Add [[nodiscard]] to various operations that return pointers/sizes.

Migrate existing ABSL_MUST_USE_RESULTs to C++17 [[nodiscard]].

PiperOrigin-RevId: 685737414
Change-Id: Ie7a6f171c16400150caed8c0875d2e4f8459374f

tcmalloc/central_freelist.h

@@ -67,8 +67,9 @@ class StaticForwarder {
   static size_t class_to_size(int size_class);
   static Length class_to_pages(int size_class);
   static void MapObjectsToSpans(absl::Span<void*> batch, Span** spans);
-  static Span* AllocateSpan(int size_class, size_t objects_per_span,
-                            Length pages_per_span)
+  [[nodiscard]] static Span* AllocateSpan(int size_class,
+                                          size_t objects_per_span,
+                                          Length pages_per_span)
       ABSL_LOCKS_EXCLUDED(pageheap_lock);
   static void DeallocateSpans(size_t objects_per_span,
                               absl::Span<Span*> free_spans)
@@ -112,7 +113,7 @@ class CentralFreeList {
 
   // Fill a prefix of batch[0..N-1] with up to N elements removed from central
   // freelist.  Return the number of elements removed.
-  ABSL_MUST_USE_RESULT int RemoveRange(absl::Span<void*> batch)
+  [[nodiscard]] int RemoveRange(absl::Span<void*> batch)
       ABSL_LOCKS_EXCLUDED(lock_);
 
   // Returns the number of free objects in cache.

tcmalloc/central_freelist_test.cc

@@ -121,8 +121,9 @@ TEST_P(StaticForwarderTest, Simple) {
   }
 
   for (void* ptr : batch) {
-    span->FreelistPush(ptr, object_size_, size_reciprocal_,
-                       max_span_cache_size);
+    EXPECT_EQ(span->FreelistPush(ptr, object_size_, size_reciprocal_,
+                                 max_span_cache_size),
+              ptr != batch.back());
   }
 
   StaticForwarder::DeallocateSpans(objects_per_span_, absl::MakeSpan(&span, 1));

tcmalloc/cpu_cache.h

@@ -90,13 +90,14 @@ constexpr inline uint8_t kTotalPossibleSlabs =
 // testing.
 class StaticForwarder {
  public:
-  static void* Alloc(size_t size, std::align_val_t alignment)
+  [[nodiscard]] static void* Alloc(size_t size, std::align_val_t alignment)
       ABSL_LOCKS_EXCLUDED(pageheap_lock) {
     TC_ASSERT(tc_globals.IsInited());
     PageHeapSpinLockHolder l;
     return tc_globals.arena().Alloc(size, alignment);
   }
-  static void* AllocReportedImpending(size_t size, std::align_val_t alignment)
+  [[nodiscard]] static void* AllocReportedImpending(size_t size,
+                                                    std::align_val_t alignment)
       ABSL_LOCKS_EXCLUDED(pageheap_lock) {
     TC_ASSERT(tc_globals.IsInited());
     PageHeapSpinLockHolder l;
@@ -304,15 +305,15 @@ class CpuCache {
 
   // Allocate an object of the given size class.
   // Returns nullptr when allocation fails.
-  void* Allocate(size_t size_class);
+  [[nodiscard]] void* Allocate(size_t size_class);
   // Separate allocation fast/slow paths.
   // The fast path succeeds iff the thread has already cached the slab pointer
   // (done by AllocateSlow) and there is an available object in the slab.
-  void* AllocateFast(size_t size_class);
-  void* AllocateSlow(size_t size_class);
+  [[nodiscard]] void* AllocateFast(size_t size_class);
+  [[nodiscard]] void* AllocateSlow(size_t size_class);
   // A slightly faster version of AllocateSlow that may be called only
   // when it's known that no hooks are installed.
-  void* AllocateSlowNoHooks(size_t size_class);
+  [[nodiscard]] void* AllocateSlowNoHooks(size_t size_class);
 
   // Free an object of the given class.
   void Deallocate(void* ptr, size_t size_class);
@@ -603,12 +604,13 @@ class CpuCache {
   GetShiftMaxCapacity GetMaxCapacityFunctor(uint8_t shift) const;
 
   // Fetches objects from backing transfer cache.
-  int FetchFromBackingCache(size_t size_class, absl::Span<void*> batch);
+  [[nodiscard]] int FetchFromBackingCache(size_t size_class,
+                                          absl::Span<void*> batch);
 
   // Releases free batch of objects to the backing transfer cache.
   void ReleaseToBackingCache(size_t size_class, absl::Span<void*> batch);
 
-  void* Refill(int cpu, size_t size_class);
+  [[nodiscard]] void* Refill(int cpu, size_t size_class);
   std::pair<int, bool> CacheCpuSlab();
   void Populate(int cpu);
 
