Track large array buffer allocations.

Adds a counter for large array buffers. Used to give an indication of
how common large array buffers are allocated in V8.

For the moment, we assume a 1Mb cutoff for the notion of large array
buffers.  We also use log2(length) to cleanly bucket sizes into a
histogram.

BUG=chromium:704922
R=bbudge@chromium.org,bradnelson@chromium.org,mtrofin@chromium.org

Review-Url: https://codereview.chromium.org/2792623002
Cr-Commit-Position: refs/heads/master@{#44317}

src/counters.h

@@ -928,7 +928,8 @@ class RuntimeCallTimerScope {
   HR(wasm_functions_per_asm_module, V8.WasmFunctionsPerModule.asm, 1, 100000, \
      51)                                                                      \
   HR(wasm_functions_per_wasm_module, V8.WasmFunctionsPerModule.wasm, 1,       \
-     100000, 51)
+     100000, 51)                                                              \
+  HR(array_buffer_big_allocations, V8.ArrayBufferBigAllocations, 1, 32, 32)
 
 #define HISTOGRAM_TIMER_LIST(HT)                                               \
   /* Garbage collection timers. */                                             \

src/objects.cc

@@ -19473,6 +19473,12 @@ bool JSArrayBuffer::SetupAllocatingData(Handle<JSArrayBuffer> array_buffer,
   // Prevent creating array buffers when serializing.
   DCHECK(!isolate->serializer_enabled());
   if (allocated_length != 0) {
+    constexpr size_t kMinBigAllocation = 1 << 20;
+    if (allocated_length >= kMinBigAllocation) {
+      isolate->counters()->array_buffer_big_allocations()->AddSample(
+          sizeof(uint64_t) * kBitsPerByte -
+          base::bits::CountLeadingZeros64(allocated_length));
+    }
     if (initialize) {
       data = isolate->array_buffer_allocator()->Allocate(allocated_length);
     } else {