Export of internal Abseil changes

--
8e04df6fcbd062e5eaf179a6ec9b0a26f8aa8a39 by Abseil Team <absl-team@google.com>:

Use a floating point type for the example usage of absl::uniform_real_distribution.

PiperOrigin-RevId: 271167776

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5f8f1dfea50bc16a9d9af3e50c4636500a938b29 by Abseil Team <absl-team@google.com>:

the llvm wasm backend does not support this data symbol in text section, so remove it from the test.

PiperOrigin-RevId: 271138100

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2874542cb212962ac3093fd78fd5e1eb85c126c0 by Xiaoyi Zhang <zhangxy@google.com>:

Work around MSVC 2019 compiler bug related to constexpr in optional_test.
The change in optional_test is necessary to avoid another bug on MSVC
complaining about accessing invalid member of union, and also makes the test
more reasonale by checking the value of a non-static member.

Filed a bug against MSVC
https://developercommunity.visualstudio.com/content/problem/743998/internal-compiler-error-related-to-constexpr-and-u.html.

PiperOrigin-RevId: 271129805

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3a5d56f0c3362aabf68938fb95c4e2d3eca59538 by Abseil Team <absl-team@google.com>:

Improve precision of absl::GetCurrentTimeNanos() by adjusting
cycle time sooner.

PiperOrigin-RevId: 271007945

--
1e044a6dec7c0ca150fff1aee52dbdb16aa43ed7 by Abseil Team <absl-team@google.com>:

Internal change.

PiperOrigin-RevId: 270962690
GitOrigin-RevId: 8e04df6fcbd062e5eaf179a6ec9b0a26f8aa8a39
Change-Id: Icb05423a7e93ebdae16baadd59a60b75b5cfa249

7 files changed, 23 insertions, 11 deletions

diff --git a/absl/container/btree_test.cc b/absl/container/btree_test.cc
index 4edb2775..48f70cb0 100644
--- a/absl/container/btree_test.cc
+++ b/absl/container/btree_test.cc
@@ -2239,6 +2239,13 @@ TEST(Btree, MoveAssignmentAllocatorPropagation) {
   }
 }
 
+TEST(Btree, EmptyTree) {
+  absl::btree_set<int> s;
+  EXPECT_TRUE(s.empty());
+  EXPECT_EQ(s.size(), 0);
+  EXPECT_GT(s.max_size(), 0);
+}
+
 }  // namespace
 }  // namespace container_internal
 }  // namespace absl
diff --git a/absl/container/internal/btree.h b/absl/container/internal/btree.h
index b2559841..cf7ef02c 100644
--- a/absl/container/internal/btree.h
+++ b/absl/container/internal/btree.h
@@ -1226,7 +1226,7 @@ class btree {
   // The height of the btree. An empty tree will have height 0.
   size_type height() const {
     size_type h = 0;
-    if (root()) {
+    if (!empty()) {
       // Count the length of the chain from the leftmost node up to the
       // root. We actually count from the root back around to the level below
       // the root, but the calculation is the same because of the circularity
@@ -1277,16 +1277,17 @@ class btree {
   // divided by the maximum number of elements a tree with the current number
   // of nodes could hold. A value of 1 indicates perfect space
   // utilization. Smaller values indicate space wastage.
+  // Returns 0 for empty trees.
   double fullness() const {
+    if (empty()) return 0.0;
     return static_cast<double>(size()) / (nodes() * kNodeValues);
   }
   // The overhead of the btree structure in bytes per node. Computed as the
   // total number of bytes used by the btree minus the number of bytes used for
   // storing elements divided by the number of elements.
+  // Returns 0 for empty trees.
   double overhead() const {
-    if (empty()) {
-      return 0.0;
-    }
+    if (empty()) return 0.0;
     return (bytes_used() - size() * sizeof(value_type)) /
            static_cast<double>(size());
   }
diff --git a/absl/debugging/symbolize_test.cc b/absl/debugging/symbolize_test.cc
index 08068c30..533799a6 100644
--- a/absl/debugging/symbolize_test.cc
+++ b/absl/debugging/symbolize_test.cc
@@ -80,6 +80,7 @@ static ABSL_PER_THREAD_TLS_KEYWORD char
     symbolize_test_thread_big[2 * 1024 * 1024];
 #endif
 
