[ADT] restrict bit_cast to trivially-constructible To

[jfb]

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.

[dblaikie]

Looks good - thanks a bunch!

[jfb]

Had to revert, this was in GCC 5.1 only. I'll fix tomorrow.

[jfb]

Re-landed as r342739.

include/llvm/ADT/bit.h

 #define LLVM_ADT_BIT_H
 
 #include "llvm/Support/Compiler.h"
 #include <cstring>
 #include <type_traits>
 
 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
           , typename = typename std::enable_if<std::is_trivially_copyable<From>::value>::type
 #elif __has_feature(is_trivially_copyable)
           , typename = typename std::enable_if<__is_trivially_copyable(To)>::type
           , typename = typename std::enable_if<__is_trivially_copyable(From)>::type
 #else
   // This case is GCC 4.x. clang with libc++ or libstdc++ never get here. Unlike
   // llvm/Support/type_traits.h's isPodLike we don't want to provide a
   // good-enough answer here: developers in that configuration will hit
   // compilation failures on the bots instead of locally. That's acceptable
   // because it's very few developers, and only until we move past C++11.
 #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
 
 #endif