Export of internal Abseil changes

--
7f5caec21a1a88db6486b8bc2e5f6d5baba076ed by Derek Mauro <dmauro@google.com>:

Tag absl::StatusOr with [[nodiscard]] when it is available

[[nodiscard]] provides better diagnostics on classes than the current
ABSL_MUST_USE_RESULT, which expands to
__attribute__((warn_unused_result))

Ideally we would make ABSL_MUST_USE_RESULT expand to [[nodiscard]], but
we would need to fix all code to build with [[nodiscard]] first.

PiperOrigin-RevId: 422628161

--
236be69f0f34ccaa3adc831075613847797e6557 by Jorg Brown <jorg@google.com>:

Make sure all of absl/strings compiles even with -Wsign-compare and -Wconversion warnings.

PiperOrigin-RevId: 422573904

--
005669883a8794e6d19dc4bbb4f0e18032e2fbc9 by Chris Kennelly <ckennelly@google.com>:

Include sampling stride in hashtable sampling data.

This allows us to weight each sample to control for our sampling rate.

PiperOrigin-RevId: 421886935

--
78046ce6b429b9f5b6c97b05e8448a791db93bbe by Abseil Team <absl-team@google.com>:

Use __builtin_bit_cast for absl::bit_cast when possible

This makes absl::bit_cast match the requirements of the standard when compiler support exists.

PiperOrigin-RevId: 421883999

--
f397461f4bbeabd32437df0f2275663aeb51adb2 by Derek Mauro <dmauro@google.com>:

Tag absl::Status with [[nodiscard]] when it is available

[[nodiscard]] provides better diagnostics on classes than the current
ABSL_MUST_USE_RESULT, which expands to
__attribute__((warn_unused_result))

Ideally we would make ABSL_MUST_USE_RESULT expand to [[nodiscard]], but
we would need to fix all code to build with [[nodiscard]] first.

PiperOrigin-RevId: 421825565
GitOrigin-RevId: 7f5caec21a1a88db6486b8bc2e5f6d5baba076ed
Change-Id: I760b45b68f6012809c70c2a584e4144a99f98733

11 files changed, 139 insertions, 65 deletions

diff --git a/absl/base/casts.h b/absl/base/casts.h
index b16af233..bf100efa 100644
--- a/absl/base/casts.h
+++ b/absl/base/casts.h
@@ -44,8 +44,12 @@ struct is_bitcastable
           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> {};
+              type_traits_internal::is_trivially_copyable<Dest>::value
+#if !ABSL_HAVE_BUILTIN(__builtin_bit_cast)
+              && std::is_default_constructible<Dest>::value
+#endif
+          > {
+};
 
 }  // namespace internal_casts
 
@@ -147,20 +151,26 @@ constexpr To implicit_cast(typename absl::internal::identity_t<To> to) {
 // introducing this undefined behavior (since the original value is never
 // accessed in the wrong way).
 //
-// NOTE: The requirements here are stricter than the bit_cast of standard
-// proposal P0476 due to the need for workarounds and lack of intrinsics.
+// NOTE: The requirements here are more strict than the bit_cast of standard
+// proposal P0476 when __builtin_bit_cast is not available.
 // 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>
+#if ABSL_HAVE_BUILTIN(__builtin_bit_cast)
+inline constexpr Dest bit_cast(const Source& source) {
+  return __builtin_bit_cast(Dest, source);
+}
+#else
 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
 
 // 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
diff --git a/absl/container/internal/hashtablez_sampler.cc b/absl/container/internal/hashtablez_sampler.cc
index 1d24db5f..322e0547 100644
--- a/absl/container/internal/hashtablez_sampler.cc
+++ b/absl/container/internal/hashtablez_sampler.cc
@@ -27,6 +27,7 @@
 #include "absl/profiling/internal/exponential_biased.h"
 #include "absl/profiling/internal/sample_recorder.h"
 #include "absl/synchronization/mutex.h"
+#include "absl/utility/utility.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
@@ -53,7 +54,7 @@ void TriggerHashtablezConfigListener() {
 }  // namespace
 
 #if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
-ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample = 0;
+ABSL_PER_THREAD_TLS_KEYWORD SamplingState global_next_sample = {0, 0};
 #endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
 
 HashtablezSampler& GlobalHashtablezSampler() {
@@ -64,7 +65,8 @@ HashtablezSampler& GlobalHashtablezSampler() {
 HashtablezInfo::HashtablezInfo() = default;
 HashtablezInfo::~HashtablezInfo() = default;
 
