diff --git a/flang/lib/Optimizer/CodeGen/CMakeLists.txt b/flang/lib/Optimizer/CodeGen/CMakeLists.txt
index 711bb402b40bd..016544ef870a5 100644
--- a/flang/lib/Optimizer/CodeGen/CMakeLists.txt
+++ b/flang/lib/Optimizer/CodeGen/CMakeLists.txt
@@ -6,6 +6,7 @@ add_flang_library(FIRCodeGen
   TBAABuilder.cpp
   Target.cpp
   TargetRewrite.cpp
+  TypeConverter.cpp
 
   DEPENDS
   FIRDialect
diff --git a/flang/lib/Optimizer/CodeGen/TypeConverter.cpp b/flang/lib/Optimizer/CodeGen/TypeConverter.cpp
new file mode 100644
index 0000000000000..e6d8f62e7ec57
--- /dev/null
+++ b/flang/lib/Optimizer/CodeGen/TypeConverter.cpp
@@ -0,0 +1,344 @@
+//===-- TypeConverter.cpp -- type conversion --------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Coding style: https://mlir.llvm.org/getting_started/DeveloperGuide/
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "flang-type-conversion"
+
+#include "TypeConverter.h"
+#include "DescriptorModel.h"
+#include "TBAABuilder.h"
+#include "Target.h"
+#include "flang/Optimizer/Builder/Todo.h" // remove when TODO's are done
+#include "flang/Optimizer/Dialect/FIRType.h"
+#include "flang/Optimizer/Dialect/Support/FIRContext.h"
+#include "flang/Optimizer/Dialect/Support/KindMapping.h"
+#include "mlir/Conversion/LLVMCommon/TypeConverter.h"
+#include "llvm/Support/Debug.h"
+
+namespace fir {
+
+LLVMTypeConverter::LLVMTypeConverter(mlir::ModuleOp module, bool applyTBAA)
+    : mlir::LLVMTypeConverter(module.getContext(),
+                              [&] {
+                                mlir::LowerToLLVMOptions options(
+                                    module.getContext());
+                                options.useOpaquePointers = false;
+                                return options;
+                              }()),
+      kindMapping(getKindMapping(module)),
+      specifics(CodeGenSpecifics::get(module.getContext(),
+                                      getTargetTriple(module),
+                                      getKindMapping(module))),
+      tbaaBuilder(module, applyTBAA) {
+  LLVM_DEBUG(llvm::dbgs() << "FIR type converter\n");
+
+  // Each conversion should return a value of type mlir::Type.
+  addConversion([&](BoxType box) { return convertBoxType(box); });
+  addConversion([&](BoxCharType boxchar) {
+    LLVM_DEBUG(llvm::dbgs() << "type convert: " << boxchar << '\n');
+    return convertType(specifics->boxcharMemoryType(boxchar.getEleTy()));
+  });
+  addConversion([&](BoxProcType boxproc) {
+    // TODO: Support for this type will be added later when the Fortran 2003
+    // procedure pointer feature is implemented.
+    return std::nullopt;
+  });
+  addConversion(
+      [&](fir::ClassType classTy) { return convertBoxType(classTy); });
+  addConversion(
+      [&](fir::CharacterType charTy) { return convertCharType(charTy); });
+  addConversion(
+      [&](fir::ComplexType cmplx) { return convertComplexType(cmplx); });
+  addConversion([&](fir::FieldType field) {
+    // Convert to i32 because of LLVM GEP indexing restriction.
+    return mlir::IntegerType::get(field.getContext(), 32);
+  });
+  addConversion([&](HeapType heap) { return convertPointerLike(heap); });
+  addConversion([&](fir::IntegerType intTy) {
+    return mlir::IntegerType::get(
+        &getContext(), kindMapping.getIntegerBitsize(intTy.getFKind()));
+  });
+  addConversion([&](fir::LenType field) {
+    // Get size of len paramter from the descriptor.
+    return getModel<Fortran::runtime::typeInfo::TypeParameterValue>()(
+        &getContext());
+  });
+  addConversion([&](fir::LogicalType boolTy) {
+    return mlir::IntegerType::get(
+        &getContext(), kindMapping.getLogicalBitsize(boolTy.getFKind()));
+  });
+  addConversion([&](fir::LLVMPointerType pointer) {
+    return convertPointerLike(pointer);
+  });
+  addConversion(
+      [&](fir::PointerType pointer) { return convertPointerLike(pointer); });
+  addConversion([&](fir::RecordType derived,
