nghttpx: Use faster version of power

In our use case, x and y is quite small, and there is no chance for
overflow, and y is always integer.

src/shrpx_connect_blocker.cc

@@ -80,8 +80,8 @@ void ConnectBlocker::on_failure() {
 
   ++fail_count_;
 
-  auto base_backoff = pow(
+  auto base_backoff =
-      MULTIPLIER, static_cast<double>(std::min(MAX_BACKOFF_EXP, fail_count_)));
+      util::int_pow(MULTIPLIER, std::min(MAX_BACKOFF_EXP, fail_count_));
   auto dist = std::uniform_real_distribution<>(-JITTER * base_backoff,
                                                JITTER * base_backoff);
 

src/shrpx_live_check.cc

@@ -157,8 +157,8 @@ constexpr auto JITTER = 0.2;
 } // namespace
 
 void LiveCheck::schedule() {
-  auto base_backoff = pow(
+  auto base_backoff =
-      MULTIPLIER, static_cast<double>(std::min(fail_count_, MAX_BACKOFF_EXP)));
+      util::int_pow(MULTIPLIER, std::min(fail_count_, MAX_BACKOFF_EXP));
   auto dist = std::uniform_real_distribution<>(-JITTER * base_backoff,
                                                JITTER * base_backoff);
 

src/util.cc

@@ -1316,6 +1316,15 @@ StringRef percent_decode(BlockAllocator &balloc, const StringRef &src) {
   return StringRef{iov.base, p};
 }
 
+// Returns x**y
+double int_pow(double x, size_t y) {
+  auto res = 1.;
+  for (; y; --y) {
+    res *= x;
+  }
+  return res;
+}
+
 } // namespace util
 
 } // namespace nghttp2

src/util.h

@@ -677,6 +677,9 @@ OutputIterator copy_lit(OutputIterator it, CharT(&s)[N]) {
   return std::copy_n(s, N - 1, it);
 }
 
+// Returns x**y
+double int_pow(double x, size_t y);
+
 } // namespace util
 
 } // namespace nghttp2