-void HashtablezInfo::PrepareForSampling(size_t inline_element_size_value) {
+void HashtablezInfo::PrepareForSampling(int64_t stride,
+                                        size_t inline_element_size_value) {
   capacity.store(0, std::memory_order_relaxed);
   size.store(0, std::memory_order_relaxed);
   num_erases.store(0, std::memory_order_relaxed);
@@ -77,6 +79,7 @@ void HashtablezInfo::PrepareForSampling(size_t inline_element_size_value) {
   max_reserve.store(0, std::memory_order_relaxed);
 
   create_time = absl::Now();
+  weight = stride;
   // The inliner makes hardcoded skip_count difficult (especially when combined
   // with LTO).  We use the ability to exclude stacks by regex when encoding
   // instead.
@@ -105,23 +108,32 @@ static bool ShouldForceSampling() {
   return state == kForce;
 }
 
-HashtablezInfo* SampleSlow(int64_t* next_sample, size_t inline_element_size) {
+HashtablezInfo* SampleSlow(SamplingState& next_sample,
+                           size_t inline_element_size) {
   if (ABSL_PREDICT_FALSE(ShouldForceSampling())) {
-    *next_sample = 1;
+    next_sample.next_sample = 1;
+    const int64_t old_stride = exchange(next_sample.sample_stride, 1);
     HashtablezInfo* result =
-        GlobalHashtablezSampler().Register(inline_element_size);
+        GlobalHashtablezSampler().Register(old_stride, inline_element_size);
     return result;
   }
 
 #if !defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
-  *next_sample = std::numeric_limits<int64_t>::max();
+  next_sample = {
+      std::numeric_limits<int64_t>::max(),
+      std::numeric_limits<int64_t>::max(),
+  };
   return nullptr;
 #else
-  bool first = *next_sample < 0;
-  *next_sample = g_exponential_biased_generator.GetStride(
+  bool first = next_sample.next_sample < 0;
+
+  const int64_t next_stride = g_exponential_biased_generator.GetStride(
       g_hashtablez_sample_parameter.load(std::memory_order_relaxed));
+
+  next_sample.next_sample = next_stride;
+  const int64_t old_stride = exchange(next_sample.sample_stride, next_stride);
   // Small values of interval are equivalent to just sampling next time.
-  ABSL_ASSERT(*next_sample >= 1);
+  ABSL_ASSERT(next_stride >= 1);
 
   // g_hashtablez_enabled can be dynamically flipped, we need to set a threshold
   // low enough that we will start sampling in a reasonable time, so we just use
@@ -131,11 +143,11 @@ HashtablezInfo* SampleSlow(int64_t* next_sample, size_t inline_element_size) {
   // We will only be negative on our first count, so we should just retry in
   // that case.
   if (first) {
-    if (ABSL_PREDICT_TRUE(--*next_sample > 0)) return nullptr;
+    if (ABSL_PREDICT_TRUE(--next_sample.next_sample > 0)) return nullptr;
     return SampleSlow(next_sample, inline_element_size);
   }
 
-  return GlobalHashtablezSampler().Register(inline_element_size);
+  return GlobalHashtablezSampler().Register(old_stride, inline_element_size);
 #endif
 }
 
diff --git a/absl/container/internal/hashtablez_sampler.h b/absl/container/internal/hashtablez_sampler.h
index 6738786c..e7c204ee 100644
--- a/absl/container/internal/hashtablez_sampler.h
+++ b/absl/container/internal/hashtablez_sampler.h
@@ -67,7 +67,7 @@ struct HashtablezInfo : public profiling_internal::Sample<HashtablezInfo> {
 
   // Puts the object into a clean state, fills in the logically `const` members,
   // blocking for any readers that are currently sampling the object.
-  void PrepareForSampling(size_t inline_element_size_value)
+  void PrepareForSampling(int64_t stride, size_t inline_element_size_value)
       ABSL_EXCLUSIVE_LOCKS_REQUIRED(init_mu);
 
   // These fields are mutated by the various Record* APIs and need to be
@@ -145,7 +145,15 @@ inline void RecordEraseSlow(HashtablezInfo* info) {
       std::memory_order_relaxed);
 }
 
