctx, const RecordDecl *RD, const FieldDecl *FAMDecl,
+    uint64_t &Offset) {
   const LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel =
       getLangOpts().getStrictFlexArraysLevel();
   uint32_t FieldNo = 0;
@@ -832,7 +833,7 @@ const FieldDecl *CodeGenFunction::FindFlexibleArrayMemberField(
     return nullptr;
 
   for (const FieldDecl *FD : RD->fields()) {
-    if ((Name.empty() || FD->getNameAsString() == Name) &&
+    if ((!FAMDecl || FD == FAMDecl) &&
         Decl::isFlexibleArrayMemberLike(
             Ctx, FD, FD->getType(), StrictFlexArraysLevel,
             /*IgnoreTemplateOrMacroSubstitution=*/true)) {
@@ -843,8 +844,8 @@ const FieldDecl *CodeGenFunction::FindFlexibleArrayMemberField(
 
     QualType Ty = FD->getType();
     if (Ty->isRecordType()) {
-      if (const FieldDecl *Field = FindFlexibleArrayMemberField(
-              Ctx, Ty->getAsRecordDecl(), Name, Offset)) {
+      if (const FieldDecl *Field = FindFlexibleArrayMemberFieldAndOffset(
+              Ctx, Ty->getAsRecordDecl(), FAMDecl, Offset)) {
         const ASTRecordLayout &Layout = Ctx.getASTRecordLayout(RD);
         Offset += Layout.getFieldOffset(FieldNo);
         return Field;
@@ -930,12 +931,14 @@ CodeGenFunction::emitFlexibleArrayMemberSize(const Expr *E, unsigned Type,
 
   // Get the flexible array member Decl.
   const RecordDecl *OuterRD = nullptr;
-  std::string FAMName;
+  const FieldDecl *FAMDecl = nullptr;
   if (const auto *ME = dyn_cast<MemberExpr>(Base)) {
     // Check if \p Base is referencing the FAM itself.
     const ValueDecl *VD = ME->getMemberDecl();
     OuterRD = VD->getDeclContext()->getOuterLexicalRecordContext();
-    FAMName = VD->getNameAsString();
+    FAMDecl = dyn_cast<FieldDecl>(VD);
+    if (!FAMDecl)
+      return nullptr;
   } else if (const auto *DRE = dyn_cast<DeclRefExpr>(Base)) {
     // Check if we're pointing to the whole struct.
     QualType Ty = DRE->getDecl()->getType();
@@ -974,9 +977,11 @@ CodeGenFunction::emitFlexibleArrayMemberSize(const Expr *E, unsigned Type,
   if (!OuterRD)
     return nullptr;
 
+  // We call FindFlexibleArrayMemberAndOffset even if FAMDecl is non-null to
+  // get its offset.
   uint64_t Offset = 0;
-  const FieldDecl *FAMDecl =
-      FindFlexibleArrayMemberField(Ctx, OuterRD, FAMName, Offset);
+  FAMDecl =
+      FindFlexibleArrayMemberFieldAndOffset(Ctx, OuterRD, FAMDecl, Offset);
   Offset = Ctx.toCharUnitsFromBits(Offset).getQuantity();
 
   if (!FAMDecl || !FAMDecl->getType()->isCountAttributedType())

clang/lib/CodeGen/CodeGenFunction.h

@@ -3204,12 +3204,12 @@ class CodeGenFunction : public CodeGenTypeCache {
                            llvm::Value *Index, QualType IndexType,
                            QualType IndexedType, bool Accessed);
 
-  // Find a struct's flexible array member. It may be embedded inside multiple
-  // sub-structs, but must still be the last field.
-  const FieldDecl *FindFlexibleArrayMemberField(ASTContext &Ctx,
-                                                const RecordDecl *RD,
-                                                StringRef Name,
-                                                uint64_t &Offset);
+  // Find a struct's flexible array member and get its offset. It may be
+  // embedded inside multiple sub-structs, but must still be the last field.
+  const FieldDecl *
+  FindFlexibleArrayMemberFieldAndOffset(ASTContext &Ctx, const RecordDecl *RD,
+                                        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.