8235669: G1: Stack walking API can expose AS_NO_KEEPALIVE oops

Reviewed-by: kbarrett, tschatzl

src/hotspot/share/code/debugInfo.cpp

@@ -51,7 +51,15 @@ void DebugInfoWriteStream::write_metadata(Metadata* h) {
 }
 
 oop DebugInfoReadStream::read_oop() {
-  oop o = code()->oop_at(read_int());
+  nmethod* nm = const_cast<CompiledMethod*>(code())->as_nmethod_or_null();
+  oop o;
+  if (nm != NULL) {
+    // Despite these oops being found inside nmethods that are on-stack,
+    // they are not kept alive by all GCs (e.g. G1 and Shenandoah).
+    o = nm->oop_at_phantom(read_int());
+  } else {
+    o = code()->oop_at(read_int());
+  }
   assert(oopDesc::is_oop_or_null(o), "oop only");
   return o;
 }