-HashtablezInfo* SampleSlow(int64_t* next_sample, size_t inline_element_size);
+struct SamplingState {
+  int64_t next_sample;
+  // When we make a sampling decision, we record that distance so we can weight
+  // each sample.
+  int64_t sample_stride;
+};
+
+HashtablezInfo* SampleSlow(SamplingState& next_sample,
+                           size_t inline_element_size);
 void UnsampleSlow(HashtablezInfo* info);
 
 #if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
@@ -235,7 +243,7 @@ class HashtablezInfoHandle {
 #endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
 
 #if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
-extern ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample;
+extern ABSL_PER_THREAD_TLS_KEYWORD SamplingState global_next_sample;
 #endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
 
 // Returns an RAII sampling handle that manages registration and unregistation
@@ -243,11 +251,11 @@ extern ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample;
 inline HashtablezInfoHandle Sample(
     size_t inline_element_size ABSL_ATTRIBUTE_UNUSED) {
 #if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
-  if (ABSL_PREDICT_TRUE(--global_next_sample > 0)) {
+  if (ABSL_PREDICT_TRUE(--global_next_sample.next_sample > 0)) {
     return HashtablezInfoHandle(nullptr);
   }
   return HashtablezInfoHandle(
-      SampleSlow(&global_next_sample, inline_element_size));
+      SampleSlow(global_next_sample, inline_element_size));
 #else
   return HashtablezInfoHandle(nullptr);
 #endif  // !ABSL_PER_THREAD_TLS
diff --git a/absl/container/internal/hashtablez_sampler_test.cc b/absl/container/internal/hashtablez_sampler_test.cc
index ac6ed9b1..77cdf2fd 100644
--- a/absl/container/internal/hashtablez_sampler_test.cc
+++ b/absl/container/internal/hashtablez_sampler_test.cc
@@ -70,8 +70,9 @@ std::vector<size_t> GetSizes(HashtablezSampler* s) {
 }
 
 HashtablezInfo* Register(HashtablezSampler* s, size_t size) {
+  const int64_t test_stride = 123;
   const size_t test_element_size = 17;
-  auto* info = s->Register(test_element_size);
+  auto* info = s->Register(test_stride, test_element_size);
   assert(info != nullptr);
   info->size.store(size);
   return info;
@@ -79,10 +80,11 @@ HashtablezInfo* Register(HashtablezSampler* s, size_t size) {
 
 TEST(HashtablezInfoTest, PrepareForSampling) {
   absl::Time test_start = absl::Now();
+  const int64_t test_stride = 123;
   const size_t test_element_size = 17;
   HashtablezInfo info;
   absl::MutexLock l(&info.init_mu);
-  info.PrepareForSampling(test_element_size);
+  info.PrepareForSampling(test_stride, test_element_size);
 
   EXPECT_EQ(info.capacity.load(), 0);
   EXPECT_EQ(info.size.load(), 0);
@@ -95,6 +97,7 @@ TEST(HashtablezInfoTest, PrepareForSampling) {
   EXPECT_EQ(info.hashes_bitwise_xor.load(), 0);
   EXPECT_EQ(info.max_reserve.load(), 0);
   EXPECT_GE(info.create_time, test_start);
+  EXPECT_EQ(info.weight, test_stride);
   EXPECT_EQ(info.inline_element_size, test_element_size);
 
   info.capacity.store(1, std::memory_order_relaxed);
@@ -108,7 +111,7 @@ TEST(HashtablezInfoTest, PrepareForSampling) {
   info.max_reserve.store(1, std::memory_order_relaxed);
   info.create_time = test_start - absl::Hours(20);
 
