From 95cd300bbdb4538635a5dc7f08c184aff45637ad Mon Sep 17 00:00:00 2001
From: Joshua Warner <joshua.warner@readytalk.com>
Date: Thu, 26 May 2016 14:35:13 +0000
Subject: [PATCH] Don't access potentially mis-aligned, non-existent length
 field for non-array objects (fixes #486)

---
 src/avian/machine.h | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/avian/machine.h b/src/avian/machine.h
index 40b627cdbc..3011dd47ed 100644
--- a/src/avian/machine.h
+++ b/src/avian/machine.h
@@ -1987,11 +1987,14 @@ inline unsigned baseSize(Thread* t UNUSED, object o, GcClass* class_)
 {
   assertT(t, class_->fixedSize() >= BytesPerWord);
 
-  return ceilingDivide(class_->fixedSize(), BytesPerWord)
-         + ceilingDivide(class_->arrayElementSize()
-                         * fieldAtOffset<uintptr_t>(
-                               o, class_->fixedSize() - BytesPerWord),
-                         BytesPerWord);
+  unsigned size = ceilingDivide(class_->fixedSize(), BytesPerWord);
+  if (class_->arrayElementSize() > 0) {
+    size += ceilingDivide(class_->arrayElementSize()
+                       * fieldAtOffset<uintptr_t>(
+                             o, class_->fixedSize() - BytesPerWord),
+                       BytesPerWord);
+  }
+  return size;
 }
 
 object makeTrace(Thread* t, Processor::StackWalker* walker);