Add more TODOs

src/metric.cpp

@@ -105,8 +105,6 @@ template <> struct hash<metric::LabelsKey> {
 
 namespace metric {
 
-using AtomicWord = std::atomic<uint64_t>;
-
 // DESIGN: Store doubles in atomic<uint64_t> for lock-free operations
 // - Preserves full IEEE 754 double precision (no truncation)
 // - Allows atomic load/store without locks
@@ -154,9 +152,10 @@ struct Counter::State {
   friend struct Metric;
 };
 
-// Gauge: Global, can increase/decrease, multiple writers (uses atomic CAS)
+// Gauge: Global, can increase/decrease, multiple writers (uses atomic CAS).
+// TODO slow under contention.
 struct Gauge::State {
-  AtomicWord value; // Stores double as uint64_t bits, lock-free
+  std::atomic<uint64_t> value; // Stores double as uint64_t bits, lock-free
   friend struct Metric;
 };
 
@@ -166,8 +165,10 @@ struct Histogram::State {
       thresholds; // Bucket boundaries (sorted, deduplicated, includes +Inf)
   std::vector<uint64_t> counts; // Count per bucket - single writer, malloc
                                 // provides 16-byte alignment
-  AtomicWord sum; // Sum of observations (double stored as uint64_t bits)
-  AtomicWord observations; // Total observation count (uint64_t)
+  // TODO this should just be a double like in counter
+  std::atomic<uint64_t>
+      sum; // Sum of observations (double stored as uint64_t bits)
+  std::atomic<uint64_t> observations; // Total observation count (uint64_t)
   friend struct Metric;
 };
 
@@ -190,6 +191,9 @@ struct Metric {
       // Thread registration happens lazily when metrics are created
     }
     ~ThreadInit() {
+      // TODO we need to accumulate this threads counts into a global. Otherwise
+      // it can go backwards when we destroy a thread.
+
       //  Clean up this thread's storage from all families
       std::unique_lock<std::mutex> _{mutex};
       auto thread_id = std::this_thread::get_id();
@@ -375,6 +379,8 @@ update_histogram_buckets(const std::vector<double> &thresholds,
         : "x"(updated_counts)            // Input: xmm register
         : "memory"                       // Memory clobber
     );
+    // We don't actually need it to be atomic across 128-bits, but that's
+    // sufficient to guarantee each 64 bit half is atomic.
   }
 
   // Handle remainder with atomic stores