-  info.PrepareForSampling(test_element_size);
+  info.PrepareForSampling(test_stride * 2, test_element_size);
   EXPECT_EQ(info.capacity.load(), 0);
   EXPECT_EQ(info.size.load(), 0);
   EXPECT_EQ(info.num_erases.load(), 0);
@@ -119,6 +122,7 @@ TEST(HashtablezInfoTest, PrepareForSampling) {
   EXPECT_EQ(info.hashes_bitwise_and.load(), ~size_t{});
   EXPECT_EQ(info.hashes_bitwise_xor.load(), 0);
   EXPECT_EQ(info.max_reserve.load(), 0);
+  EXPECT_EQ(info.weight, 2 * test_stride);
   EXPECT_EQ(info.inline_element_size, test_element_size);
   EXPECT_GE(info.create_time, test_start);
 }
@@ -126,8 +130,9 @@ TEST(HashtablezInfoTest, PrepareForSampling) {
 TEST(HashtablezInfoTest, RecordStorageChanged) {
   HashtablezInfo info;
   absl::MutexLock l(&info.init_mu);
+  const int64_t test_stride = 21;
   const size_t test_element_size = 19;
-  info.PrepareForSampling(test_element_size);
+  info.PrepareForSampling(test_stride, test_element_size);
   RecordStorageChangedSlow(&info, 17, 47);
   EXPECT_EQ(info.size.load(), 17);
   EXPECT_EQ(info.capacity.load(), 47);
@@ -139,8 +144,9 @@ TEST(HashtablezInfoTest, RecordStorageChanged) {
 TEST(HashtablezInfoTest, RecordInsert) {
   HashtablezInfo info;
   absl::MutexLock l(&info.init_mu);
+  const int64_t test_stride = 25;
   const size_t test_element_size = 23;
-  info.PrepareForSampling(test_element_size);
+  info.PrepareForSampling(test_stride, test_element_size);
   EXPECT_EQ(info.max_probe_length.load(), 0);
   RecordInsertSlow(&info, 0x0000FF00, 6 * kProbeLength);
   EXPECT_EQ(info.max_probe_length.load(), 6);
@@ -160,10 +166,11 @@ TEST(HashtablezInfoTest, RecordInsert) {
 }
 
 TEST(HashtablezInfoTest, RecordErase) {
+  const int64_t test_stride = 31;
   const size_t test_element_size = 29;
   HashtablezInfo info;
   absl::MutexLock l(&info.init_mu);
-  info.PrepareForSampling(test_element_size);
+  info.PrepareForSampling(test_stride, test_element_size);
   EXPECT_EQ(info.num_erases.load(), 0);
   EXPECT_EQ(info.size.load(), 0);
   RecordInsertSlow(&info, 0x0000FF00, 6 * kProbeLength);
@@ -175,10 +182,11 @@ TEST(HashtablezInfoTest, RecordErase) {
 }
 
 TEST(HashtablezInfoTest, RecordRehash) {
+  const int64_t test_stride = 33;
   const size_t test_element_size = 31;
   HashtablezInfo info;
   absl::MutexLock l(&info.init_mu);
-  info.PrepareForSampling(test_element_size);
+  info.PrepareForSampling(test_stride, test_element_size);
   RecordInsertSlow(&info, 0x1, 0);
   RecordInsertSlow(&info, 0x2, kProbeLength);
   RecordInsertSlow(&info, 0x4, kProbeLength);
@@ -203,8 +211,9 @@ TEST(HashtablezInfoTest, RecordRehash) {
 TEST(HashtablezInfoTest, RecordReservation) {
   HashtablezInfo info;
   absl::MutexLock l(&info.init_mu);
+  const int64_t test_stride = 35;
   const size_t test_element_size = 33;
-  info.PrepareForSampling(test_element_size);
+  info.PrepareForSampling(test_stride, test_element_size);
   RecordReservationSlow(&info, 3);
   EXPECT_EQ(info.max_reserve.load(), 3);
 
@@ -224,9 +233,10 @@ TEST(HashtablezSamplerTest, SmallSampleParameter) {
   SetHashtablezSampleParameter(100);
 
   for (int i = 0; i < 1000; ++i) {
-    int64_t next_sample = 0;
-    HashtablezInfo* sample = SampleSlow(&next_sample, test_element_size);
-    EXPECT_GT(next_sample, 0);
+    SamplingState next_sample = {0, 0};
+    HashtablezInfo* sample = SampleSlow(next_sample, test_element_size);
+    EXPECT_GT(next_sample.next_sample, 0);
+    EXPECT_EQ(next_sample.next_sample, next_sample.sample_stride);
     EXPECT_NE(sample, nullptr);
     UnsampleSlow(sample);
   }
@@ -238,9 +248,10 @@ TEST(HashtablezSamplerTest, LargeSampleParameter) {
   SetHashtablezSampleParameter(std::numeric_limits<int32_t>::max());
 
   for (int i = 0; i < 1000; ++i) {
-    int64_t next_sample = 0;
-    HashtablezInfo* sample = SampleSlow(&next_sample, test_element_size);
-    EXPECT_GT(next_sample, 0);
+    SamplingState next_sample = {0, 0};
+    HashtablezInfo* sample = SampleSlow(next_sample, test_element_size);
+    EXPECT_GT(next_sample.next_sample, 0);
+    EXPECT_EQ(next_sample.next_sample, next_sample.sample_stride);
     EXPECT_NE(sample, nullptr);
     UnsampleSlow(sample);
   }
@@ -267,14 +278,16 @@ TEST(HashtablezSamplerTest, Sample) {
 
