Explain union safety in MicroLock comment

Reviewed By: ot

Differential Revision: D3045850

fb-gh-sync-id: ee19b146c43410d8d8804c9bfe79c3811394b10e
shipit-source-id: ee19b146c43410d8d8804c9bfe79c3811394b10e

diff --git a/folly/MicroLock.h b/folly/MicroLock.h
index 07aa40b8df592a08ac557fc3c47e5b1e649d5443..159e3341c6dce3beb2ad2881193e4afeefb34f60 100644 (file)
--- a/folly/MicroLock.h
+++ b/folly/MicroLock.h
@@ -43,6 +43,10 @@ namespace folly {
  * limit yourself to MicroLock slot zero, which will use the two
  * least-significant bits in the bottom byte.
  *
+ * (Note that such a union is safe only because MicroLock is based on
+ * a character type, and even under a strict interpretation of C++'s
+ * aliasing rules, character types may alias anything.)
+ *
  * MicroLock uses a dirty trick: it actually operates on the full
  * word-size, word-aligned bit of memory into which it is embedded.
  * It never modifies bits outside the ones it's defined to modify, but