From 7bbef1fafc2ce4d10633ba72bde7c714bee9aef6 Mon Sep 17 00:00:00 2001
From: Jeff Walden <jwalden@mit.edu>
Date: Wed, 6 Jun 2018 16:03:47 -0700
Subject: [PATCH] Bug 1465981 - Use memcpy instead of union-arm-punning (which
 has implementation-defined, desired behavior with gcc and presumably clang,
 and is not known to have problems on MSVC, but potentially could with other
 compilers) in BitwiseCast.  r=froydnj

--HG--
extra : rebase_source : 686363576c84710ae0181afc32b05dee8b40a59b
---
 mfbt/Casting.h | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/mfbt/Casting.h b/mfbt/Casting.h
index adf2c90456b4..6648b1488db2 100644
--- a/mfbt/Casting.h
+++ b/mfbt/Casting.h
@@ -12,7 +12,9 @@
 #include "mozilla/Assertions.h"
 #include "mozilla/TypeTraits.h"
 
+#include <cstring>
 #include <limits.h>
+#include <type_traits>
 
 namespace mozilla {
 
@@ -43,13 +45,19 @@ BitwiseCast(const From aFrom, To* aResult)
 {
   static_assert(sizeof(From) == sizeof(To),
                 "To and From must have the same size");
-  union
-  {
-    From mFrom;
-    To mTo;
-  } u;
-  u.mFrom = aFrom;
-  *aResult = u.mTo;
+
+  // We could maybe downgrade these to std::is_trivially_copyable, but the
+  // various STLs we use don't all provide it.
+  static_assert(std::is_trivial<From>::value,
+                "shouldn't bitwise-copy a type having non-trivial "
+                "initialization");
+  static_assert(std::is_trivial<To>::value,
+                "shouldn't bitwise-copy a type having non-trivial "
+                "initialization");
+
+  std::memcpy(static_cast<void*>(aResult),
+              static_cast<const void*>(&aFrom),
+              sizeof(From));
 }
 
 template<typename To, typename From>