[PATCH] D51888: [ADT] bit_cast: check for is_trivially_copyable more portably

[JF Bastien via Phabricator via llvm-commits]

jfb created this revision.
Herald added subscribers: llvm-commits, dexonsmith.

It turns out that isPodLike isn't a good workaround for is_trivially_copyable for bit_cast's purpose. In https://reviews.llvm.org/D51872 Louis points out that tuple and pair really aren't a good fit, and for bit_cast I want to capture array. This patch instead checks is_trivially_copyable directly in bit_cast for all but GCC 4.x. In GCC 4.x developers only check for sizeof match, which means any mistake they make will succeed locally and fail on the bots. Realistically that's few developers and they'll be left behind once we upgrade past C++11.

This will allow using bit_cast with std::array.


Repository:
  rL LLVM

https://reviews.llvm.org/D51888

Files:
  include/llvm/ADT/bit.h


Index: include/llvm/ADT/bit.h
===================================================================
--- include/llvm/ADT/bit.h
+++ include/llvm/ADT/bit.h
@@ -14,15 +14,29 @@
 #ifndef LLVM_ADT_BIT_H
 #define LLVM_ADT_BIT_H
 
-#include "llvm/Support/type_traits.h"
+#include "llvm/Support/Compiler.h"
 #include <cstring>
+#include <type_traits>
 
 namespace llvm {
 
-template <typename To, typename From,
-          typename = typename std::enable_if<sizeof(To) == sizeof(From)>::type,
-          typename = typename std::enable_if<isPodLike<To>::value>::type,
-          typename = typename std::enable_if<isPodLike<From>::value>::type>
+template <typename To, typename From
+          , typename = typename std::enable_if<sizeof(To) == sizeof(From)>::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>::value>::type
+          , typename = typename std::enable_if<__is_trivially_copyable<From>::value>::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));


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