 TEST(HashtablezSamplerTest, Handle) {
   auto& sampler = GlobalHashtablezSampler();
+  const int64_t test_stride = 41;
   const size_t test_element_size = 39;
-  HashtablezInfoHandle h(sampler.Register(test_element_size));
+  HashtablezInfoHandle h(sampler.Register(test_stride, test_element_size));
   auto* info = HashtablezInfoHandlePeer::GetInfo(&h);
   info->hashes_bitwise_and.store(0x12345678, std::memory_order_relaxed);
 
   bool found = false;
   sampler.Iterate([&](const HashtablezInfo& h) {
     if (&h == info) {
+      EXPECT_EQ(h.weight, test_stride);
       EXPECT_EQ(h.hashes_bitwise_and.load(), 0x12345678);
       found = true;
     }
@@ -340,19 +353,20 @@ TEST(HashtablezSamplerTest, MultiThreaded) {
   ThreadPool pool(10);
 
   for (int i = 0; i < 10; ++i) {
+    const int64_t sampling_stride = 11 + i % 3;
     const size_t elt_size = 10 + i % 2;
-    pool.Schedule([&sampler, &stop, elt_size]() {
+    pool.Schedule([&sampler, &stop, sampling_stride, elt_size]() {
       std::random_device rd;
       std::mt19937 gen(rd());
 
       std::vector<HashtablezInfo*> infoz;
       while (!stop.HasBeenNotified()) {
         if (infoz.empty()) {
-          infoz.push_back(sampler.Register(elt_size));
+          infoz.push_back(sampler.Register(sampling_stride, elt_size));
         }
         switch (std::uniform_int_distribution<>(0, 2)(gen)) {
           case 0: {
-            infoz.push_back(sampler.Register(elt_size));
+            infoz.push_back(sampler.Register(sampling_stride, elt_size));
             break;
           }
           case 1: {
@@ -361,6 +375,7 @@ TEST(HashtablezSamplerTest, MultiThreaded) {
             HashtablezInfo* info = infoz[p];
             infoz[p] = infoz.back();
             infoz.pop_back();
+            EXPECT_EQ(info->weight, sampling_stride);
             sampler.Unregister(info);
             break;
           }
diff --git a/absl/profiling/internal/sample_recorder.h b/absl/profiling/internal/sample_recorder.h
index fc269bdd..5f65983b 100644
--- a/absl/profiling/internal/sample_recorder.h
+++ b/absl/profiling/internal/sample_recorder.h
@@ -46,6 +46,7 @@ struct Sample {
   absl::Mutex init_mu;
   T* next = nullptr;
   T* dead ABSL_GUARDED_BY(init_mu) = nullptr;
+  int64_t weight;  // How many sampling events were required to sample this one.
 };
 
 // Holds samples and their associated stack traces with a soft limit of
diff --git a/absl/profiling/internal/sample_recorder_test.cc b/absl/profiling/internal/sample_recorder_test.cc
index ec6e0fa2..3373329a 100644
--- a/absl/profiling/internal/sample_recorder_test.cc
+++ b/absl/profiling/internal/sample_recorder_test.cc
@@ -36,7 +36,7 @@ using ::testing::UnorderedElementsAre;
 
 struct Info : public Sample<Info> {
  public:
-  void PrepareForSampling() {}
+  void PrepareForSampling(int64_t w) { weight = w; }
   std::atomic<size_t> size;
   absl::Time create_time;
 };
@@ -49,8 +49,14 @@ std::vector<size_t> GetSizes(SampleRecorder<Info>* s) {
   return res;
 }
 
-Info* Register(SampleRecorder<Info>* s, size_t size) {
-  auto* info = s->Register();
+std::vector<int64_t> GetWeights(SampleRecorder<Info>* s) {
+  std::vector<int64_t> res;
+  s->Iterate([&](const Info& info) { res.push_back(info.weight); });
+  return res;
+}
+
+Info* Register(SampleRecorder<Info>* s, int64_t weight, size_t size) {
+  auto* info = s->Register(weight);
   assert(info != nullptr);
   info->size.store(size);
   return info;
@@ -58,13 +64,15 @@ Info* Register(SampleRecorder<Info>* s, size_t size) {
 