@@ -684,7 +686,7 @@ class CpuCache {
   // <shift_offset> is the offset of the shift in slabs_by_shift_. Note that we
   // can't calculate this from `shift` directly due to numa shift.
   // Returns the allocated slabs and the number of reused bytes.
-  ABSL_MUST_USE_RESULT std::pair<void*, size_t> AllocOrReuseSlabs(
+  [[nodiscard]] std::pair<void*, size_t> AllocOrReuseSlabs(
       absl::FunctionRef<void*(size_t, std::align_val_t)> alloc,
       subtle::percpu::Shift shift, int num_cpus, uint8_t shift_offset,
       uint8_t resize_offset);
@@ -1078,7 +1080,12 @@ void* CpuCache<Forwarder>::AllocateSlowNoHooks(size_t size_class) {
     if (ABSL_PREDICT_FALSE(cpu < 0)) {
       // The cpu is stopped.
       void* ptr = nullptr;
-      FetchFromBackingCache(size_class, absl::MakeSpan(&ptr, 1));
+      int r = FetchFromBackingCache(size_class, absl::MakeSpan(&ptr, 1));
+#ifndef NDEBUG
+      TC_ASSERT(r == 1 || ptr == nullptr);
+#else
+      (void)r;
+#endif
       return ptr;
     }
     if (void* ret = AllocateFast(size_class)) {

tcmalloc/mock_central_freelist.h

@@ -53,7 +53,7 @@ class FakeCentralFreeListBase {
 class FakeCentralFreeList : public FakeCentralFreeListBase {
  public:
   void InsertRange(absl::Span<void*> batch);
-  int RemoveRange(absl::Span<void*> batch);
+  [[nodiscard]] int RemoveRange(absl::Span<void*> batch);
 
   void AllocateBatch(absl::Span<void*> batch);
   void FreeBatch(absl::Span<void*> batch);
@@ -66,7 +66,7 @@ class FakeCentralFreeList : public FakeCentralFreeListBase {
 class MinimalFakeCentralFreeList : public FakeCentralFreeListBase {
  public:
   void InsertRange(absl::Span<void*> batch);
-  int RemoveRange(absl::Span<void*> batch);
+  [[nodiscard]] int RemoveRange(absl::Span<void*> batch);
 
   void AllocateBatch(absl::Span<void*> batch);
   void FreeBatch(absl::Span<void*> batch);

tcmalloc/mock_static_forwarder.h

@@ -66,7 +66,7 @@ class FakeStaticForwarder {
     }
   }
 
-  Span* MapObjectToSpan(const void* object) {
+  [[nodiscard]] Span* MapObjectToSpan(const void* object) {
     const PageId page = PageIdContaining(object);
 
     absl::MutexLock l(&mu_);
@@ -82,7 +82,8 @@ class FakeStaticForwarder {
     return nullptr;
   }
 
-  Span* AllocateSpan(int, size_t objects_per_span, Length pages_per_span) {
+  [[nodiscard]] Span* AllocateSpan(int, size_t objects_per_span,
+                                   Length pages_per_span) {
     void* backing =
         ::operator new(pages_per_span.in_bytes(), std::align_val_t(kPageSize));
     PageId page = PageIdContaining(backing);

tcmalloc/span.h

@@ -139,7 +139,7 @@ class Span final : public SpanList::Elem {
   // non-sampling span. It will also update the global counters on the total
   // size of sampled allocations.
   // REQUIRES: this is a SAMPLED span.
-  SampledAllocation* Unsample();
+  [[nodiscard]] SampledAllocation* Unsample();
 
   // Returns the sampled allocation of the span.
   // pageheap_lock is not required, but caller either needs to hold the lock or
@@ -213,21 +213,22 @@ class Span final : public SpanList::Elem {
   // just return false.
   //
   // If the freelist becomes full, we do not push the object onto the freelist.
-  bool FreelistPush(void* ptr, size_t size, uint32_t reciprocal,
-                    uint32_t max_cache_size);
+  [[nodiscard]] bool FreelistPush(void* ptr, size_t size, uint32_t reciprocal,
+                                  uint32_t max_cache_size);
 
   // Pops up to N objects from the freelist and returns them in the batch array.
   // Returns number of objects actually popped.
-  size_t FreelistPopBatch(absl::Span<void*> batch, size_t size);
+  [[nodiscard]] size_t FreelistPopBatch(absl::Span<void*> batch, size_t size);
 
   // Reset a Span object to track the range [p, p + n).
   void Init(PageId p, Length n);
 
   // Initialize freelist to contain all objects in the span.
   // Pops up to N objects from the freelist and returns them in the batch array.
   // Returns number of objects actually popped.
-  int BuildFreelist(size_t size, size_t count, absl::Span<void*> batch,
-                    uint32_t max_cache_size, uint64_t alloc_time);
+  [[nodiscard]] int BuildFreelist(size_t size, size_t count,
+                                  absl::Span<void*> batch,
+                                  uint32_t max_cache_size, uint64_t alloc_time);
 
   // Prefetch cacheline containing most important span information.
   void Prefetch();

tcmalloc/span_benchmark.cc

@@ -49,8 +49,9 @@ class RawSpan {
     int res = posix_memalign(&mem, kPageSize, npages.in_bytes());
     TC_CHECK_EQ(res, 0);
     span_.Init(PageIdContaining(mem), npages);
-    span_.BuildFreelist(size, objects_per_span, {}, kMaxCacheSize,
-                        kSpanAllocTime);
+    TC_CHECK_EQ(span_.BuildFreelist(size, objects_per_span, {}, kMaxCacheSize,
+                                    kSpanAllocTime),
+                0);
   }
 
