[libc-commits] [PATCH] D148739: [libc] Use __builtin_bit_cast only when src and dest types are trivially copyable

[Mikhail Ramalho via Phabricator via libc-commits]

mikhail.ramalho updated this revision to Diff 515524.
mikhail.ramalho added a comment.

Addressed comments


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D148739/new/

https://reviews.llvm.org/D148739

Files:
  libc/src/__support/CPP/bit.h
  libc/src/__support/CPP/type_traits.h


Index: libc/src/__support/CPP/type_traits.h
===================================================================
--- libc/src/__support/CPP/type_traits.h
+++ libc/src/__support/CPP/type_traits.h
@@ -12,9 +12,7 @@
 namespace __llvm_libc {
 namespace cpp {
 
-template <typename T> struct type_identity {
-  using type = T;
-};
+template <typename T> struct type_identity { using type = T; };
 
 template <bool B, typename T> struct enable_if;
 template <typename T> struct enable_if<true, T> : type_identity<T> {};
@@ -28,6 +26,14 @@
 using true_type = cpp::integral_constant<bool, true>;
 using false_type = cpp::integral_constant<bool, false>;
 
+template <class T>
+struct is_trivially_copyable
+    : public integral_constant<bool, __is_trivially_copyable(T)> {};
+
+template <class T, class... Args>
+struct is_trivially_constructible
+    : integral_constant<bool, __is_trivially_constructible(T, Args...)> {};
+
 template <typename T, typename U> struct is_same : cpp::false_type {};
 template <typename T> struct is_same<T, T> : cpp::true_type {};
 template <typename T, typename U>
Index: libc/src/__support/CPP/bit.h
===================================================================
--- libc/src/__support/CPP/bit.h
+++ libc/src/__support/CPP/bit.h
@@ -9,6 +9,7 @@
 #ifndef LLVM_LIBC_SUPPORT_CPP_BIT_H
 #define LLVM_LIBC_SUPPORT_CPP_BIT_H
 
+#include "src/__support/CPP/type_traits.h"
 #include "src/__support/macros/config.h" // LIBC_HAS_BUILTIN
 
 namespace __llvm_libc::cpp {
@@ -25,9 +26,15 @@
 // GCC >= 8 and Clang >= 6.
 template <class To, class From> constexpr To bit_cast(const From &from) {
   static_assert(sizeof(To) == sizeof(From), "To and From must be of same size");
+  static_assert(cpp::is_trivially_copyable<To>::value &&
+                    cpp::is_trivially_copyable<From>::value,
+                "Cannot bit-cast instances of non-trivially copyable classes.");
 #if defined(LLVM_LIBC_HAS_BUILTIN_BIT_CAST)
   return __builtin_bit_cast(To, from);
 #else
+  static_assert(cpp::is_trivially_constructible<To>::value,
+                "This implementation additionally requires destination type to "
+                "be trivially constructible");
   To to;
   char *dst = reinterpret_cast<char *>(&to);
   const char *src = reinterpret_cast<const char *>(&from);


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