Add an sfmt19937 benchmark (again)

Summary: `__gnu_cxx::sfmt19937` is 3.5x faster than `std::mt19937` according to the benchmark.

Reviewed By: yfeldblum

Differential Revision: D5410705

fbshipit-source-id: d503544a0b9b1b0c614a27466a297a4e6902fc15

folly/test/RandomBenchmark.cpp

@@ -24,6 +24,10 @@
 #include <folly/Benchmark.h>
 #include <folly/Foreach.h>
 
+#if FOLLY_HAVE_EXTRANDOM_SFMT19937
+#include <ext/random>
+#endif
+
 using namespace folly;
 
 BENCHMARK(minstdrand, n) {
@@ -46,6 +50,18 @@ BENCHMARK(mt19937, n) {
   FOR_EACH_RANGE(i, 0, n) { doNotOptimizeAway(rng()); }
 }
 
+#if FOLLY_HAVE_EXTRANDOM_SFMT19937
+BENCHMARK(sfmt19937, n) {
+  BenchmarkSuspender braces;
+  std::random_device rd;
+  __gnu_cxx::sfmt19937 rng(rd());
+
+  braces.dismiss();
+
+  FOR_EACH_RANGE(i, 0, n) { doNotOptimizeAway(rng()); }
+}
+#endif
+
 BENCHMARK(threadprng, n) {
   BenchmarkSuspender braces;
   ThreadLocalPRNG tprng;