micros() optimisation and flto build fix (#53)

Author: mh-dm

cores/arduino/arduino.cpp

@@ -31,8 +31,6 @@
 
 #include <Arduino.h>
 
-extern uint64_t _millis;
-
 // Interrupts
 void cli(void) { __disable_irq(); } // Disable
 void sei(void) { __enable_irq(); }  // Enable
@@ -45,14 +43,6 @@ void _delay_ms(const int delay_ms) {
   delay(delay_ms);
 }
 
-uint32_t millis() {
-  return _millis;
-}
-
-uint32_t micros() {
-  return (_millis * 1000) + ((SysTick->LOAD - SysTick->VAL) / (SystemCoreClock / 1000000) );
-}
-
 // This is required for some Arduino libraries we are using
 void delayMicroseconds(uint32_t us) {
   LPC176x::delay_us(us);
@@ -89,7 +79,7 @@ void analogWrite(pin_t pin, int pwm_value) {  // 1 - 254: pwm_value, 0: LOW, 255
   }
 }
 
-uint8_t analog_read_resolution = 10;
+volatile static uint8_t analog_read_resolution = 10;
 void analogReadResolution(uint8_t resolution) {
   analog_read_resolution = resolution > 32 ? 32 : resolution;
 }

cores/arduino/main.cpp

@@ -14,11 +14,14 @@ extern void setup();
 extern void loop();
 
 extern "C" {
-  volatile uint64_t _millis;
+  [[gnu::section(".noinit")]] volatile uint32_t _millis;
+  [[gnu::section(".noinit")]] volatile static uint32_t _micros_per_tick_scaled; // Units: us / (2^16 ticks)
 
   uint32_t SysTick_Config(uint32_t ticks) {
     if (ticks > SysTick_LOAD_RELOAD_Msk) return 1;
 
+    _micros_per_tick_scaled = (1000 << 16) / ticks;               // microseconds per system clock cycle, scaled by 2^16
+
     SysTick->LOAD = (ticks & SysTick_LOAD_RELOAD_Msk) - 1;        // Set reload register
     SysTick->VAL  = 0;                                            // Load the SysTick Counter Value
     SysTick->CTRL = SysTick_CTRL_CLKSOURCE_Msk |
@@ -41,6 +44,14 @@ extern "C" {
   }
 }
 
+uint32_t millis() {
+  return _millis;
+}
+
+uint32_t micros() {
+  return (_millis * 1000) + (((SysTick->LOAD - SysTick->VAL) * _micros_per_tick_scaled) >> 16); // Shifted to remove scale factor
+}
+
 int main(void) {
   // Turn on and initialise ADC in burst mode
   LPC176x::adc_hardware.init();