+                    llvm::SmallVectorImpl<mlir::Type> &results,
+                    llvm::ArrayRef<mlir::Type> callStack) {
+    return convertRecordType(derived, results, callStack);
+  });
+  addConversion(
+      [&](fir::RealType real) { return convertRealType(real.getFKind()); });
+  addConversion(
+      [&](fir::ReferenceType ref) { return convertPointerLike(ref); });
+  addConversion([&](fir::SequenceType sequence) {
+    return convertSequenceType(sequence);
+  });
+  addConversion([&](fir::TypeDescType tdesc) {
+    return convertTypeDescType(tdesc.getContext());
+  });
+  addConversion([&](fir::VectorType vecTy) {
+    return mlir::VectorType::get(llvm::ArrayRef<int64_t>(vecTy.getLen()),
+                                 convertType(vecTy.getEleTy()));
+  });
+  addConversion([&](mlir::TupleType tuple) {
+    LLVM_DEBUG(llvm::dbgs() << "type convert: " << tuple << '\n');
+    llvm::SmallVector<mlir::Type> members;
+    for (auto mem : tuple.getTypes()) {
+      // Prevent fir.box from degenerating to a pointer to a descriptor in the
+      // context of a tuple type.
+      if (auto box = mem.dyn_cast<fir::BaseBoxType>())
+        members.push_back(convertBoxTypeAsStruct(box));
+      else
+        members.push_back(convertType(mem).cast<mlir::Type>());
+    }
+    return mlir::LLVM::LLVMStructType::getLiteral(&getContext(), members,
+                                                  /*isPacked=*/false);
+  });
+  addConversion([&](mlir::NoneType none) {
+    return mlir::LLVM::LLVMStructType::getLiteral(
+        none.getContext(), std::nullopt, /*isPacked=*/false);
+  });
+  // FIXME: https://reviews.llvm.org/D82831 introduced an automatic
+  // materialization of conversion around function calls that is not working
+  // well with fir lowering to llvm (incorrect llvm.mlir.cast are inserted).
+  // Workaround until better analysis: register a handler that does not insert
+  // any conversions.
+  addSourceMaterialization(
+      [&](mlir::OpBuilder &builder, mlir::Type resultType,
+          mlir::ValueRange inputs,
+          mlir::Location loc) -> std::optional<mlir::Value> {
+        if (inputs.size() != 1)
+          return std::nullopt;
+        return inputs[0];
+      });
+  // Similar FIXME workaround here (needed for compare.fir/select-type.fir
+  // as well as rebox-global.fir tests). This is needed to cope with the
+  // the fact that codegen does not lower some operation results to the LLVM
+  // type produced by this LLVMTypeConverter. For instance, inside FIR
+  // globals, fir.box are lowered to llvm.struct, while the fir.box type
+  // conversion translates it into an llvm.ptr<llvm.struct<>> because
+  // descriptors are manipulated in memory outside of global initializers
+  // where this is not possible. Hence, MLIR inserts
+  // builtin.unrealized_conversion_cast after the translation of operations
+  // producing fir.box in fir.global codegen. addSourceMaterialization and
+  // addTargetMaterialization allow ignoring these ops and removing them
+  // after codegen assuming the type discrepencies are intended (like for
+  // fir.box inside globals).
+  addTargetMaterialization(
+      [&](mlir::OpBuilder &builder, mlir::Type resultType,
+          mlir::ValueRange inputs,
+          mlir::Location loc) -> std::optional<mlir::Value> {
+        if (inputs.size() != 1)
+          return std::nullopt;
+        return inputs[0];
+      });
+}
+
+// i32 is used here because LLVM wants i32 constants when indexing into struct
+// types. Indexing into other aggregate types is more flexible.
+mlir::Type LLVMTypeConverter::offsetType() {
+  return mlir::IntegerType::get(&getContext(), 32);
+}
+
+// i64 can be used to index into aggregates like arrays
+mlir::Type LLVMTypeConverter::indexType() {
+  return mlir::IntegerType::get(&getContext(), 64);
+}
+
+// fir.type<name(p : TY'...){f : TY...}>  -->  llvm<"%name = { ty... }">
