[Runtime] Don't write to rodata if values already match.

initClassFieldOffsetVector writes the instanceStart and size to the class's rodata. In some cases they already match, and this write will dirty memory unnecessarily, and prevent the compiler from emitting those rodatas into read-only memory.

rdar://problem/71119533

Author: mikeash

stdlib/public/runtime/Metadata.cpp

@@ -2721,7 +2721,12 @@ static void initClassFieldOffsetVector(ClassMetadata *self,
   // situations where our entire superclass hierarchy is defined
   // in Swift.  (But note that ObjC might think we have a superclass
   // even if Swift doesn't, because of SwiftObject.)
-  rodata->InstanceStart = size;
+  //
+  // The rodata may be in read-only memory if the compiler knows that the size
+  // it generates is already definitely correct. Don't write to this value
+  // unless it's necessary.
+  if (rodata->InstanceStart != size)
+    rodata->InstanceStart = size;
 #endif
 
   // Okay, now do layout.
@@ -2745,7 +2750,8 @@ static void initClassFieldOffsetVector(ClassMetadata *self,
 
 #if SWIFT_OBJC_INTEROP
   // Save the size into the Objective-C metadata as well.
-  rodata->InstanceSize = size;
+  if (rodata->InstanceSize != size)
+    rodata->InstanceSize = size;
 #endif
 }