   ~RawSpan() { free(span_.start_address()); }
@@ -81,7 +82,7 @@ void BM_single_span(benchmark::State& state) {
     processed += n;
 
     for (int j = 0; j < n; j++) {
-      span.FreelistPush(batch[j], size, reciprocal, kMaxCacheSize);
+      (void)span.FreelistPush(batch[j], size, reciprocal, kMaxCacheSize);
     }
   }
 
@@ -203,8 +204,8 @@ void BM_multiple_spans(benchmark::State& state) {
     processed += n;
 
     for (int j = 0; j < n; j++) {
-      spans[current_span].span().FreelistPush(batch[j], size, reciprocal,
-                                              kMaxCacheSize);
+      (void)spans[current_span].span().FreelistPush(batch[j], size, reciprocal,
+                                                    kMaxCacheSize);
     }
   }
 

tcmalloc/span_fuzz.cc

@@ -70,8 +70,9 @@ void FuzzSpan(const std::string& s) {
   Span* span = new (buf) Span();
   span->Init(PageIdContaining(mem), pages);
   // TODO(b/271282540): Fuzz the initial allocation during freelist building.
-  span->BuildFreelist(object_size, objects_per_span, {}, max_span_cache_size,
-                      alloc_time);
+  TC_CHECK_EQ(span->BuildFreelist(object_size, objects_per_span, {},
+                                  max_span_cache_size, alloc_time),
+              0);
 
   TC_CHECK_EQ(span->Allocated(), 0);
 

tcmalloc/span_test.cc

@@ -62,8 +62,9 @@ class RawSpan {
 
     span_ = new (buf_) Span();
     span_->Init(PageIdContaining(mem_), npages);
-    span_->BuildFreelist(size, objects_per_span, {}, max_cache_size,
-                         kSpanAllocTime);
+    TC_CHECK_EQ(span_->BuildFreelist(size, objects_per_span, {}, max_cache_size,
+                                     kSpanAllocTime),
+                0);
   }
 
   ~RawSpan() {

tcmalloc/transfer_cache.h

@@ -112,7 +112,7 @@ class BackingTransferCache {
  public:
   void Init(int size_class) { size_class_ = size_class; }
   void InsertRange(absl::Span<void *> batch) const;
-  ABSL_MUST_USE_RESULT int RemoveRange(absl::Span<void *> batch) const;
+  [[nodiscard]] int RemoveRange(absl::Span<void *> batch) const;
   int size_class() const { return size_class_; }
 
  private:
@@ -194,7 +194,7 @@ class ShardedTransferCacheManagerBase {
     return objects;
   }
 
-  void *Pop(int size_class) {
+  [[nodiscard]] void *Pop(int size_class) {
     TC_ASSERT(subtle::percpu::IsFastNoInit());
     void *batch[1];
     const int got =
@@ -276,7 +276,7 @@ class ShardedTransferCacheManagerBase {
     return stats;
   }
 
-  int RemoveRange(int size_class, absl::Span<void *> batch) {
+  [[nodiscard]] int RemoveRange(int size_class, absl::Span<void *> batch) {
     return get_cache(size_class).RemoveRange(size_class, batch);
   }
 
@@ -423,8 +423,7 @@ class TransferCacheManager : public StaticForwarder {
     cache_[size_class].tc.InsertRange(size_class, batch);
   }
 
-  ABSL_MUST_USE_RESULT int RemoveRange(int size_class,
-                                       absl::Span<void *> batch) {
+  [[nodiscard]] int RemoveRange(int size_class, absl::Span<void *> batch) {
     return cache_[size_class].tc.RemoveRange(size_class, batch);
   }
 
@@ -558,8 +557,7 @@ class TransferCacheManager {
     freelist_[size_class].InsertRange(batch);
   }
 
-  ABSL_MUST_USE_RESULT int RemoveRange(int size_class,
-                                       absl::Span<void*> batch) {
+  [[nodiscard]] int RemoveRange(int size_class, absl::Span<void*> batch) {
     return freelist_[size_class].RemoveRange(batch);
   }
 
@@ -587,9 +585,10 @@ struct ShardedTransferCacheManager {
   constexpr ShardedTransferCacheManager(std::nullptr_t, std::nullptr_t) {}
   static constexpr void Init() {}
   static constexpr bool should_use(int size_class) { return false; }
-  static constexpr void* Pop(int size_class) { return nullptr; }
+  [[nodiscard]] static constexpr void* Pop(int size_class) { return nullptr; }
   static constexpr void Push(int size_class, void* ptr) {}
-  static constexpr int RemoveRange(int size_class, absl::Span<void*> batch) {
+  [[nodiscard]] static constexpr int RemoveRange(int size_class,
+                                                 absl::Span<void*> batch) {
     return 0;
   }
   static constexpr void InsertRange(int size_class, absl::Span<void*> batch) {}