Fixups following addition of KeepRunningBatch (296ec5693) (#526)

* Support State::KeepRunningBatch().

State::KeepRunning() can take large amounts of time relative to quick
operations (on the order of 1ns, depending on hardware). For such
sensitive operations, it is recommended to run batches of repeated
operations.

This commit simplifies handling of total_iterations_. Rather than
predecrementing such that total_iterations_ == 1 signals that
KeepRunning() should exit, total_iterations_ == 0 now signals the
intention for the benchmark to exit.

* Create better fast path in State::KeepRunningBatch()

* Replace int parameter with size_t to fix signed mismatch warnings

* Ensure benchmark State has been started even on error.

* Simplify KeepRunningBatch()

* Implement KeepRunning() in terms of KeepRunningBatch().

* Improve codegen by helping the compiler undestand dead code.

* Dummy commit for build bots' benefit.

include/benchmark/benchmark.h

@@ -433,6 +433,7 @@ class State {
   bool KeepRunning();
 
   // Returns true iff the benchmark should run n more iterations.
+  // REQUIRES: 'n' > 0.
   // NOTE: A benchmark must not return from the test until KeepRunningBatch()
   // has returned false.
   // NOTE: KeepRunningBatch() may overshoot by up to 'n' iterations.
@@ -609,6 +610,9 @@ class State {
 
  private:
   void StartKeepRunning();
+  // Implementation of KeepRunning() and KeepRunningBatch().
+  // is_batch must be true unless n is 1.
+  bool KeepRunningInternal(size_t n, bool is_batch);
   void FinishKeepRunning();
   internal::ThreadTimer* timer_;
   internal::ThreadManager* manager_;
@@ -617,28 +621,21 @@ class State {
 
 inline BENCHMARK_ALWAYS_INLINE
 bool State::KeepRunning() {
-  // total_iterations_ is set to 0 by the constructor, and always set to a
+  return KeepRunningInternal(1, /*is_batch=*/ false);
-  // nonzero value by StartKepRunning().
-  if (BENCHMARK_BUILTIN_EXPECT(total_iterations_ != 0, true)) {
-    --total_iterations_;
-    return true;
-  }
-  if (!started_) {
-    StartKeepRunning();
-    if (!error_occurred_) {
-      // max_iterations > 0. The first iteration is always valid.
-      --total_iterations_;
-      return true;
-    }
-  }
-  FinishKeepRunning();
-  return false;
 }
 
 inline BENCHMARK_ALWAYS_INLINE
 bool State::KeepRunningBatch(size_t n) {
+  return KeepRunningInternal(n, /*is_batch=*/ true);
+}
+
+inline BENCHMARK_ALWAYS_INLINE
+bool State::KeepRunningInternal(size_t n, bool is_batch) {
   // total_iterations_ is set to 0 by the constructor, and always set to a
   // nonzero value by StartKepRunning().
+  assert(n > 0);
+  // n must be 1 unless is_batch is true.
+  assert(is_batch || n == 1);
   if (BENCHMARK_BUILTIN_EXPECT(total_iterations_ >= n, true)) {
     total_iterations_ -= n;
     return true;
@@ -650,7 +647,8 @@ bool State::KeepRunningBatch(size_t n) {
       return true;
     }
   }
-  if (total_iterations_ != 0) {
+  // For non-batch runs, total_iterations_ must be 0 by now.
+  if (is_batch && total_iterations_ != 0) {
     batch_leftover_  = n - total_iterations_;
     total_iterations_ = 0;
     return true;