+std::optional<mlir::LogicalResult>
+LLVMTypeConverter::convertRecordType(fir::RecordType derived,
+                                     llvm::SmallVectorImpl<mlir::Type> &results,
+                                     llvm::ArrayRef<mlir::Type> callStack) {
+  auto name = derived.getName();
+  auto st = mlir::LLVM::LLVMStructType::getIdentified(&getContext(), name);
+  if (llvm::count(callStack, derived) > 1) {
+    results.push_back(st);
+    return mlir::success();
+  }
+  llvm::SmallVector<mlir::Type> members;
+  for (auto mem : derived.getTypeList()) {
+    // Prevent fir.box from degenerating to a pointer to a descriptor in the
+    // context of a record type.
+    if (auto box = mem.second.dyn_cast<fir::BaseBoxType>())
+      members.push_back(convertBoxTypeAsStruct(box));
+    else
+      members.push_back(convertType(mem.second).cast<mlir::Type>());
+  }
+  if (mlir::failed(st.setBody(members, /*isPacked=*/false)))
+    return mlir::failure();
+  results.push_back(st);
+  return mlir::success();
+}
+
+// Is an extended descriptor needed given the element type of a fir.box type ?
+// Extended descriptors are required for derived types.
+bool LLVMTypeConverter::requiresExtendedDesc(mlir::Type boxElementType) {
+  auto eleTy = fir::unwrapSequenceType(boxElementType);
+  return eleTy.isa<fir::RecordType>();
+}
+
+// This corresponds to the descriptor as defined in ISO_Fortran_binding.h and
+// the addendum defined in descriptor.h.
+mlir::Type LLVMTypeConverter::convertBoxType(BaseBoxType box, int rank) {
+  // (base_addr*, elem_len, version, rank, type, attribute, f18Addendum, [dim]
+  llvm::SmallVector<mlir::Type> dataDescFields;
+  mlir::Type ele = box.getEleTy();
+  // remove fir.heap/fir.ref/fir.ptr
+  if (auto removeIndirection = fir::dyn_cast_ptrEleTy(ele))
+    ele = removeIndirection;
+  auto eleTy = convertType(ele);
+  // base_addr*
+  if (ele.isa<SequenceType>() && eleTy.isa<mlir::LLVM::LLVMPointerType>())
+    dataDescFields.push_back(eleTy);
+  else
+    dataDescFields.push_back(mlir::LLVM::LLVMPointerType::get(eleTy));
+  // elem_len
+  dataDescFields.push_back(
+      getDescFieldTypeModel<kElemLenPosInBox>()(&getContext()));
+  // version
+  dataDescFields.push_back(
+      getDescFieldTypeModel<kVersionPosInBox>()(&getContext()));
+  // rank
+  dataDescFields.push_back(
+      getDescFieldTypeModel<kRankPosInBox>()(&getContext()));
+  // type
+  dataDescFields.push_back(
+      getDescFieldTypeModel<kTypePosInBox>()(&getContext()));
+  // attribute
+  dataDescFields.push_back(
+      getDescFieldTypeModel<kAttributePosInBox>()(&getContext()));
+  // f18Addendum
+  dataDescFields.push_back(
+      getDescFieldTypeModel<kF18AddendumPosInBox>()(&getContext()));
+  // [dims]
+  if (rank == unknownRank()) {
+    if (auto seqTy = ele.dyn_cast<SequenceType>())
+      rank = seqTy.getDimension();
+    else
+      rank = 0;
+  }
+  if (rank > 0) {
+    auto rowTy = getDescFieldTypeModel<kDimsPosInBox>()(&getContext());
+    dataDescFields.push_back(mlir::LLVM::LLVMArrayType::get(rowTy, rank));
+  }
+  // opt-type-ptr: i8* (see fir.tdesc)
+  if (requiresExtendedDesc(ele) || fir::isUnlimitedPolymorphicType(box)) {
+    dataDescFields.push_back(
+        getExtendedDescFieldTypeModel<kOptTypePtrPosInBox>()(&getContext()));
+    auto rowTy =
+        getExtendedDescFieldTypeModel<kOptRowTypePosInBox>()(&getContext());
+    dataDescFields.push_back(mlir::LLVM::LLVMArrayType::get(rowTy, 1));
+    if (auto recTy = fir::unwrapSequenceType(ele).dyn_cast<fir::RecordType>())
+      if (recTy.getNumLenParams() > 0) {
+        // The descriptor design needs to be clarified regarding the number of
+        // length parameters in the addendum. Since it can change for
+        // polymorphic allocatables, it seems all length parameters cannot
+        // always possibly be placed in the addendum.
+        TODO_NOLOC("extended descriptor derived with length parameters");