 TEST(SampleRecorderTest, Registration) {
   SampleRecorder<Info> sampler;
-  auto* info1 = Register(&sampler, 1);
+  auto* info1 = Register(&sampler, 31, 1);
   EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(1));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(31));
 
-  auto* info2 = Register(&sampler, 2);
+  auto* info2 = Register(&sampler, 32, 2);
   EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(1, 2));
   info1->size.store(3);
   EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(3, 2));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(31, 32));
 
   sampler.Unregister(info1);
   sampler.Unregister(info2);
@@ -74,18 +82,22 @@ TEST(SampleRecorderTest, Unregistration) {
   SampleRecorder<Info> sampler;
   std::vector<Info*> infos;
   for (size_t i = 0; i < 3; ++i) {
-    infos.push_back(Register(&sampler, i));
+    infos.push_back(Register(&sampler, 33 + i, i));
   }
   EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(0, 1, 2));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(33, 34, 35));
 
   sampler.Unregister(infos[1]);
   EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(0, 2));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(33, 35));
 
-  infos.push_back(Register(&sampler, 3));
-  infos.push_back(Register(&sampler, 4));
+  infos.push_back(Register(&sampler, 36, 3));
+  infos.push_back(Register(&sampler, 37, 4));
   EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(0, 2, 3, 4));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(33, 35, 36, 37));
   sampler.Unregister(infos[3]);
   EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(0, 2, 4));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(33, 35, 37));
 
   sampler.Unregister(infos[0]);
   sampler.Unregister(infos[2]);
@@ -99,18 +111,18 @@ TEST(SampleRecorderTest, MultiThreaded) {
   ThreadPool pool(10);
 
   for (int i = 0; i < 10; ++i) {
-    pool.Schedule([&sampler, &stop]() {
+    pool.Schedule([&sampler, &stop, i]() {
       std::random_device rd;
       std::mt19937 gen(rd());
 
       std::vector<Info*> infoz;
       while (!stop.HasBeenNotified()) {
         if (infoz.empty()) {
-          infoz.push_back(sampler.Register());
+          infoz.push_back(sampler.Register(i));
         }
         switch (std::uniform_int_distribution<>(0, 2)(gen)) {
           case 0: {
-            infoz.push_back(sampler.Register());
+            infoz.push_back(sampler.Register(i));
             break;
           }
           case 1: {
@@ -119,6 +131,7 @@ TEST(SampleRecorderTest, MultiThreaded) {
             Info* info = infoz[p];
             infoz[p] = infoz.back();
             infoz.pop_back();
+            EXPECT_EQ(info->weight, i);
             sampler.Unregister(info);
             break;
           }
@@ -143,8 +156,8 @@ TEST(SampleRecorderTest, MultiThreaded) {
 TEST(SampleRecorderTest, Callback) {
   SampleRecorder<Info> sampler;
 
-  auto* info1 = Register(&sampler, 1);
-  auto* info2 = Register(&sampler, 2);
+  auto* info1 = Register(&sampler, 39, 1);
+  auto* info2 = Register(&sampler, 40, 2);
 
   static const Info* expected;
 
diff --git a/absl/status/internal/status_internal.h b/absl/status/internal/status_internal.h
index ac12940a..34914d2e 100644
--- a/absl/status/internal/status_internal.h
+++ b/absl/status/internal/status_internal.h
@@ -16,6 +16,7 @@
 
 #include <string>
 
+#include "absl/base/attributes.h"
 #include "absl/container/inlined_vector.h"
 #include "absl/strings/cord.h"
 
@@ -25,7 +26,14 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 // Returned Status objects may not be ignored. Codesearch doesn't handle ifdefs
 // as part of a class definitions (b/6995610), so we use a forward declaration.
+//
+// TODO(b/176172494): ABSL_MUST_USE_RESULT should expand to the more strict
+// [[nodiscard]]. For now, just use [[nodiscard]] directly when it is available.
+#if ABSL_HAVE_CPP_ATTRIBUTE(nodiscard)
+class [[nodiscard]] Status;
+#else
 class ABSL_MUST_USE_RESULT Status;
+#endif
 ABSL_NAMESPACE_END
 }  // namespace absl
 #endif  // !SWIG
diff --git a/absl/status/statusor.h b/absl/status/statusor.h
index c051fbb3..d6ebdc2b 100644
--- a/absl/status/statusor.h
+++ b/absl/status/statusor.h
@@ -106,7 +106,13 @@ class BadStatusOrAccess : public std::exception {
 