+#if !defined(__EMSCRIPTEN__)
 // Used below to hopefully inhibit some compiler/linker optimizations
 // that may remove kHpageTextPadding, kPadding0, and kPadding1 from
 // the binary.
@@ -89,6 +90,7 @@ static volatile bool volatile_bool = false;
 static constexpr size_t kHpageSize = 1 << 21;
 const char kHpageTextPadding[kHpageSize * 4] ABSL_ATTRIBUTE_SECTION_VARIABLE(
     .text) = "";
+#endif  // !defined(__EMSCRIPTEN__)
 
 static char try_symbolize_buffer[4096];
 
@@ -498,10 +500,12 @@ TEST(Symbolize, Unimplemented) {
 #endif
 
 int main(int argc, char **argv) {
+#if !defined(__EMSCRIPTEN__)
   // Make sure kHpageTextPadding is linked into the binary.
   if (volatile_bool) {
     ABSL_RAW_LOG(INFO, "%s", kHpageTextPadding);
   }
+#endif  // !defined(__EMSCRIPTEN__)
 
 #if ABSL_PER_THREAD_TLS
   // Touch the per-thread variables.
diff --git a/absl/random/uniform_real_distribution.h b/absl/random/uniform_real_distribution.h
index 600f915b..336abb39 100644
--- a/absl/random/uniform_real_distribution.h
+++ b/absl/random/uniform_real_distribution.h
@@ -56,7 +56,7 @@ namespace absl {
 //
 //   // Use the distribution to produce a value between 0.0 (inclusive)
 //   // and 1.0 (exclusive).
-//   int value = absl::uniform_real_distribution<double>(0, 1)(gen);
+//   double value = absl::uniform_real_distribution<double>(0, 1)(gen);
 //
 template <typename RealType = double>
 class uniform_real_distribution {
diff --git a/absl/time/clock.cc b/absl/time/clock.cc
index fa0ed34d..bc6f5ccf 100644
--- a/absl/time/clock.cc
+++ b/absl/time/clock.cc
@@ -449,7 +449,7 @@ static uint64_t UpdateLastSample(uint64_t now_cycles, uint64_t now_ns,
     last_sample.min_cycles_per_sample.store(0, std::memory_order_relaxed);
     stats_initializations++;
   } else if (sample->raw_ns + 500 * 1000 * 1000 < now_ns &&
-             sample->base_cycles + 100 < now_cycles) {
+             sample->base_cycles + 50 < now_cycles) {
     // Enough time has passed to compute the cycle time.
     if (sample->nsscaled_per_cycle != 0) {  // Have a cycle time estimate.
       // Compute time from counter reading, but avoiding overflow
diff --git a/absl/types/internal/optional.h b/absl/types/internal/optional.h
index 8acbda20..d41ccc75 100644
--- a/absl/types/internal/optional.h
+++ b/absl/types/internal/optional.h
@@ -84,8 +84,8 @@ class optional_data_dtor_base {
   bool engaged_;
   // Data storage
   union {
-    dummy_type dummy_;
     T data_;
+    dummy_type dummy_;
   };
 
   void destruct() noexcept {
@@ -119,8 +119,8 @@ class optional_data_dtor_base<T, true> {
   bool engaged_;
   // Data storage
   union {
-    dummy_type dummy_;
     T data_;
+    dummy_type dummy_;
   };
   void destruct() noexcept { engaged_ = false; }
 
diff --git a/absl/types/optional_test.cc b/absl/types/optional_test.cc
index e005affb..35efa863 100644
--- a/absl/types/optional_test.cc
+++ b/absl/types/optional_test.cc
@@ -944,7 +944,7 @@ TEST(optionalTest, Swap) {
 
 template <int v>
 struct DeletedOpAddr {
-  constexpr static const int value = v;
+  int value = v;
   constexpr DeletedOpAddr() = default;
   constexpr const DeletedOpAddr<v>* operator&() const = delete;  // NOLINT
   DeletedOpAddr<v>* operator&() = delete;                        // NOLINT
@@ -954,9 +954,9 @@ struct DeletedOpAddr {
 // to document the fact that the current implementation of absl::optional<T>
 // expects such usecases to be malformed and not compile.
 TEST(optionalTest, OperatorAddr) {
-  constexpr const int v = -1;
+  constexpr int v = -1;
   {  // constexpr
-    constexpr const absl::optional<DeletedOpAddr<v>> opt(absl::in_place_t{});
+    constexpr absl::optional<DeletedOpAddr<v>> opt(absl::in_place_t{});
     static_assert(opt.has_value(), "");
     // static_assert(opt->value == v, "");
     static_assert((*opt).value == v, "");