Cache the result of FuncDeclaration.isTypeIsolated() because it is potentially a very expensive check..

This resulted in a large compile-time improvement on the large codebase at Weka.io (11min --> 3.5min).

Author: JohanEngelen

src/dmd/declaration.h

@@ -535,6 +535,8 @@ class FuncDeclaration : public Declaration
     VarDeclaration *vresult;            // result variable for out contracts
     LabelDsymbol *returnLabel;          // where the return goes
 
+    void *isTypeIsolatedCache;          // An AA on the D side to cache an expensive check result
+
     // used to prevent symbols in different
     // scopes from having the same name
     DsymbolTable *localsymtab;

src/dmd/func.d

@@ -262,6 +262,8 @@ extern (C++) class FuncDeclaration : Declaration
     VarDeclaration vresult;             /// result variable for out contracts
     LabelDsymbol returnLabel;           /// where the return goes
 
+    bool[size_t] isTypeIsolatedCache;   /// cache for the potentially very expensive isTypeIsolated check
+
     // used to prevent symbols in different
     // scopes from having the same name
     DsymbolTable localsymtab;
@@ -1530,8 +1532,23 @@ extern (C++) class FuncDeclaration : Declaration
     extern (D) final bool isTypeIsolated(Type t)
     {
         StringTable!Type parentTypes;
-        parentTypes._init();
-        return isTypeIsolated(t, parentTypes);
+        const uniqueTypeID = t.getUniqueID();
+        if (uniqueTypeID)
+        {
+            const cacheResultPtr = uniqueTypeID in isTypeIsolatedCache;
+            if (cacheResultPtr !is null)
+                return *cacheResultPtr;
+
+            parentTypes._init();
+            const isIsolated = isTypeIsolated(t, parentTypes);
+            isTypeIsolatedCache[uniqueTypeID] = isIsolated;
+            return isIsolated;
+        }
+        else
+        {
+            parentTypes._init();
+            return isTypeIsolated(t, parentTypes);
+        }
     }
 
     ///ditto

src/dmd/mtype.d

@@ -426,6 +426,13 @@ extern (C++) abstract class Type : ASTNode
         return DYNCAST.type;
     }
 
+    /// Returns a non-zero unique ID for this Type, or returns 0 if the Type does not (yet) have a unique ID.
+    /// If `semantic()` has not been run, 0 is returned.
+    final size_t getUniqueID() const
+    {
+        return cast(size_t) deco;
+    }
+
     extern (D)
     final Mcache* getMcache()
     {