[benchmark] Add a benchmark for our dtoa implementation

Make sure it is faster than std::to_value(). And it is, by ~30% for
the normal use case, and much faster when close to the double limits.

util::dtoa also offers a much better precision.

benchmark/util/dtoa.benchmark.cpp

@@ -0,0 +1,66 @@
+#include <benchmark/benchmark.h>
+
+#include <mbgl/util/dtoa.hpp>
+
+#include <cfloat>
+#include <cmath>
+
+using namespace mbgl;
+
+static void Util_dtoa(::benchmark::State& state) {
+    while (state.KeepRunning()) {
+        util::dtoa(0.);
+        util::dtoa(M_E);
+        util::dtoa(M_LOG2E);
+        util::dtoa(M_LOG10E);
+        util::dtoa(M_LN2);
+        util::dtoa(M_LN10);
+        util::dtoa(M_PI);
+        util::dtoa(M_PI_2);
+        util::dtoa(M_PI_4);
+        util::dtoa(M_1_PI);
+        util::dtoa(M_2_PI);
+        util::dtoa(M_2_SQRTPI);
+        util::dtoa(M_SQRT2);
+        util::dtoa(M_SQRT1_2);
+    }
+}
+
+static void Util_standardDtoa(::benchmark::State& state) {
+    while (state.KeepRunning()) {
+        std::to_string(0.);
+        std::to_string(M_E);
+        std::to_string(M_LOG2E);
+        std::to_string(M_LOG10E);
+        std::to_string(M_LN2);
+        std::to_string(M_LN10);
+        std::to_string(M_PI);
+        std::to_string(M_PI_2);
+        std::to_string(M_PI_4);
+        std::to_string(M_1_PI);
+        std::to_string(M_2_PI);
+        std::to_string(M_2_SQRTPI);
+        std::to_string(M_SQRT2);
+        std::to_string(M_SQRT1_2);
+    }
+}
+
+static void Util_dtoaLimits(::benchmark::State& state) {
+    while (state.KeepRunning()) {
+        util::dtoa(DBL_MIN);
+        util::dtoa(DBL_MAX);
+    }
+}
+
+static void Util_standardDtoaLimits(::benchmark::State& state) {
+    while (state.KeepRunning()) {
+        std::to_string(DBL_MIN);
+        std::to_string(DBL_MAX);
+    }
+}
+
+BENCHMARK(Util_dtoa);
+BENCHMARK(Util_standardDtoa);
+
+BENCHMARK(Util_dtoaLimits);
+BENCHMARK(Util_standardDtoaLimits);

cmake/benchmark-files.cmake

@@ -19,4 +19,7 @@ set(MBGL_BENCHMARK_FILES
     benchmark/src/mbgl/benchmark/benchmark.cpp
     benchmark/src/mbgl/benchmark/util.cpp
     benchmark/src/mbgl/benchmark/util.hpp
+
+    # util
+    benchmark/util/dtoa.benchmark.cpp
 )