Avoid atomic operation on the last refcount

Summary:
There's no need in the atomic refcount decrement from 1->0, because we're the last live reference and using it to bump 1->2 is already undefined behavior.

Added benchmark:

Before:
```
stepan@devvm329:~/fbsource/fbcode$ buck-out/opt/gen/folly/io/test/iobuf_benchmark --bm_min_usec=1000000
============================================================================
folly/io/test/IOBufBenchmark.cpp                relative  time/iter  iters/s
============================================================================
createAndDestroy                                            36.35ns   27.51M
cloneOneBenchmark                                           40.40ns   24.75M
cloneOneIntoBenchmark                                       27.03ns   37.00M
cloneBenchmark                                              42.23ns   23.68M
cloneIntoBenchmark                                          28.35ns   35.27M
moveBenchmark                                               15.20ns   65.79M
copyBenchmark                                               35.00ns   28.57M
cloneCoalescedBaseline                                     334.59ns    2.99M
cloneCoalescedBenchmark                          660.01%    50.69ns   19.73M
takeOwnershipBenchmark                                      50.57ns   19.78M
============================================================================
```

After:
```
stepan@devvm329:~/fbsource/fbcode$ buck-out/opt/gen/folly/io/test/iobuf_benchmark --bm_min_usec=1000000
============================================================================
folly/io/test/IOBufBenchmark.cpp                relative  time/iter  iters/s
============================================================================
createAndDestroy                                            30.04ns   33.29M
cloneOneBenchmark                                           41.27ns   24.23M
cloneOneIntoBenchmark                                       26.37ns   37.92M
cloneBenchmark                                              43.91ns   22.77M
cloneIntoBenchmark                                          28.49ns   35.10M
moveBenchmark                                               15.50ns   64.52M
copyBenchmark                                               35.85ns   27.89M
cloneCoalescedBaseline                                     318.49ns    3.14M
cloneCoalescedBenchmark                          643.69%    49.48ns   20.21M
takeOwnershipBenchmark                                      45.15ns   22.15M
============================================================================
```

Reviewed By: yfeldblum, davidtgoldblatt

Differential Revision: D14715579

fbshipit-source-id: 3c0373b8423dda680920860979cfa240bf3d8d7a

folly/io/IOBuf.cpp

@@ -799,13 +799,18 @@ void IOBuf::decrementRefcount() {
     return;
   }
 
-  // Decrement the refcount
-  uint32_t newcnt = info->refcount.fetch_sub(1, std::memory_order_acq_rel);
-  // Note that fetch_sub() returns the value before we decremented.
-  // If it is 1, we were the only remaining user; if it is greater there are
-  // still other users.
-  if (newcnt > 1) {
-    return;
+  // Avoid doing atomic decrement if the refcount is 1.
+  // This is safe, because it means that we're the last reference and destroying
+  // the object. Anything trying to copy it is already undefined behavior.
+  if (info->refcount.load(std::memory_order_acquire) > 1) {
+    // Decrement the refcount
+    uint32_t newcnt = info->refcount.fetch_sub(1, std::memory_order_acq_rel);
+    // Note that fetch_sub() returns the value before we decremented.
+    // If it is 1, we were the only remaining user; if it is greater there are
+    // still other users.
+    if (newcnt > 1) {
+      return;
+    }
   }
 
   // save the useHeapFullStorage flag here since

folly/io/test/IOBufBenchmark.cpp

@@ -19,6 +19,13 @@
 
 using folly::IOBuf;
 
+BENCHMARK(createAndDestroy, iters) {
+  while (iters--) {
+    IOBuf buf(IOBuf::CREATE, 10);
+    folly::doNotOptimizeAway(buf.capacity());
+  }
+}
+
 BENCHMARK(cloneOneBenchmark, iters) {
   IOBuf buf(IOBuf::CREATE, 10);
   while (iters--) {