file: Use lighter access to map of fs-info-s

Every time make_file_impl() creates a file it first counts the number of
entries for the device, then tries to lookup the entry. Instead of
relying on compiler to optimize it, it's nicer to lookup the entry once.

Signed-off-by: Pavel Emelyanov <[email protected]>

src/core/file.cc

@@ -1078,7 +1078,8 @@ make_file_impl(int fd, file_open_options options, int flags, struct stat st) noe
  auto st_dev = st.st_dev;
  static thread_local std::unordered_map<decltype(st_dev), internal::fs_info> s_fstype;
 
- if (s_fstype.count(st_dev) == 0) {
+ auto i = s_fstype.find(st_dev);
+ if (i == s_fstype.end()) [[unlikely]] {
  return engine().fstatfs(fd).then([fd, options = std::move(options), flags, st = std::move(st)] (struct statfs sfs) {
  internal::fs_info fsi;
  fsi.block_size = sfs.f_bsize;
@@ -1126,13 +1127,12 @@ make_file_impl(int fd, file_open_options options, int flags, struct stat st) noe
  fsi.fsync_is_exclusive = true;
  fsi.nowait_works = false;
  }
- auto st_dev = st.st_dev;
- s_fstype[st_dev] = std::move(fsi);
+ s_fstype.insert(std::make_pair(st.st_dev, std::move(fsi)));
  return make_file_impl(fd, std::move(options), flags, std::move(st));
  });
  }
 
- const internal::fs_info& fsi = s_fstype[st_dev];
+ const internal::fs_info& fsi = i->second;
  if (!fsi.append_challenged || options.append_is_unlikely || ((flags & O_ACCMODE) == O_RDONLY)) {
  return make_ready_future<shared_ptr<file_impl>>(make_shared<posix_file_real_impl>(fd, open_flags(flags), std::move(options), fsi, st_dev));
  }