+        unsigned numLenParams = recTy.getNumLenParams();
+        dataDescFields.push_back(
+            mlir::LLVM::LLVMArrayType::get(rowTy, numLenParams));
+      }
+  }
+  // TODO: send the box type and the converted LLVM structure layout
+  // to tbaaBuilder for proper creation of TBAATypeDescriptorOp.
+  return mlir::LLVM::LLVMPointerType::get(
+      mlir::LLVM::LLVMStructType::getLiteral(&getContext(), dataDescFields,
+                                             /*isPacked=*/false));
+}
+
+/// Convert fir.box type to the corresponding llvm struct type instead of a
+/// pointer to this struct type.
+mlir::Type LLVMTypeConverter::convertBoxTypeAsStruct(BaseBoxType box) {
+  return convertBoxType(box)
+      .cast<mlir::LLVM::LLVMPointerType>()
+      .getElementType();
+}
+
+// fir.boxproc<any>  -->  llvm<"{ any*, i8* }">
+mlir::Type LLVMTypeConverter::convertBoxProcType(BoxProcType boxproc) {
+  auto funcTy = convertType(boxproc.getEleTy());
+  auto i8PtrTy = mlir::LLVM::LLVMPointerType::get(
+      mlir::IntegerType::get(&getContext(), 8));
+  llvm::SmallVector<mlir::Type, 2> tuple = {funcTy, i8PtrTy};
+  return mlir::LLVM::LLVMStructType::getLiteral(&getContext(), tuple,
+                                                /*isPacked=*/false);
+}
+
+unsigned LLVMTypeConverter::characterBitsize(fir::CharacterType charTy) {
+  return kindMapping.getCharacterBitsize(charTy.getFKind());
+}
+
+// fir.char<k,?>  -->  llvm<"ix">          where ix is scaled by kind mapping
+// fir.char<k,n>  -->  llvm.array<n x "ix">
+mlir::Type LLVMTypeConverter::convertCharType(fir::CharacterType charTy) {
+  auto iTy = mlir::IntegerType::get(&getContext(), characterBitsize(charTy));
+  if (charTy.getLen() == fir::CharacterType::unknownLen())
+    return iTy;
+  return mlir::LLVM::LLVMArrayType::get(iTy, charTy.getLen());
+}
+
+// convert a front-end kind value to either a std or LLVM IR dialect type
+// fir.real<n>  -->  llvm.anyfloat  where anyfloat is a kind mapping
+mlir::Type LLVMTypeConverter::convertRealType(fir::KindTy kind) {
+  return fir::fromRealTypeID(&getContext(), kindMapping.getRealTypeID(kind),
+                             kind);
+}
+
+// fir.array<c ... :any>  -->  llvm<"[...[c x any]]">
+mlir::Type LLVMTypeConverter::convertSequenceType(SequenceType seq) {
+  auto baseTy = convertType(seq.getEleTy());
+  if (characterWithDynamicLen(seq.getEleTy()))
+    return mlir::LLVM::LLVMPointerType::get(baseTy);
+  auto shape = seq.getShape();
+  auto constRows = seq.getConstantRows();
+  if (constRows) {
+    decltype(constRows) i = constRows;
+    for (auto e : shape) {
+      baseTy = mlir::LLVM::LLVMArrayType::get(baseTy, e);
+      if (--i == 0)
+        break;
+    }
+    if (!seq.hasDynamicExtents())
+      return baseTy;
+  }
+  return mlir::LLVM::LLVMPointerType::get(baseTy);
+}
+
+// fir.tdesc<any>  -->  llvm<"i8*">
+// TODO: For now use a void*, however pointer identity is not sufficient for
+// the f18 object v. class distinction (F2003).
+mlir::Type LLVMTypeConverter::convertTypeDescType(mlir::MLIRContext *ctx) {
+  return mlir::LLVM::LLVMPointerType::get(
+      mlir::IntegerType::get(&getContext(), 8));
+}
+
+// Relay TBAA tag attachment to TBAABuilder.
+void LLVMTypeConverter::attachTBAATag(mlir::LLVM::AliasAnalysisOpInterface op,
+                                      mlir::Type baseFIRType,
+                                      mlir::Type accessFIRType,
+                                      mlir::LLVM::GEPOp gep) {
+  tbaaBuilder.attachTBAATag(op, baseFIRType, accessFIRType, gep);
+}
+
+} // namespace fir
diff --git a/flang/lib/Optimizer/CodeGen/TypeConverter.h b/flang/lib/Optimizer/CodeGen/TypeConverter.h
index 36c02b8ff71c8..d2bcfd14162e7 100644
--- a/flang/lib/Optimizer/CodeGen/TypeConverter.h
+++ b/flang/lib/Optimizer/CodeGen/TypeConverter.h
@@ -13,7 +13,6 @@
 #ifndef FORTRAN_OPTIMIZER_CODEGEN_TYPECONVERTER_H
 #define FORTRAN_OPTIMIZER_CODEGEN_TYPECONVERTER_H
 
