[fir] Use contralized values for indexing box

[clementval]

Summary

Add constant to index the different values in a box so that
they can be reused for the codegen part as well.

[awarzynski]

Big thank you for this patch! IMHO it would be helpful to expand the comment, but it's already a big improvement as is!

flang/lib/Optimizer/CodeGen/TypeConverter.h

24

[nit] I appreciate that we do know what box this is referring to, but it might be unclear to somebody completely new to this code-base. Perhaps it's worth referring to the CFI struct? And fir.box instead of box?

flang/lib/Optimizer/CodeGen/TypeConverter.h

 #include "DescriptorModel.h"
 #include "Target.h"
 #include "flang/Lower/Todo.h" // remove when TODO's are done
 #include "flang/Optimizer/Support/FIRContext.h"
 #include "flang/Optimizer/Support/KindMapping.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/Support/Debug.h"
 
+// Position of the different values in a `fir.box`.
+static constexpr unsigned kAddrPosInBox = 0;
+static constexpr unsigned kElemLenPosInBox = 1;
+static constexpr unsigned kVersionPosInBox = 2;
+static constexpr unsigned kRankPosInBox = 3;
+static constexpr unsigned kTypePosInBox = 4;
+static constexpr unsigned kAttributePosInBox = 5;
+static constexpr unsigned kF18AddendumPosInBox = 6;
+static constexpr unsigned kDimsPosInBox = 7;
+static constexpr unsigned kOptTypePtrPosInBox = 8;
+static constexpr unsigned kOptRowTypePosInBox = 9;
+
 namespace fir {
 
 /// FIR type converter
 /// This converts FIR types to LLVM types (for now)
 class LLVMTypeConverter : public mlir::LLVMTypeConverter {
 public:
   LLVMTypeConverter(mlir::ModuleOp module)
       : mlir::LLVMTypeConverter(module.getContext()),
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<1>()(&getContext()));
+    dataDescFields.push_back(
+        getDescFieldTypeModel<kElemLenPosInBox>()(&getContext()));
     // version
-    dataDescFields.push_back(getDescFieldTypeModel<2>()(&getContext()));
+    dataDescFields.push_back(
+        getDescFieldTypeModel<kVersionPosInBox>()(&getContext()));
     // rank
-    dataDescFields.push_back(getDescFieldTypeModel<3>()(&getContext()));
+    dataDescFields.push_back(
+        getDescFieldTypeModel<kRankPosInBox>()(&getContext()));
     // type
-    dataDescFields.push_back(getDescFieldTypeModel<4>()(&getContext()));
+    dataDescFields.push_back(
+        getDescFieldTypeModel<kTypePosInBox>()(&getContext()));
     // attribute
-    dataDescFields.push_back(getDescFieldTypeModel<5>()(&getContext()));
+    dataDescFields.push_back(
+        getDescFieldTypeModel<kAttributePosInBox>()(&getContext()));
     // f18Addendum
-    dataDescFields.push_back(getDescFieldTypeModel<6>()(&getContext()));
+    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<7>()(&getContext());
+      auto rowTy = getDescFieldTypeModel<kDimsPosInBox>()(&getContext());
       dataDescFields.push_back(mlir::LLVM::LLVMArrayType::get(rowTy, rank));
     }
     // opt-type-ptr: i8* (see fir.tdesc)
     if (requiresExtendedDesc(ele)) {
       dataDescFields.push_back(
-          getExtendedDescFieldTypeModel<8>()(&getContext()));
-      auto rowTy = getExtendedDescFieldTypeModel<9>()(&getContext());
+          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");