[Runtime] Lazily copy ivar list in initObjCClass.

There's often nothing that needs to be fixed up in the ivar list. In that case, we can avoid copying it. This saves time and memory, and allows the class rodata to be in immutable memory.

rdar://116189946

Author: mikeash

stdlib/public/runtime/Metadata.cpp

@@ -3441,38 +3441,45 @@ static void initObjCClass(ClassMetadata *self,
                           size_t *fieldOffsets) {
   ClassROData *rodata = getROData(self);
 
-  // Always clone the ivar descriptors.
-  if (numFields) {
-    const ClassIvarList *dependentIvars = rodata->IvarList;
-    assert(dependentIvars->Count == numFields);
-    assert(dependentIvars->EntrySize == sizeof(ClassIvarEntry));
-
-    auto ivarListSize = sizeof(ClassIvarList) +
-                        numFields * sizeof(ClassIvarEntry);
-    auto ivars = (ClassIvarList*) getResilientMetadataAllocator()
-      .Allocate(ivarListSize, alignof(ClassIvarList));
-    memcpy(ivars, dependentIvars, ivarListSize);
-    rodata->IvarList = ivars;
+  ClassIvarList *ivars = rodata->IvarList;
+  if (!ivars) {
+    assert(numFields == 0);
+    return;
+  }
 
-    for (unsigned i = 0; i != numFields; ++i) {
-      auto *eltLayout = fieldTypes[i];
+  assert(ivars->Count == numFields);
+  assert(ivars->EntrySize == sizeof(ClassIvarEntry));
 
-      ClassIvarEntry &ivar = ivars->getIvars()[i];
+  bool copiedIvarList = false;
 
-      // Fill in the field offset global, if this ivar has one.
-      if (ivar.Offset) {
-        if (*ivar.Offset != fieldOffsets[i])
-          *ivar.Offset = fieldOffsets[i];
-      }
+  for (unsigned i = 0; i != numFields; ++i) {
+    auto *eltLayout = fieldTypes[i];
 
-      // If the ivar's size doesn't match the field layout we
-      // computed, overwrite it and give it better type information.
-      if (ivar.Size != eltLayout->size) {
-        ivar.Size = eltLayout->size;
-        ivar.Type = nullptr;
-        ivar.Log2Alignment =
-          getLog2AlignmentFromMask(eltLayout->flags.getAlignmentMask());
+    ClassIvarEntry &ivar = ivars->getIvars()[i];
+
+    // Fill in the field offset global, if this ivar has one.
+    if (ivar.Offset) {
+      if (*ivar.Offset != fieldOffsets[i])
+        *ivar.Offset = fieldOffsets[i];
+    }
+
+    // If the ivar's size doesn't match the field layout we
+    // computed, overwrite it and give it better type information.
+    if (ivar.Size != eltLayout->size) {
+      // If we're going to modify the ivar list, we need to copy it first.
+      if (!copiedIvarList) {
+        auto ivarListSize = sizeof(ClassIvarList) +
+                            numFields * sizeof(ClassIvarEntry);
+        ivars = (ClassIvarList*) getResilientMetadataAllocator()
+          .Allocate(ivarListSize, alignof(ClassIvarList));
+        memcpy(ivars, rodata->IvarList, ivarListSize);
+        rodata->IvarList = ivars;
+        copiedIvarList = true;
       }
+      ivar.Size = eltLayout->size;
+      ivar.Type = nullptr;
+      ivar.Log2Alignment =
+        getLog2AlignmentFromMask(eltLayout->flags.getAlignmentMask());
     }
   }
 }