New intrinsic "unreachable()" to indicate unreachable code points

This allows better compiler optimisation, while also including an
ASSERT() for debug builds.

Refs #7

inc/intrinsics.h

@@ -27,6 +27,8 @@ struct exception_frame {
 #define likely(x)     __builtin_expect(!!(x),1)
 #define unlikely(x)   __builtin_expect(!!(x),0)
 
+#define unreachable() do { ASSERT(0); __builtin_unreachable(); } while (0)
+
 #define illegal() asm volatile (".short 0xde00");
 
 #define barrier() asm volatile ("" ::: "memory")

src/board.c

@@ -26,8 +26,7 @@ GPIO gpio_from_id(uint8_t id)
     case _I: return gpioi;
 #endif
     }
-    ASSERT(0);
-    return NULL;
+    unreachable();
 }
 
 uint8_t write_mapped_pin(

src/usb/hw_dwc_otg.c

@@ -410,9 +410,8 @@ static void handle_iepint(uint8_t epnr)
             handle_tx_ep0();
     }
 
-    if (iepint & OTG_DIEPINT_TXFE) {
-        ASSERT(0);
-    }
+    /* We don't set DIEPEMPMSK bits so TXFE notification is impossible. */
+    ASSERT(!(iepint & OTG_DIEPINT_TXFE));
 }
 
 static void dwc_otg_process(void)

src/usb/hw_f7.c

@@ -65,7 +65,7 @@ void hw_usb_init(void)
         }
         break;
     default:
-        ASSERT(0);
+        unreachable();
     }
 
     peripheral_clock_delay();
@@ -109,7 +109,7 @@ void hw_usb_deinit(void)
         rcc->ahb1enr &= ~RCC_AHB1ENR_OTGHSEN;
         break;
     default:
-        ASSERT(0);
+        unreachable();
     }
 }