Get rid of tail padding within `absl::Duration`. This reduces memory usage needs when storing duration in containers (e.g. `vector<absl::Duration>` uses 25% less memory), and allows classes with `absl::Duration` fields to fit other stuff in memory previously used by tail padding (e.g. `std::optional<absl::Duration>` is now 16 bytes instead of 24).

PiperOrigin-RevId: 506568782
Change-Id: Ic9e077f02a80da013fb2d312aff77761b970c07a

1 files changed, 3 insertions, 70 deletions

diff --git a/absl/time/time.h b/absl/time/time.h
index bf1007c2..cc390082 100644
--- a/absl/time/time.h
+++ b/absl/time/time.h
@@ -84,7 +84,6 @@ struct timeval;
 #include <type_traits>
 #include <utility>
 
-#include "absl/base/config.h"
 #include "absl/base/macros.h"
 #include "absl/strings/string_view.h"
 #include "absl/time/civil_time.h"
@@ -215,7 +214,7 @@ class Duration {
 
   template <typename H>
   friend H AbslHashValue(H h, Duration d) {
-    return H::combine(std::move(h), d.rep_hi_.Get(), d.rep_lo_);
+    return H::combine(std::move(h), d.rep_hi_, d.rep_lo_);
   }
 
  private:
@@ -224,73 +223,7 @@ class Duration {
   friend constexpr Duration time_internal::MakeDuration(int64_t hi,
                                                         uint32_t lo);
   constexpr Duration(int64_t hi, uint32_t lo) : rep_hi_(hi), rep_lo_(lo) {}
-
-  // We store `hi_rep_` 4-byte rather than 8-byte aligned to avoid 4 bytes of
-  // tail padding.
-  class HiRep {
-   public:
-    // Default constructor default-initializes `hi_`, which has the same
-    // semantics as default-initializing an `int64_t` (undetermined value).
-    HiRep() = default;
-
-    HiRep(const HiRep&) = default;
-    HiRep& operator=(const HiRep&) = default;
-
-    explicit constexpr HiRep(const int64_t value)
-        :  // C++17 forbids default-initialization in constexpr contexts. We can
-           // remove this in C++20.
-          lo_(0),
-          hi_(0) {
-      *this = value;
-    }
-
-    constexpr int64_t Get() const {
-      const uint64_t unsigned_value =
-          (static_cast<uint64_t>(hi_) << 32) | static_cast<uint64_t>(lo_);
-      // `static_cast<int64_t>(unsigned_value)` is implementation-defined
-      // before c++20. On all supported platforms the behaviour is that mandated
-      // by c++20, i.e. "If the destination type is signed, [...] the result is
-      // the unique value of the destination type equal to the source value
-      // modulo 2^n, where n is the number of bits used to represent the
-      // destination type."
-      static_assert(
-          (static_cast<int64_t>((std::numeric_limits<uint64_t>::max)()) ==
-           int64_t{-1}) &&
-              (static_cast<int64_t>(static_cast<uint64_t>(
-                                        (std::numeric_limits<int64_t>::max)()) +
-                                    1) ==
-               (std::numeric_limits<int64_t>::min)()),
-          "static_cast<int64_t>(uint64_t) does not have c++20 semantics");
-      return static_cast<int64_t>(unsigned_value);
-    }
-
-    constexpr HiRep& operator=(const int64_t value) {
-      // "If the destination type is unsigned, the resulting value is the
-      // smallest unsigned value equal to the source value modulo 2^n
-      // where `n` is the number of bits used to represent the destination
-      // type".
-      const auto unsigned_value = static_cast<uint64_t>(value);
-      hi_ = static_cast<uint32_t>(unsigned_value >> 32);
-      lo_ = static_cast<uint32_t>(unsigned_value);
-      return *this;
-    }
-
-   private:
-    // Notes:
-    //  - Ideally we would use a `char[]` and `std::bitcast`, but the latter
-    //    does not exist (and is not constexpr in `absl`) before c++20.
-    //  - Order is optimized depending on endianness so that the compiler can
-    //    turn `Get()` (resp. `operator=()`) into a single 8-byte load (resp.
-    //    store).
-#if defined(ABSL_IS_BIG_ENDIAN) && ABSL_IS_BIG_ENDIAN
-    uint32_t hi_;
-    uint32_t lo_;
-#else
-    uint32_t lo_;
-    uint32_t hi_;
-#endif
-  };
-  HiRep rep_hi_;
+  int64_t rep_hi_;
   uint32_t rep_lo_;
 };
 
@@ -1558,7 +1491,7 @@ ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration MakeNormalizedDuration(
 
 // Provide access to the Duration representation.
 ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t GetRepHi(Duration d) {
-  return d.rep_hi_.Get();
+  return d.rep_hi_;
 }
 ABSL_ATTRIBUTE_CONST_FUNCTION constexpr uint32_t GetRepLo(Duration d) {
   return d.rep_lo_;