-#include "DescriptorModel.h"
 #include "TBAABuilder.h"
 #include "Target.h"
 #include "flang/Optimizer/Builder/Todo.h" // remove when TODO's are done
@@ -46,173 +45,24 @@ namespace fir {
 /// This converts FIR types to LLVM types (for now)
 class LLVMTypeConverter : public mlir::LLVMTypeConverter {
 public:
-  LLVMTypeConverter(mlir::ModuleOp module, bool applyTBAA)
-      : mlir::LLVMTypeConverter(module.getContext(),
-                                [&] {
-                                  mlir::LowerToLLVMOptions options(
-                                      module.getContext());
-                                  options.useOpaquePointers = false;
-                                  return options;
-                                }()),
-        kindMapping(getKindMapping(module)),
-        specifics(CodeGenSpecifics::get(module.getContext(),
-                                        getTargetTriple(module),
-                                        getKindMapping(module))),
-        tbaaBuilder(module, applyTBAA) {
-    LLVM_DEBUG(llvm::dbgs() << "FIR type converter\n");
-
-    // Each conversion should return a value of type mlir::Type.
-    addConversion([&](BoxType box) { return convertBoxType(box); });
-    addConversion([&](BoxCharType boxchar) {
-      LLVM_DEBUG(llvm::dbgs() << "type convert: " << boxchar << '\n');
-      return convertType(specifics->boxcharMemoryType(boxchar.getEleTy()));
-    });
-    addConversion([&](BoxProcType boxproc) {
-      // TODO: Support for this type will be added later when the Fortran 2003
-      // procedure pointer feature is implemented.
-      return std::nullopt;
-    });
-    addConversion(
-        [&](fir::ClassType classTy) { return convertBoxType(classTy); });
-    addConversion(
-        [&](fir::CharacterType charTy) { return convertCharType(charTy); });
-    addConversion(
-        [&](fir::ComplexType cmplx) { return convertComplexType(cmplx); });
-    addConversion([&](fir::FieldType field) {
-      // Convert to i32 because of LLVM GEP indexing restriction.
-      return mlir::IntegerType::get(field.getContext(), 32);
-    });
-    addConversion([&](HeapType heap) { return convertPointerLike(heap); });
-    addConversion([&](fir::IntegerType intTy) {
-      return mlir::IntegerType::get(
-          &getContext(), kindMapping.getIntegerBitsize(intTy.getFKind()));
-    });
-    addConversion([&](fir::LenType field) {
-      // Get size of len paramter from the descriptor.
-      return getModel<Fortran::runtime::typeInfo::TypeParameterValue>()(
-          &getContext());
-    });
-    addConversion([&](fir::LogicalType boolTy) {
-      return mlir::IntegerType::get(
-          &getContext(), kindMapping.getLogicalBitsize(boolTy.getFKind()));
-    });
-    addConversion([&](fir::LLVMPointerType pointer) {
-      return convertPointerLike(pointer);
-    });
-    addConversion(
-        [&](fir::PointerType pointer) { return convertPointerLike(pointer); });
-    addConversion([&](fir::RecordType derived,
-                      llvm::SmallVectorImpl<mlir::Type> &results,
-                      llvm::ArrayRef<mlir::Type> callStack) {
-      return convertRecordType(derived, results, callStack);
-    });
-    addConversion(
-        [&](fir::RealType real) { return convertRealType(real.getFKind()); });
-    addConversion(
-        [&](fir::ReferenceType ref) { return convertPointerLike(ref); });
-    addConversion([&](fir::SequenceType sequence) {
-      return convertSequenceType(sequence);
-    });
-    addConversion([&](fir::TypeDescType tdesc) {
-      return convertTypeDescType(tdesc.getContext());
-    });
-    addConversion([&](fir::VectorType vecTy) {
-      return mlir::VectorType::get(llvm::ArrayRef<int64_t>(vecTy.getLen()),
-                                   convertType(vecTy.getEleTy()));
-    });
-    addConversion([&](mlir::TupleType tuple) {
-      LLVM_DEBUG(llvm::dbgs() << "type convert: " << tuple << '\n');
-      llvm::SmallVector<mlir::Type> members;
-      for (auto mem : tuple.getTypes()) {
-        // Prevent fir.box from degenerating to a pointer to a descriptor in the
-        // context of a tuple type.
-        if (auto box = mem.dyn_cast<fir::BaseBoxType>())
-          members.push_back(convertBoxTypeAsStruct(box));
-        else
-          members.push_back(convertType(mem).cast<mlir::Type>());
-      }
-      return mlir::LLVM::LLVMStructType::getLiteral(&getContext(), members,
-                                                    /*isPacked=*/false);
-    });
-    addConversion([&](mlir::NoneType none) {
-      return mlir::LLVM::LLVMStructType::getLiteral(
-          none.getContext(), std::nullopt, /*isPacked=*/false);
-    });
-    // FIXME: https://reviews.llvm.org/D82831 introduced an automatic
-    // materialization of conversion around function calls that is not working
-    // well with fir lowering to llvm (incorrect llvm.mlir.cast are inserted).
-    // Workaround until better analysis: register a handler that does not insert
-    // any conversions.
-    addSourceMaterialization(
-        [&](mlir::OpBuilder &builder, mlir::Type resultType,
-            mlir::ValueRange inputs,
-            mlir::Location loc) -> std::optional<mlir::Value> {
-          if (inputs.size() != 1)
-            return std::nullopt;
-          return inputs[0];
-        });
-    // Similar FIXME workaround here (needed for compare.fir/select-type.fir
-    // as well as rebox-global.fir tests). This is needed to cope with the
-    // the fact that codegen does not lower some operation results to the LLVM
-    // type produced by this LLVMTypeConverter. For instance, inside FIR
-    // globals, fir.box are lowered to llvm.struct, while the fir.box type
-    // conversion translates it into an llvm.ptr<llvm.struct<>> because
-    // descriptors are manipulated in memory outside of global initializers
-    // where this is not possible. Hence, MLIR inserts
-    // builtin.unrealized_conversion_cast after the translation of operations
-    // producing fir.box in fir.global codegen. addSourceMaterialization and
-    // addTargetMaterialization allow ignoring these ops and removing them
-    // after codegen assuming the type discrepencies are intended (like for
-    // fir.box inside globals).
-    addTargetMaterialization(
-        [&](mlir::OpBuilder &builder, mlir::Type resultType,
-            mlir::ValueRange inputs,
-            mlir::Location loc) -> std::optional<mlir::Value> {
-          if (inputs.size() != 1)
-            return std::nullopt;
-          return inputs[0];
-        });
-  }
+  LLVMTypeConverter(mlir::ModuleOp module, bool applyTBAA);
 
