#if defined => #ifdef / PIN_EXISTS

Marlin/MarlinSerial.cpp

     }
   }
 
-  #if defined(M_USARTx_UDRE_vect)
+  #ifdef M_USARTx_UDRE_vect
     ISR(M_USARTx_UDRE_vect) {
       _tx_udr_empty_irq();
     }
 
 #endif // TX_BUFFER_SIZE
 
-#if defined(M_USARTx_RX_vect)
+#ifdef M_USARTx_RX_vect
   ISR(M_USARTx_RX_vect) {
     unsigned char c  =  M_UDRx;
     store_char(c);

Marlin/Marlin_main.cpp

   void setPwmFrequency(uint8_t pin, int val) {
     val &= 0x07;
     switch (digitalPinToTimer(pin)) {
-      #if defined(TCCR0A)
+      #ifdef TCCR0A
         case TIMER0A:
         case TIMER0B:
           // TCCR0B &= ~(_BV(CS00) | _BV(CS01) | _BV(CS02));
           // TCCR0B |= val;
           break;
       #endif
-      #if defined(TCCR1A)
+      #ifdef TCCR1A
         case TIMER1A:
         case TIMER1B:
           // TCCR1B &= ~(_BV(CS10) | _BV(CS11) | _BV(CS12));
           // TCCR1B |= val;
           break;
       #endif
-      #if defined(TCCR2)
+      #ifdef TCCR2
         case TIMER2:
         case TIMER2:
           TCCR2 &= ~(_BV(CS10) | _BV(CS11) | _BV(CS12));
           TCCR2 |= val;
           break;
       #endif
-      #if defined(TCCR2A)
+      #ifdef TCCR2A
         case TIMER2A:
         case TIMER2B:
           TCCR2B &= ~(_BV(CS20) | _BV(CS21) | _BV(CS22));
           TCCR2B |= val;
           break;
       #endif
-      #if defined(TCCR3A)
+      #ifdef TCCR3A
         case TIMER3A:
         case TIMER3B:
         case TIMER3C:
           TCCR3B |= val;
           break;
       #endif
-      #if defined(TCCR4A)
+      #ifdef TCCR4A
         case TIMER4A:
         case TIMER4B:
         case TIMER4C:
           TCCR4B |= val;
           break;
       #endif
-      #if defined(TCCR5A)
+      #ifdef TCCR5A
         case TIMER5A:
         case TIMER5B:
         case TIMER5C:

Marlin/pinsDebug.h

     #if defined(TCCR3A) && defined(COM3A1)
       PWM_CASE(3,A);
       PWM_CASE(3,B);
-      #if defined(COM3C1)
+      #ifdef COM3C1
         PWM_CASE(3,C);
       #endif
     #endif
         else if (TIMSK3 & (_BV(TOIE3) | _BV(ICIE3))) err_prob_interrupt();
         else can_be_used();
         break;
-      #if defined(COM3C1)
+      #ifdef COM3C1
       case TIMER3C:
-          TIMER_PREFIX(3,C,3);
-          if (WGM_TEST2) err_is_counter();
-          else if (TEST(TIMSK3, OCIE3C)) err_is_interrupt();
-          else if (TIMSK3 & (_BV(TOIE3) | _BV(ICIE3))) err_prob_interrupt();
-          else can_be_used();
-          break;
+        TIMER_PREFIX(3,C,3);
+        if (WGM_TEST2) err_is_counter();
+        else if (TEST(TIMSK3, OCIE3C)) err_is_interrupt();
+        else if (TIMSK3 & (_BV(TOIE3) | _BV(ICIE3))) err_prob_interrupt();
+        else can_be_used();
+        break;
       #endif
     #endif
 

Marlin/pins_MELZI.h

 
 #define BOARD_NAME "Melzi"
 
-#if defined(__AVR_ATmega1284P__)
+#ifdef __AVR_ATmega1284P__
   #define LARGE_FLASH true
 #endif
 

Marlin/pins_MELZI_MAKR3D.h

 
 #define BOARD_NAME "Melzi ATmega1284"
 
-#if defined(__AVR_ATmega1284P__)
+#ifdef __AVR_ATmega1284P__
   #define LARGE_FLASH true
 #endif
 

Marlin/pins_SANGUINOLOLU_12.h

 
 #define BOARD_NAME "Sanguinololu 1.2"
 
-#if defined(__AVR_ATmega1284P__)
+#ifdef __AVR_ATmega1284P__
   #define LARGE_FLASH true
 #endif
 

Marlin/pins_STB_11.h

 
 #define BOARD_NAME "STB V1.1"
 
-#if defined(__AVR_ATmega1284P__)
+#ifdef __AVR_ATmega1284P__
   #define LARGE_FLASH true
 #endif
 

Marlin/planner.cpp

 
   #if FAN_COUNT > 0
 
-    #if defined(FAN_MIN_PWM)
+    #ifdef FAN_MIN_PWM
       #define CALC_FAN_SPEED(f) (tail_fan_speed[f] ? ( FAN_MIN_PWM + (tail_fan_speed[f] * (255 - FAN_MIN_PWM)) / 255 ) : 0)
     #else
       #define CALC_FAN_SPEED(f) tail_fan_speed[f]

Marlin/ultralcd_impl_HD44780.h

   #include <LCD.h>
   #include <LiquidCrystal_SR.h>
   #define LCD_CLASS LiquidCrystal_SR
-  #if defined(SR_STROBE_PIN)
+  #if PIN_EXISTS(SR_STROBE)
     LCD_CLASS lcd(SR_DATA_PIN, SR_CLK_PIN, SR_STROBE_PIN);
   #else
     LCD_CLASS lcd(SR_DATA_PIN, SR_CLK_PIN);