Reland "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: 524256689
Change-Id: Ibf40d9e5411020179fa34c972349c7b58aa9d908
diff --git a/absl/time/duration.cc b/absl/time/duration.cc
index 26fadb3..634e5d5 100644
--- a/absl/time/duration.cc
+++ b/absl/time/duration.cc
@@ -400,16 +400,18 @@
Duration& Duration::operator+=(Duration rhs) {
if (time_internal::IsInfiniteDuration(*this)) return *this;
if (time_internal::IsInfiniteDuration(rhs)) return *this = rhs;
- const int64_t orig_rep_hi = rep_hi_;
- rep_hi_ =
- DecodeTwosComp(EncodeTwosComp(rep_hi_) + EncodeTwosComp(rhs.rep_hi_));
+ const int64_t orig_rep_hi = rep_hi_.Get();
+ rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_.Get()) +
+ EncodeTwosComp(rhs.rep_hi_.Get()));
if (rep_lo_ >= kTicksPerSecond - rhs.rep_lo_) {
- rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_) + 1);
+ rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_.Get()) + 1);
rep_lo_ -= kTicksPerSecond;
}
rep_lo_ += rhs.rep_lo_;
- if (rhs.rep_hi_ < 0 ? rep_hi_ > orig_rep_hi : rep_hi_ < orig_rep_hi) {
- return *this = rhs.rep_hi_ < 0 ? -InfiniteDuration() : InfiniteDuration();
+ if (rhs.rep_hi_.Get() < 0 ? rep_hi_.Get() > orig_rep_hi
+ : rep_hi_.Get() < orig_rep_hi) {
+ return *this =
+ rhs.rep_hi_.Get() < 0 ? -InfiniteDuration() : InfiniteDuration();
}
return *this;
}
@@ -417,18 +419,21 @@
Duration& Duration::operator-=(Duration rhs) {
if (time_internal::IsInfiniteDuration(*this)) return *this;
if (time_internal::IsInfiniteDuration(rhs)) {
- return *this = rhs.rep_hi_ >= 0 ? -InfiniteDuration() : InfiniteDuration();
+ return *this = rhs.rep_hi_.Get() >= 0 ? -InfiniteDuration()
+ : InfiniteDuration();
}
- const int64_t orig_rep_hi = rep_hi_;
- rep_hi_ =
- DecodeTwosComp(EncodeTwosComp(rep_hi_) - EncodeTwosComp(rhs.rep_hi_));
+ const int64_t orig_rep_hi = rep_hi_.Get();
+ rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_.Get()) -
+ EncodeTwosComp(rhs.rep_hi_.Get()));
if (rep_lo_ < rhs.rep_lo_) {
- rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_) - 1);
+ rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_.Get()) - 1);
rep_lo_ += kTicksPerSecond;
}
rep_lo_ -= rhs.rep_lo_;
- if (rhs.rep_hi_ < 0 ? rep_hi_ < orig_rep_hi : rep_hi_ > orig_rep_hi) {
- return *this = rhs.rep_hi_ >= 0 ? -InfiniteDuration() : InfiniteDuration();
+ if (rhs.rep_hi_.Get() < 0 ? rep_hi_.Get() < orig_rep_hi
+ : rep_hi_.Get() > orig_rep_hi) {
+ return *this = rhs.rep_hi_.Get() >= 0 ? -InfiniteDuration()
+ : InfiniteDuration();
}
return *this;
}
@@ -439,7 +444,7 @@
Duration& Duration::operator*=(int64_t r) {
if (time_internal::IsInfiniteDuration(*this)) {
- const bool is_neg = (r < 0) != (rep_hi_ < 0);
+ const bool is_neg = (r < 0) != (rep_hi_.Get() < 0);
return *this = is_neg ? -InfiniteDuration() : InfiniteDuration();
}
return *this = ScaleFixed<SafeMultiply>(*this, r);
@@ -447,7 +452,7 @@
Duration& Duration::operator*=(double r) {
if (time_internal::IsInfiniteDuration(*this) || !IsFinite(r)) {
- const bool is_neg = (std::signbit(r) != 0) != (rep_hi_ < 0);
+ const bool is_neg = std::signbit(r) != (rep_hi_.Get() < 0);
return *this = is_neg ? -InfiniteDuration() : InfiniteDuration();
}
return *this = ScaleDouble<std::multiplies>(*this, r);
@@ -455,7 +460,7 @@
Duration& Duration::operator/=(int64_t r) {
if (time_internal::IsInfiniteDuration(*this) || r == 0) {
- const bool is_neg = (r < 0) != (rep_hi_ < 0);
+ const bool is_neg = (r < 0) != (rep_hi_.Get() < 0);
return *this = is_neg ? -InfiniteDuration() : InfiniteDuration();
}
return *this = ScaleFixed<std::divides>(*this, r);
@@ -463,7 +468,7 @@
Duration& Duration::operator/=(double r) {
if (time_internal::IsInfiniteDuration(*this) || !IsValidDivisor(r)) {
- const bool is_neg = (std::signbit(r) != 0) != (rep_hi_ < 0);
+ const bool is_neg = std::signbit(r) != (rep_hi_.Get() < 0);
return *this = is_neg ? -InfiniteDuration() : InfiniteDuration();
}
return *this = ScaleDouble<std::divides>(*this, r);
diff --git a/absl/time/duration_test.cc b/absl/time/duration_test.cc
index 3bb6081..dcf7aad 100644
--- a/absl/time/duration_test.cc
+++ b/absl/time/duration_test.cc
@@ -16,6 +16,7 @@
#include <winsock2.h> // for timeval
#endif
+#include <array>
#include <cfloat>
#include <chrono> // NOLINT(build/c++11)
#include <cmath>
@@ -1826,4 +1827,11 @@
EXPECT_EQ(absl::StrFormat("%v", d), absl::FormatDuration(d));
}
+TEST(Duration, NoPadding) {
+ // Should match the size of a struct with uint32_t alignment and no padding.
+ using NoPadding = std::array<uint32_t, 3>;
+ EXPECT_EQ(sizeof(NoPadding), sizeof(absl::Duration));
+ EXPECT_EQ(alignof(NoPadding), alignof(absl::Duration));
+}
+
} // namespace
diff --git a/absl/time/time.h b/absl/time/time.h
index f255593..29178c7 100644
--- a/absl/time/time.h
+++ b/absl/time/time.h
@@ -84,6 +84,7 @@
#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"
@@ -219,7 +220,7 @@
template <typename H>
friend H AbslHashValue(H h, Duration d) {
- return H::combine(std::move(h), d.rep_hi_, d.rep_lo_);
+ return H::combine(std::move(h), d.rep_hi_.Get(), d.rep_lo_);
}
private:
@@ -228,7 +229,79 @@
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) {}
- int64_t rep_hi_;
+
+ // We store `rep_hi_` 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.
+#if defined(ABSL_IS_BIG_ENDIAN) && ABSL_IS_BIG_ENDIAN
+ hi_(0),
+ lo_(0)
+#else
+ lo_(0),
+ hi_(0)
+#endif
+ {
+ *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_;
uint32_t rep_lo_;
};
@@ -1511,7 +1584,7 @@
// Provide access to the Duration representation.
ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t GetRepHi(Duration d) {
- return d.rep_hi_;
+ return d.rep_hi_.Get();
}
ABSL_ATTRIBUTE_CONST_FUNCTION constexpr uint32_t GetRepLo(Duration d) {
return d.rep_lo_;
