Merge tag '20220623.1-1' into bullseye-backports-staging

1 files changed, 61 insertions, 68 deletions

diff --git a/absl/base/casts.h b/absl/base/casts.h
index 83c69126..b99adb06 100644
--- a/absl/base/casts.h
+++ b/absl/base/casts.h
@@ -29,6 +29,10 @@
 #include <type_traits>
 #include <utility>
 
+#if defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
+#include <bit>  // For std::bit_cast.
+#endif  // defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
+
 #include "absl/base/internal/identity.h"
 #include "absl/base/macros.h"
 #include "absl/meta/type_traits.h"
@@ -36,19 +40,6 @@
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
-namespace internal_casts {
-
-template <class Dest, class Source>
-struct is_bitcastable
-    : std::integral_constant<
-          bool,
-          sizeof(Dest) == sizeof(Source) &&
-              type_traits_internal::is_trivially_copyable<Source>::value &&
-              type_traits_internal::is_trivially_copyable<Dest>::value &&
-              std::is_default_constructible<Dest>::value> {};
-
-}  // namespace internal_casts
-
 // implicit_cast()
 //
 // Performs an implicit conversion between types following the language
@@ -105,81 +96,83 @@ constexpr To implicit_cast(typename absl::internal::identity_t<To> to) {
 
 // bit_cast()
 //
-// Performs a bitwise cast on a type without changing the underlying bit
-// representation of that type's value. The two types must be of the same size
-// and both types must be trivially copyable. As with most casts, use with
-// caution. A `bit_cast()` might be needed when you need to temporarily treat a
-// type as some other type, such as in the following cases:
+// Creates a value of the new type `Dest` whose representation is the same as
+// that of the argument, which is of (deduced) type `Source` (a "bitwise cast";
+// every bit in the value representation of the result is equal to the
+// corresponding bit in the object representation of the source). Source and
+// destination types must be of the same size, and both types must be trivially
+// copyable.
 //
-//    * Serialization (casting temporarily to `char *` for those purposes is
-//      always allowed by the C++ standard)
-//    * Managing the individual bits of a type within mathematical operations
-//      that are not normally accessible through that type
-//    * Casting non-pointer types to pointer types (casting the other way is
-//      allowed by `reinterpret_cast()` but round-trips cannot occur the other
-//      way).
-//
-// Example:
+// As with most casts, use with caution. A `bit_cast()` might be needed when you
+// need to treat a value as the value of some other type, for example, to access
+// the individual bits of an object which are not normally accessible through
+// the object's type, such as for working with the binary representation of a
+// floating point value:
 //
 //   float f = 3.14159265358979;
-//   int i = bit_cast<int32_t>(f);
+//   int i = bit_cast<int>(f);
 //   // i = 0x40490fdb
 //
-// Casting non-pointer types to pointer types and then dereferencing them
-// traditionally produces undefined behavior.
+// Reinterpreting and accessing a value directly as a different type (as shown
+// below) usually results in undefined behavior.
 //
 // Example:
 //
 //   // WRONG
-//   float f = 3.14159265358979;            // WRONG
-//   int i = * reinterpret_cast<int*>(&f);  // WRONG
+//   float f = 3.14159265358979;
+//   int i = reinterpret_cast<int&>(f);    // Wrong
+//   int j = *reinterpret_cast<int*>(&f);  // Equally wrong
+//   int k = *bit_cast<int*>(&f);          // Equally wrong
 //
-// The address-casting method produces undefined behavior according to the ISO
-// C++ specification section [basic.lval]. Roughly, this section says: if an
-// object in memory has one type, and a program accesses it with a different
-// type, the result is undefined behavior for most values of "different type".
+// Reinterpret-casting results in undefined behavior according to the ISO C++
+// specification, section [basic.lval]. Roughly, this section says: if an object
+// in memory has one type, and a program accesses it with a different type, the
+// result is undefined behavior for most "different type".
+//
+// Using bit_cast on a pointer and then dereferencing it is no better than using
+// reinterpret_cast. You should only use bit_cast on the value itself.
 //
 // Such casting results in type punning: holding an object in memory of one type
 // and reading its bits back using a different type. A `bit_cast()` avoids this
-// issue by implementing its casts using `memcpy()`, which avoids introducing
-// this undefined behavior.
-//
-// NOTE: The requirements here are more strict than the bit_cast of standard
-// proposal p0476 due to the need for workarounds and lack of intrinsics.
-// Specifically, this implementation also requires `Dest` to be
-// default-constructible.
-template <
-    typename Dest, typename Source,
-    typename std::enable_if<internal_casts::is_bitcastable<Dest, Source>::value,
-                            int>::type = 0>
+// issue by copying the object representation to a new value, which avoids
+// introducing this undefined behavior (since the original value is never
+// accessed in the wrong way).
+//
+// The requirements of `absl::bit_cast` are more strict than that of
+// `std::bit_cast` unless compiler support is available. Specifically, without
+// compiler support, this implementation also requires `Dest` to be
+// default-constructible. In C++20, `absl::bit_cast` is replaced by
+// `std::bit_cast`.
+#if defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
+
+using std::bit_cast;
+
+#else  // defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
+
+template <typename Dest, typename Source,
+          typename std::enable_if<
+              sizeof(Dest) == sizeof(Source) &&
+                  type_traits_internal::is_trivially_copyable<Source>::value &&
+                  type_traits_internal::is_trivially_copyable<Dest>::value
+#if !ABSL_HAVE_BUILTIN(__builtin_bit_cast)
+                  && std::is_default_constructible<Dest>::value
+#endif  // !ABSL_HAVE_BUILTIN(__builtin_bit_cast)
+              ,
+              int>::type = 0>
+#if ABSL_HAVE_BUILTIN(__builtin_bit_cast)
+inline constexpr Dest bit_cast(const Source& source) {
+  return __builtin_bit_cast(Dest, source);
+}
+#else  // ABSL_HAVE_BUILTIN(__builtin_bit_cast)
 inline Dest bit_cast(const Source& source) {
   Dest dest;
   memcpy(static_cast<void*>(std::addressof(dest)),
          static_cast<const void*>(std::addressof(source)), sizeof(dest));
   return dest;
 }
+#endif  // ABSL_HAVE_BUILTIN(__builtin_bit_cast)
 
-// NOTE: This overload is only picked if the requirements of bit_cast are
-// not met. It is therefore UB, but is provided temporarily as previous
-// versions of this function template were unchecked. Do not use this in
-// new code.
-template <
-    typename Dest, typename Source,
-    typename std::enable_if<
-        !internal_casts::is_bitcastable<Dest, Source>::value,
-        int>::type = 0>
-ABSL_DEPRECATED(
-    "absl::bit_cast type requirements were violated. Update the types "
-    "being used such that they are the same size and are both "
-    "TriviallyCopyable.")
-inline Dest bit_cast(const Source& source) {
-  static_assert(sizeof(Dest) == sizeof(Source),
-                "Source and destination types should have equal sizes.");
-
-  Dest dest;
-  memcpy(&dest, &source, sizeof(dest));
-  return dest;
-}
+#endif  // defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
 
 ABSL_NAMESPACE_END
 }  // namespace absl