 // Returned StatusOr objects may not be ignored.
 template <typename T>
+#if ABSL_HAVE_CPP_ATTRIBUTE(nodiscard)
+// TODO(b/176172494): ABSL_MUST_USE_RESULT should expand to the more strict
+// [[nodiscard]]. For now, just use [[nodiscard]] directly when it is available.
+class [[nodiscard]] StatusOr;
+#else
 class ABSL_MUST_USE_RESULT StatusOr;
+#endif  // ABSL_HAVE_CPP_ATTRIBUTE(nodiscard)
 
 // absl::StatusOr<T>
 //
diff --git a/absl/strings/internal/escaping.cc b/absl/strings/internal/escaping.cc
index c5271286..7f87e124 100644
--- a/absl/strings/internal/escaping.cc
+++ b/absl/strings/internal/escaping.cc
@@ -102,8 +102,8 @@ size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
     }
   }
   // To save time, we didn't update szdest or szsrc in the loop.  So do it now.
-  szdest = limit_dest - cur_dest;
-  szsrc = limit_src - cur_src;
+  szdest = static_cast<size_t>(limit_dest - cur_dest);
+  szsrc = static_cast<size_t>(limit_src - cur_src);
 
   /* now deal with the tail (<=3 bytes) */
   switch (szsrc) {
@@ -154,7 +154,8 @@ size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
       // the loop because the loop above always reads 4 bytes, and the fourth
       // byte is past the end of the input.
       if (szdest < 4) return 0;
-      uint32_t in = (cur_src[0] << 16) + absl::big_endian::Load16(cur_src + 1);
+      uint32_t in =
+          (uint32_t{cur_src[0]} << 16) + absl::big_endian::Load16(cur_src + 1);
       cur_dest[0] = base64[in >> 18];
       in &= 0x3FFFF;
       cur_dest[1] = base64[in >> 12];
@@ -172,7 +173,7 @@ size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
       ABSL_RAW_LOG(FATAL, "Logic problem? szsrc = %zu", szsrc);
       break;
   }
-  return (cur_dest - dest);
+  return static_cast<size_t>(cur_dest - dest);
 }
 
 }  // namespace strings_internal
diff --git a/absl/strings/internal/ostringstream.cc b/absl/strings/internal/ostringstream.cc
index 05324c78..dc6cfe16 100644
--- a/absl/strings/internal/ostringstream.cc
+++ b/absl/strings/internal/ostringstream.cc
@@ -27,7 +27,7 @@ OStringStream::Buf::int_type OStringStream::overflow(int c) {
 
 std::streamsize OStringStream::xsputn(const char* s, std::streamsize n) {
   assert(s_);
-  s_->append(s, n);
+  s_->append(s, static_cast<size_t>(n));
   return n;
 }
 
diff --git a/absl/strings/internal/utf8.cc b/absl/strings/internal/utf8.cc
index 8fd8edc1..7ecb93df 100644
--- a/absl/strings/internal/utf8.cc
+++ b/absl/strings/internal/utf8.cc
@@ -25,25 +25,25 @@ size_t EncodeUTF8Char(char *buffer, char32_t utf8_char) {
     *buffer = static_cast<char>(utf8_char);
     return 1;
   } else if (utf8_char <= 0x7FF) {
-    buffer[1] = 0x80 | (utf8_char & 0x3F);
+    buffer[1] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[0] = 0xC0 | utf8_char;
+    buffer[0] = static_cast<char>(0xC0 | utf8_char);
     return 2;
   } else if (utf8_char <= 0xFFFF) {
-    buffer[2] = 0x80 | (utf8_char & 0x3F);
+    buffer[2] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[1] = 0x80 | (utf8_char & 0x3F);
+    buffer[1] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[0] = 0xE0 | utf8_char;
+    buffer[0] = static_cast<char>(0xE0 | utf8_char);
     return 3;
   } else {
-    buffer[3] = 0x80 | (utf8_char & 0x3F);
+    buffer[3] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[2] = 0x80 | (utf8_char & 0x3F);
+    buffer[2] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[1] = 0x80 | (utf8_char & 0x3F);
+    buffer[1] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[0] = 0xF0 | utf8_char;
+    buffer[0] = static_cast<char>(0xF0 | utf8_char);
     return 4;
   }
 }