`<chrono>`: optimize `chrono::steady_clock::now()` (#2086)

Co-authored-by: Bruce Dawson <brucedawson@chromium.org>
Co-authored-by: Alex Guteniev <gutenev@gmail.com>

stl/inc/chrono

@@ -691,6 +691,15 @@ namespace chrono {
             const long long _Freq = _Query_perf_frequency(); // doesn't change after system boot
             const long long _Ctr  = _Query_perf_counter();
             static_assert(period::num == 1, "This assumes period::num == 1.");
+            // 10 MHz is a very common QPC frequency on modern PCs. Optimizing for
+            // this specific frequency can double the performance of this function by
+            // avoiding the expensive frequency conversion path.
+            constexpr long long _TenMHz = 10'000'000;
+            if (_Freq == _TenMHz) {
+                static_assert(period::den % _TenMHz == 0, "It should never fail.");
+                constexpr long long _Multiplier = period::den / _TenMHz;
+                return time_point(duration(_Ctr * _Multiplier));
+            } else {
                 // Instead of just having "(_Ctr * period::den) / _Freq",
                 // the algorithm below prevents overflow when _Ctr is sufficiently large.
                 // It assumes that _Freq * period::den does not overflow, which is currently true for nano period.
@@ -700,6 +709,7 @@ namespace chrono {
                 const long long _Part  = (_Ctr % _Freq) * period::den / _Freq;
                 return time_point(duration(_Whole + _Part));
             }
+        }
     };
 
     using high_resolution_clock = steady_clock;