   // i32 is used here because LLVM wants i32 constants when indexing into struct
   // types. Indexing into other aggregate types is more flexible.
-  mlir::Type offsetType() { return mlir::IntegerType::get(&getContext(), 32); }
+  mlir::Type offsetType();
 
   // i64 can be used to index into aggregates like arrays
-  mlir::Type indexType() { return mlir::IntegerType::get(&getContext(), 64); }
+  mlir::Type indexType();
 
   // fir.type<name(p : TY'...){f : TY...}>  -->  llvm<"%name = { ty... }">
   std::optional<mlir::LogicalResult>
   convertRecordType(fir::RecordType derived,
                     llvm::SmallVectorImpl<mlir::Type> &results,
-                    llvm::ArrayRef<mlir::Type> callStack) {
-    auto name = derived.getName();
-    auto st = mlir::LLVM::LLVMStructType::getIdentified(&getContext(), name);
-    if (llvm::count(callStack, derived) > 1) {
-      results.push_back(st);
-      return mlir::success();
-    }
-    llvm::SmallVector<mlir::Type> members;
-    for (auto mem : derived.getTypeList()) {
-      // Prevent fir.box from degenerating to a pointer to a descriptor in the
-      // context of a record type.
-      if (auto box = mem.second.dyn_cast<fir::BaseBoxType>())
-        members.push_back(convertBoxTypeAsStruct(box));
-      else
-        members.push_back(convertType(mem.second).cast<mlir::Type>());
-    }
-    if (mlir::failed(st.setBody(members, /*isPacked=*/false)))
-      return mlir::failure();
-    results.push_back(st);
-    return mlir::success();
-  }
+                    llvm::ArrayRef<mlir::Type> callStack);
 
