Merge pull request swiftlang#34616 from mikeash/writers-block

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

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
 }