Make inline cache reads / writes atomic with object shapes

Prior to this commit, we were reading and writing ivar index and
shape ID in inline caches in two separate instructions when
getting and setting ivars. This meant there was a race condition
with ractors and these caches where one ractor could change
a value in the cache while another was still reading from it.

This commit instead reads and writes shape ID and ivar index to
inline caches atomically so there is no longer a race condition.

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>
Co-Authored-By: John Hawthorn <john@hawthorn.email>

1 files changed, 1 insertions, 3 deletions

diff --git a/vm_core.h b/vm_core.h
index 4dd873c1f8..aed15114e5 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -273,9 +273,7 @@ struct iseq_inline_constant_cache {
 };
 
 struct iseq_inline_iv_cache_entry {
-    shape_id_t source_shape_id;
-    shape_id_t dest_shape_id;
-    attr_index_t attr_index;
+    uintptr_t value; // attr_index in lower bits, dest_shape_id in upper bits
 };
 
 struct iseq_inline_cvar_cache_entry {