   // Is an extended descriptor needed given the element type of a fir.box type ?
   // Extended descriptors are required for derived types.
-  bool requiresExtendedDesc(mlir::Type boxElementType) {
-    auto eleTy = fir::unwrapSequenceType(boxElementType);
-    return eleTy.isa<fir::RecordType>();
-  }
+  bool requiresExtendedDesc(mlir::Type boxElementType);
 
   // Magic value to indicate we do not know the rank of an entity, either
   // because it is assumed rank or because we have not determined it yet.
@@ -220,104 +70,20 @@ class LLVMTypeConverter : public mlir::LLVMTypeConverter {
 
   // This corresponds to the descriptor as defined in ISO_Fortran_binding.h and
   // the addendum defined in descriptor.h.
-  mlir::Type convertBoxType(BaseBoxType box, int rank = unknownRank()) {
-    // (base_addr*, elem_len, version, rank, type, attribute, f18Addendum, [dim]
-    llvm::SmallVector<mlir::Type> dataDescFields;
-    mlir::Type ele = box.getEleTy();
-    // remove fir.heap/fir.ref/fir.ptr
-    if (auto removeIndirection = fir::dyn_cast_ptrEleTy(ele))
-      ele = removeIndirection;
-    auto eleTy = convertType(ele);
-    // base_addr*
-    if (ele.isa<SequenceType>() && eleTy.isa<mlir::LLVM::LLVMPointerType>())
-      dataDescFields.push_back(eleTy);
-    else
-      dataDescFields.push_back(mlir::LLVM::LLVMPointerType::get(eleTy));
-    // elem_len
-    dataDescFields.push_back(
-        getDescFieldTypeModel<kElemLenPosInBox>()(&getContext()));
-    // version
-    dataDescFields.push_back(
-        getDescFieldTypeModel<kVersionPosInBox>()(&getContext()));
-    // rank
-    dataDescFields.push_back(
-        getDescFieldTypeModel<kRankPosInBox>()(&getContext()));
-    // type
-    dataDescFields.push_back(
-        getDescFieldTypeModel<kTypePosInBox>()(&getContext()));
-    // attribute
-    dataDescFields.push_back(
-        getDescFieldTypeModel<kAttributePosInBox>()(&getContext()));
-    // f18Addendum
-    dataDescFields.push_back(
-        getDescFieldTypeModel<kF18AddendumPosInBox>()(&getContext()));
-    // [dims]
-    if (rank == unknownRank()) {
-      if (auto seqTy = ele.dyn_cast<SequenceType>())
-        rank = seqTy.getDimension();
-      else
-        rank = 0;
-    }
-    if (rank > 0) {
-      auto rowTy = getDescFieldTypeModel<kDimsPosInBox>()(&getContext());
-      dataDescFields.push_back(mlir::LLVM::LLVMArrayType::get(rowTy, rank));
-    }
-    // opt-type-ptr: i8* (see fir.tdesc)
-    if (requiresExtendedDesc(ele) || fir::isUnlimitedPolymorphicType(box)) {
-      dataDescFields.push_back(
-          getExtendedDescFieldTypeModel<kOptTypePtrPosInBox>()(&getContext()));
-      auto rowTy =
-          getExtendedDescFieldTypeModel<kOptRowTypePosInBox>()(&getContext());
-      dataDescFields.push_back(mlir::LLVM::LLVMArrayType::get(rowTy, 1));
-      if (auto recTy = fir::unwrapSequenceType(ele).dyn_cast<fir::RecordType>())
-        if (recTy.getNumLenParams() > 0) {
-          // The descriptor design needs to be clarified regarding the number of
-          // length parameters in the addendum. Since it can change for
-          // polymorphic allocatables, it seems all length parameters cannot
-          // always possibly be placed in the addendum.
-          TODO_NOLOC("extended descriptor derived with length parameters");
-          unsigned numLenParams = recTy.getNumLenParams();
-          dataDescFields.push_back(
-              mlir::LLVM::LLVMArrayType::get(rowTy, numLenParams));
-        }
-    }
-    // TODO: send the box type and the converted LLVM structure layout
-    // to tbaaBuilder for proper creation of TBAATypeDescriptorOp.
-    return mlir::LLVM::LLVMPointerType::get(
-        mlir::LLVM::LLVMStructType::getLiteral(&getContext(), dataDescFields,
-                                               /*isPacked=*/false));
-  }
+  mlir::Type convertBoxType(BaseBoxType box, int rank = unknownRank());
 
