Allow constexpr evalution to occur. optimistic_yield is always called with a literal parameter value.

Author: dok-net

cores/esp8266/Arduino.h

@@ -204,7 +204,24 @@ void setup(void);
 void loop(void);
 
 void yield(void);
-void optimistic_yield(uint32_t interval_us);
+
+#ifndef F_CPU
+// single function needed from SDK user_interface.h
+extern "C" uint8 system_get_cpu_freq(void);
+#endif
+
+void __optimistic_yield(uint32_t intvl_cycles);
+
+void inline optimistic_yield(uint32_t interval_us) __attribute__((always_inline));
+void inline optimistic_yield(uint32_t interval_us) {
+    __optimistic_yield(interval_us *
+#if defined(F_CPU)
+        clockCyclesPerMicrosecond()
+#else
+        getCpuFreqMHz()
+#endif
+    );
+}
 
 #define _PORT_GPIO16    1
 #define digitalPinToPort(pin)       (((pin)==16)?(_PORT_GPIO16):(0))

cores/esp8266/core_esp8266_main.cpp

@@ -57,7 +57,7 @@ cont_t* g_pcont __attribute__((section(".noinit")));
 /* Event queue used by the main (arduino) task */
 static os_event_t s_loop_queue[LOOP_QUEUE_SIZE];
 
-/* Used to implement optimistic_yield */
+/* Used to implement __optimistic_yield */
 static uint32_t s_cycles_since_yield_start;
 
 /* For ets_intr_lock_nest / ets_intr_unlock_nest
@@ -125,10 +125,9 @@ extern "C" void __yield() {
 
 extern "C" void yield(void) __attribute__ ((weak, alias("__yield")));
 
-extern "C" void optimistic_yield(uint32_t interval_us) {
-    const uint32_t interval_cycles = interval_us * ESP.getCpuFreqMHz();
-    if (can_yield() &&
-        (ESP.getCycleCount() - s_cycles_since_yield_start) > interval_cycles)
+extern "C" void __optimistic_yield(uint32_t intvl_cycles) {
+    if ((ESP.getCycleCount() - s_cycles_since_yield_start) > intvl_cycles &&
+        can_yield())
     {
         yield();
     }