From: JF Bastien <jfbastien@apple.com>
Date: Fri, 21 Sep 2018 05:18:49 +0000 (+0000)
Subject: [ADT] restrict bit_cast to trivially-constructible To
X-Git-Tag: llvmorg-8.0.0-rc1~8271
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=86f732a6a8809799a5e1bbae5911c7c48477cadc;p=platform%2Fupstream%2Fllvm.git

[ADT] restrict bit_cast to trivially-constructible To

Summary: As discussed in r341853 by blaikie, the reinterpret_cast was technically an aliasing violation. Restrict our bit_cast implementation to To which are trivially-constructible (and note the existing restriction to constexpr). Once we move to C++17 we can use a version of bit_cast without these restrictions, or if we care we can SFINAE a different implementation when To isn't trivially-constructible.

Reviewers: dblaikie, rsmith

Subscribers: dexonsmith, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D52332

llvm-svn: 342710
---

diff --git a/llvm/include/llvm/ADT/bit.h b/llvm/include/llvm/ADT/bit.h
index 0e38321..0a3c2ed 100644
--- a/llvm/include/llvm/ADT/bit.h
+++ b/llvm/include/llvm/ADT/bit.h
@@ -20,8 +20,12 @@
 
 namespace llvm {
 
+// This implementation of bit_cast is different from the C++17 one in two ways:
+//  - It isn't constexpr because that requires compiler support.
+//  - It requires trivially-constructible To, to avoid UB in the implementation.
 template <typename To, typename From
           , typename = typename std::enable_if<sizeof(To) == sizeof(From)>::type
+          , typename = typename std::is_trivially_constructible<To>::type
 #if (__has_feature(is_trivially_copyable) && defined(_LIBCPP_VERSION)) || \
     (defined(__GNUC__) && __GNUC__ >= 5)
           , typename = typename std::enable_if<std::is_trivially_copyable<To>::value>::type
@@ -38,17 +42,9 @@ template <typename To, typename From
 #endif
 >
 inline To bit_cast(const From &from) noexcept {
-  alignas(To) unsigned char storage[sizeof(To)];
-  std::memcpy(&storage, &from, sizeof(To));
-#if defined(__GNUC__)
-  // Before GCC 7.2, GCC thought that this violated strict aliasing.
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wstrict-aliasing"
-#endif
-  return reinterpret_cast<To &>(storage);
-#if defined(__GNUC__)
-#pragma GCC diagnostic pop
-#endif
+  To to;
+  std::memcpy(&to, &from, sizeof(To));
+  return to;
 }
 
 } // namespace llvm