   /// Convert fir.box type to the corresponding llvm struct type instead of a
   /// pointer to this struct type.
-  mlir::Type convertBoxTypeAsStruct(BaseBoxType box) {
-    return convertBoxType(box)
-        .cast<mlir::LLVM::LLVMPointerType>()
-        .getElementType();
-  }
+  mlir::Type convertBoxTypeAsStruct(BaseBoxType box);
 
   // fir.boxproc<any>  -->  llvm<"{ any*, i8* }">
-  mlir::Type convertBoxProcType(BoxProcType boxproc) {
-    auto funcTy = convertType(boxproc.getEleTy());
-    auto i8PtrTy = mlir::LLVM::LLVMPointerType::get(
-        mlir::IntegerType::get(&getContext(), 8));
-    llvm::SmallVector<mlir::Type, 2> tuple = {funcTy, i8PtrTy};
-    return mlir::LLVM::LLVMStructType::getLiteral(&getContext(), tuple,
-                                                  /*isPacked=*/false);
-  }
+  mlir::Type convertBoxProcType(BoxProcType boxproc);
 
-  unsigned characterBitsize(fir::CharacterType charTy) {
-    return kindMapping.getCharacterBitsize(charTy.getFKind());
-  }
+  unsigned characterBitsize(fir::CharacterType charTy);
 
   // fir.char<k,?>  -->  llvm<"ix">          where ix is scaled by kind mapping
   // fir.char<k,n>  -->  llvm.array<n x "ix">
-  mlir::Type convertCharType(fir::CharacterType charTy) {
-    auto iTy = mlir::IntegerType::get(&getContext(), characterBitsize(charTy));
-    if (charTy.getLen() == fir::CharacterType::unknownLen())
-      return iTy;
-    return mlir::LLVM::LLVMArrayType::get(iTy, charTy.getLen());
-  }
+  mlir::Type convertCharType(fir::CharacterType charTy);
 
   // Use the target specifics to figure out how to map complex to LLVM IR. The
   // use of complex values in function signatures is handled before conversion
@@ -360,47 +126,22 @@ class LLVMTypeConverter : public mlir::LLVMTypeConverter {
 
   // convert a front-end kind value to either a std or LLVM IR dialect type
   // fir.real<n>  -->  llvm.anyfloat  where anyfloat is a kind mapping
-  mlir::Type convertRealType(fir::KindTy kind) {
-    return fir::fromRealTypeID(&getContext(), kindMapping.getRealTypeID(kind),
-                               kind);
-  }
+  mlir::Type convertRealType(fir::KindTy kind);
 
   // fir.array<c ... :any>  -->  llvm<"[...[c x any]]">
-  mlir::Type convertSequenceType(SequenceType seq) {
-    auto baseTy = convertType(seq.getEleTy());
-    if (characterWithDynamicLen(seq.getEleTy()))
-      return mlir::LLVM::LLVMPointerType::get(baseTy);
-    auto shape = seq.getShape();
-    auto constRows = seq.getConstantRows();
-    if (constRows) {
-      decltype(constRows) i = constRows;
-      for (auto e : shape) {
-        baseTy = mlir::LLVM::LLVMArrayType::get(baseTy, e);
-        if (--i == 0)
-          break;
-      }
-      if (!seq.hasDynamicExtents())
-        return baseTy;
-    }
-    return mlir::LLVM::LLVMPointerType::get(baseTy);
-  }
+  mlir::Type convertSequenceType(SequenceType seq);
 
   // fir.tdesc<any>  -->  llvm<"i8*">
   // TODO: For now use a void*, however pointer identity is not sufficient for
   // the f18 object v. class distinction (F2003).
-  mlir::Type convertTypeDescType(mlir::MLIRContext *ctx) {
-    return mlir::LLVM::LLVMPointerType::get(
-        mlir::IntegerType::get(&getContext(), 8));
-  }
+  mlir::Type convertTypeDescType(mlir::MLIRContext *ctx);
 
   KindMapping &getKindMap() { return kindMapping; }
 
   // Relay TBAA tag attachment to TBAABuilder.
   void attachTBAATag(mlir::LLVM::AliasAnalysisOpInterface op,
                      mlir::Type baseFIRType, mlir::Type accessFIRType,
-                     mlir::LLVM::GEPOp gep) {
-    tbaaBuilder.attachTBAATag(op, baseFIRType, accessFIRType, gep);
-  }
+                     mlir::LLVM::GEPOp gep);
 
 private:
   KindMapping kindMapping;