[llvm] r342710 - [ADT] restrict bit_cast to trivially-constructible To
JF Bastien via llvm-commits
llvm-commits at lists.llvm.org
Thu Sep 20 22:18:49 PDT 2018
Author: jfb
Date: Thu Sep 20 22:18:49 2018
New Revision: 342710
URL: http://llvm.org/viewvc/llvm-project?rev=342710&view=rev
Log:
[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
Modified:
llvm/trunk/include/llvm/ADT/bit.h
Modified: llvm/trunk/include/llvm/ADT/bit.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/bit.h?rev=342710&r1=342709&r2=342710&view=diff
==============================================================================
--- llvm/trunk/include/llvm/ADT/bit.h (original)
+++ llvm/trunk/include/llvm/ADT/bit.h Thu Sep 20 22:18:49 2018
@@ -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
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