Keep GNU fastpath for jerry_port_get_local_time_zone_adjustment (#4600)

PR #4513 caused a measurable slowdown by removing the GNU specific TZA calculation.
This patch reverts this removal but also keeps the general implementation introduced in #4513

JerryScript-DCO-1.0-Signed-off-by: Robert Fancsik robert.fancsik@h-lab.eu

jerry-port/default/CMakeLists.txt

@@ -59,6 +59,22 @@ endif()
 # (should only be necessary if we used compiler default libc but not checking that)
 set(DEFINES_PORT_DEFAULT _BSD_SOURCE _DEFAULT_SOURCE)
 
+INCLUDE (CheckStructHasMember)
+# CHECK_STRUCT_HAS_MEMBER works by trying to compile some C code that accesses the
+# given field of the given struct. However, our default compiler options break this
+# C code, so turn a couple of them off for this.
+if(USING_GCC OR USING_CLANG)
+  set(CMAKE_REQUIRED_FLAGS "-Wno-error=strict-prototypes -Wno-error=old-style-definition -Wno-error=unused-value")
+endif()
+# tm.tm_gmtoff is non-standard, so glibc doesn't expose it in c99 mode
+# (our default). Define some macros to expose it anyway.
+set(CMAKE_REQUIRED_DEFINITIONS "-D_BSD_SOURCE -D_DEFAULT_SOURCE")
+CHECK_STRUCT_HAS_MEMBER ("struct tm" tm_gmtoff time.h HAVE_TM_GMTOFF)
+# localtime_r is is not threadsafe with clang on OSX
+if(HAVE_TM_GMTOFF AND NOT "${PLATFORM}" STREQUAL "DARWIN" AND NOT USING_CLANG)
+  set(DEFINES_PORT_DEFAULT ${DEFINES_PORT_DEFAULT} HAVE_TM_GMTOFF)
+endif()
+
 # Default Jerry port implementation library
 add_library(${JERRY_PORT_DEFAULT_NAME} ${SOURCE_PORT_DEFAULT})
 target_include_directories(${JERRY_PORT_DEFAULT_NAME} PUBLIC ${INCLUDE_PORT_DEFAULT})

jerry-port/default/default-date.c

@@ -59,7 +59,19 @@ static double FileTimeToUnixTimeMs (FILETIME ft)
 double jerry_port_get_local_time_zone_adjustment (double unix_ms,  /**< ms since unix epoch */
                                                   bool is_utc)  /**< is the time above in UTC? */
 {
-#ifdef _WIN32
+#if defined (HAVE_TM_GMTOFF)
+  struct tm tm;
+  time_t now = (time_t) (unix_ms / 1000);
+  localtime_r (&now, &tm);
+
+  if (!is_utc)
+  {
+    now -= tm.tm_gmtoff;
+    localtime_r (&now, &tm);
+  }
+
+  return ((double) tm.tm_gmtoff) * 1000;
+#elif defined (_WIN32)
   FILETIME fileTime, localFileTime;
   SYSTEMTIME systemTime, localSystemTime;
   ULARGE_INTEGER time, localTime;
@@ -111,11 +123,11 @@ double jerry_port_get_local_time_zone_adjustment (double unix_ms,  /**< ms since
   }
 
   return tza_s * 1000;
-#else /* !_WIN32 && !__GNUC__ && !__clang__ */
+#else /* !HAVE_TM_GMTOFF && !_WIN32 && !__GNUC__ && !__clang__ */
   (void) unix_ms; /* unused */
   (void) is_utc; /* unused */
   return 0.0;
-#endif /* _WIN32 */
+#endif /* HAVE_TM_GMTOFF */
 } /* jerry_port_get_local_time_zone_adjustment */
 
 /**