[flang][NFC] speedup BoxedProcedure for derived types with many components (#86144)

This patch speeds up the compilation time of the example in
#76478 (comment)
from 2 minutes with my builds to about 2 seconds.

MLIR timers showed more than 98% of the time was spend in BoxedProcedure
trying to figure out if a type needs to be converted.

This is because walking the fir.type members is very expansive for types
containing many components and/or components with many sub-components.

Increase the caching time of visited types from "the type being visited"
to "the whole pass". Use DenseMap since it is not ok anymore to assume
this container will only have a few elements.

Author: jeanPerier

flang/lib/Optimizer/CodeGen/BoxedProcedure.cpp

@@ -69,17 +69,32 @@ class BoxprocTypeRewriter : public mlir::TypeConverter {
       return false;
     }
     if (auto recTy = ty.dyn_cast<RecordType>()) {
-      if (llvm::is_contained(visitedTypes, recTy))
-        return false;
+      auto visited = visitedTypes.find(ty);
+      if (visited != visitedTypes.end())
+        return visited->second;
+      [[maybe_unused]] auto newIt = visitedTypes.try_emplace(ty, false);
+      assert(newIt.second && "expected ty to not be in the map");
+      bool wasAlreadyVisitingRecordType = needConversionIsVisitingRecordType;
+      needConversionIsVisitingRecordType = true;
       bool result = false;
-      visitedTypes.push_back(recTy);
       for (auto t : recTy.getTypeList()) {
         if (needsConversion(t.second)) {
           result = true;
           break;
         }
       }
-      visitedTypes.pop_back();
+      // Only keep the result cached if the fir.type visited was a "top-level
+      // type". Nested types with a recursive reference to the "top-level type"
+      // may incorrectly have been resolved as not needed conversions because it
+      // had not been determined yet if the "top-level type" needed conversion.
+      // This is not an issue to determine the "top-level type" need of
+      // conversion, but the result should not be kept and later used in other
+      // contexts.
+      needConversionIsVisitingRecordType = wasAlreadyVisitingRecordType;
+      if (needConversionIsVisitingRecordType)
+        visitedTypes.erase(ty);
+      else
+        visitedTypes.find(ty)->second = result;
       return result;
     }
     if (auto boxTy = ty.dyn_cast<BaseBoxType>())
@@ -140,9 +155,7 @@ class BoxprocTypeRewriter : public mlir::TypeConverter {
       if (rec.isFinalized())
         return rec;
       auto it = convertedTypes.try_emplace(ty, rec);
-      if (!it.second) {
-        llvm::errs() << "failed\n" << ty << "\n";
-      }
+      assert(it.second && "expected ty to not be in the map");
       std::vector<RecordType::TypePair> ps = ty.getLenParamList();
       std::vector<RecordType::TypePair> cs;
       for (auto t : ty.getTypeList()) {
@@ -171,11 +184,12 @@ class BoxprocTypeRewriter : public mlir::TypeConverter {
   void setLocation(mlir::Location location) { loc = location; }
 
 private:
-  llvm::SmallVector<mlir::Type> visitedTypes;
-  // Map to deal with recursive derived types (avoid infinite loops).
+  // Maps to deal with recursive derived types (avoid infinite loops).
   // Caching is also beneficial for apps with big types (dozens of
   // components and or parent types), so the lifetime of the cache
   // is the whole pass.
+  llvm::DenseMap<mlir::Type, bool> visitedTypes;
+  bool needConversionIsVisitingRecordType = false;
   llvm::DenseMap<mlir::Type, mlir::Type> convertedTypes;
   mlir::Location loc;
 };

flang/test/Fir/boxproc-2.fir

@@ -108,3 +108,10 @@ func.func @very_nested_type(%arg0: !fir.type<t0{t01:!fir.type<t01{t02:!fir.type<
 }
 // CHECK-LABEL: func.func @very_nested_type(
 // CHECK-SAME: !fir.type<t0UnboxProc{t01:!fir.type<t01UnboxProc{t02:!fir.type<t02UnboxProc{t3:!fir.type<t3UnboxProc{t4:!fir.type<t4UnboxProc{t5:!fir.type<t5UnboxProc{t6:!fir.type<t6UnboxProc{t7:!fir.type<t7UnboxProc{t8:!fir.type<t8UnboxProc{t9:!fir.type<t9UnboxProc{t10:!fir.type<t10UnboxProc{t11:!fir.type<t11UnboxProc{t12:!fir.type<t12UnboxProc{t13:!fir.type<t13UnboxProc{t14:!fir.type<t14UnboxProc{t15:!fir.type<t15UnboxProc{t16:!fir.type<t16UnboxProc{t17:!fir.type<t17UnboxProc{t18:!fir.type<t18UnboxProc{t19:!fir.type<t19UnboxProc{b:() -> ()}>}>}>}>}>}>}>}>}>}>}>}>}>}>}>}>}>}>}>}>)
+
+func.func @test_indirect_type_recursion() {
+    %0 = fir.zero_bits !fir.ptr<!fir.type<t001{x:!fir.ptr<!fir.type<t002{x:!fir.ptr<!fir.type<t001>>}>>,p:!fir.boxproc<() -> ()>}>>
+  return
+}
+// CHECK-LABEL: func.func @test_indirect_type_recursion(
+// CHECK: fir.zero_bits !fir.ptr<!fir.type<t001UnboxProc{x:!fir.ptr<!fir.type<t002UnboxProc{x:!fir.ptr<!fir.type<t001UnboxProc>>}>>,p:() -> ()}>>