Add C++11 Ranged For loop alternative to KeepRunning (#454)

* Add C++11 Ranged For loop alternative to KeepRunning

As pointed out by @astrelni and @dominichamon, the KeepRunning
loop requires a bunch of memory loads and stores every iterations,
which affects the measurements.

The main reason for these additional loads and stores is that the
State object is passed in by reference, making its contents externally
visible memory, and the compiler doesn't know it hasn't been changed
by non-visible code.

It's also possible the large size of the State struct is hindering
optimizations.

This patch allows the `State` object to be iterated over using
a range-based for loop. Example:

void BM_Foo(benchmark::State& state) {
	for (auto _ : state) {
		[...]
	}
}

This formulation is much more efficient, because the variable counting
the loop index is stored in the iterator produced by `State::begin()`,
which itself is stored in function-local memory and therefore not accessible
by code outside of the function. Therefore the compiler knows the iterator
hasn't been changed every iteration.

This initial patch and idea was from Alex Strelnikov.

* Fix null pointer initialization in C++03

README.md

@@ -194,6 +194,58 @@ Three macros are provided for adding benchmark templates.
 #define BENCHMARK_TEMPLATE2(func, arg1, arg2)
 ```
 
+### A Faster KeepRunning loop
+
+In C++11 mode, a ranged-based for loop should be used in preference to
+the `KeepRunning` loop for running the benchmarks. For example:
+
+```c++
+static void BM_Faste(benchmark::State &st) {
+  for (auto _ : state) {
+    FastOperation();
+  }
+}
+```
+
+The reason the ranged-based for loop is faster than using `KeepRunning`, is
+because `KeepRunning` requires a memory load and store of the iteration count
+ever iteration, whereas the ranged-for variant is able to keep the iteration count
+in a register.
+
+For example, an empty inner loop of using the ranged-based for method looks like:
+
+```asm
+# Loop Init
+  mov rbx, qword ptr [r14 + 104]
+  call benchmark::State::StartKeepRunning()
+  test rbx, rbx
+  je .LoopEnd
+.LoopHeader: # =>This Inner Loop Header: Depth=1
+  add rbx, -1
+  jne .LoopHeader
+.LoopEnd:
+```
+
+Compared to an empty `KeepRunning` loop, which looks like:
+
+```asm
+.LoopHeader: # in Loop: Header=BB0_3 Depth=1
+  cmp byte ptr [rbx], 1
+  jne .LoopInit
+.LoopBody: # =>This Inner Loop Header: Depth=1
+  mov rax, qword ptr [rbx + 8]
+  lea rcx, [rax + 1]
+  mov qword ptr [rbx + 8], rcx
+  cmp rax, qword ptr [rbx + 104]
+  jb .LoopHeader
+  jmp .LoopEnd
+.LoopInit:
+  mov rdi, rbx
+  call benchmark::State::StartKeepRunning()
+  jmp .LoopBody
+.LoopEnd:
+```
+
 ## Passing arbitrary arguments to a benchmark
 In C++11 it is possible to define a benchmark that takes an arbitrary number
 of extra arguments. The `BENCHMARK_CAPTURE(func, test_case_name, ...args)`

include/benchmark/benchmark.h

@@ -238,7 +238,6 @@ BENCHMARK(BM_test)->Unit(benchmark::kMillisecond);
 #define BENCHMARK_GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__)
 #endif
 
-
 namespace benchmark {
 class BenchmarkReporter;
 
@@ -413,6 +412,19 @@ enum ReportMode
 // benchmark to use.
 class State {
  public:
+  struct StateIterator;
+  friend struct StateIterator;
+
+  // Returns iterators used to run each iteration of a benchmark using a
+  // C++11 ranged-based for loop. These functions should not be called directly.
+  //
+  // REQUIRES: The benchmark has not started running yet. Neither begin nor end
+  // have been called previously.
+  //
+  // NOTE: KeepRunning may not be used after calling either of these functions.
+  BENCHMARK_ALWAYS_INLINE StateIterator begin();
+  BENCHMARK_ALWAYS_INLINE StateIterator end();
+
   // Returns true if the benchmark should continue through another iteration.
   // NOTE: A benchmark may not return from the test until KeepRunning() has
   // returned false.
@@ -585,6 +597,53 @@ class State {
   BENCHMARK_DISALLOW_COPY_AND_ASSIGN(State);
 };
 
+struct State::StateIterator {
+  struct BENCHMARK_UNUSED Value {};
+  typedef std::forward_iterator_tag iterator_category;
+  typedef Value value_type;
+  typedef Value reference;
+  typedef Value pointer;
+
+ private:
+  friend class State;
+  BENCHMARK_ALWAYS_INLINE
+  StateIterator() : cached_(0), parent_() {}
+
+  BENCHMARK_ALWAYS_INLINE
+  explicit StateIterator(State* st)
+      : cached_(st->max_iterations), parent_(st) {}
+
+ public:
+  BENCHMARK_ALWAYS_INLINE
+  Value operator*() const { return Value(); }
+
+  BENCHMARK_ALWAYS_INLINE
+  StateIterator& operator++() {
+    assert(cached_ > 0);
+    --cached_;
+    return *this;
+  }
+
+  BENCHMARK_ALWAYS_INLINE
+  bool operator!=(StateIterator const&) const {
+    if (BENCHMARK_BUILTIN_EXPECT(cached_ != 0, true)) return true;
+    parent_->FinishKeepRunning();
+    return false;
+  }
+
+ private:
+  size_t cached_;
+  State* const parent_;
+};
+
+BENCHMARK_ALWAYS_INLINE inline State::StateIterator State::begin() {
+  return StateIterator(this);
+}
+BENCHMARK_ALWAYS_INLINE inline State::StateIterator State::end() {
+  StartKeepRunning();
+  return StateIterator();
+}
+
 namespace internal {
 
 typedef void(Function)(State&);

test/basic_test.cc

@@ -96,4 +96,23 @@ void BM_empty_stop_start(benchmark::State& state) {
 BENCHMARK(BM_empty_stop_start);
 BENCHMARK(BM_empty_stop_start)->ThreadPerCpu();
 
+
+void BM_KeepRunning(benchmark::State& state) {
+  size_t iter_count = 0;
+  while (state.KeepRunning()) {
+    ++iter_count;
+  }
+  assert(iter_count == state.max_iterations);
+}
+BENCHMARK(BM_KeepRunning);
+
+void BM_RangedFor(benchmark::State& state) {
+  size_t iter_count = 0;
+  for (auto _ : state) {
+    ++iter_count;
+  }
+  assert(iter_count == state.max_iterations);
+}
+BENCHMARK(BM_RangedFor);
+
 BENCHMARK_MAIN()