Merge pull request #29219 from aschwaighofer/irgen_fix_zero_size_field_instanceStart_2

IRGen: Correctly compute instanceStart for a Swift class that starts …

lib/IRGen/GenClass.cpp

@@ -165,6 +165,8 @@ namespace {
 
     ClassMetadataOptions Options;
 
+    Size HeaderSize;
+
   public:
     ClassLayoutBuilder(IRGenModule &IGM, SILType classType,
                        ReferenceCounting refcounting,
@@ -178,11 +180,13 @@ namespace {
       case ReferenceCounting::Native:
         // For native classes, place a full object header.
         addHeapHeader();
+        HeaderSize = CurSize;
         break;
       case ReferenceCounting::ObjC:
         // For ObjC-inheriting classes, we don't reliably know the size of the
         // base class, but NSObject only has an `isa` pointer at most.
         addNSObjectHeader();
+        HeaderSize = CurSize;
         break;
       case ReferenceCounting::Block:
       case ReferenceCounting::Unknown:
@@ -222,7 +226,7 @@ namespace {
       auto allElements = IGM.Context.AllocateCopy(Elements);
 
       return ClassLayout(*this, Options, classTy,
-                         allStoredProps, allFieldAccesses, allElements);
+                         allStoredProps, allFieldAccesses, allElements, HeaderSize);
     }
 
   private: