Mark functions as NORETURN

jepler · jepler · commit 55b0f157c7bb · 2025-04-16T16:57:09.000+02:00
Surprisingly this leads to some code size savings on itsybitsym4,
even though LTO optimization should have let the compiler deduce
(almost) all of this.

When a compiler knows a call can never return, it is possibly able
to avoid emitting code, such as saving caller-saved registers to the
stack, or doing _ANYTHING_ affter the function returns, explaining
why code size savings is possible with this attribute.

With the exception of raise_deinited_error, these were found by checking
the functions found by `-Wmissing-noreturn` and making a case by case
decision. However, this dignostic can't be enabled unconditionally as it
has false positives, functions we do NOT want to mark as noreturn.

Testing performed: built itsybitsy m4, flash size savings 128 bytes.
didn't test on hardware as none came on this trip with me.
diff --git a/ports/atmel-samd/common-hal/alarm/touch/TouchAlarm.c b/ports/atmel-samd/common-hal/alarm/touch/TouchAlarm.c
@@ -7,6 +7,6 @@
 #include "shared-bindings/alarm/touch/TouchAlarm.h"
 #include "shared-bindings/microcontroller/__init__.h"
 
-void common_hal_alarm_touch_touchalarm_construct(alarm_touch_touchalarm_obj_t *self, const mcu_pin_obj_t *pin) {
+NORETURN void common_hal_alarm_touch_touchalarm_construct(alarm_touch_touchalarm_obj_t *self, const mcu_pin_obj_t *pin) {
     mp_raise_NotImplementedError_varg(MP_ERROR_TEXT("%q"), MP_QSTR_TouchAlarm);
 }
diff --git a/ports/atmel-samd/supervisor/port.c b/ports/atmel-samd/supervisor/port.c
@@ -686,7 +686,7 @@ void port_idle_until_interrupt(void) {
 /**
  * \brief Default interrupt handler for unused IRQs.
  */
-__attribute__((used)) void HardFault_Handler(void) {
+__attribute__((used)) NORETURN void HardFault_Handler(void) {
     #ifdef ENABLE_MICRO_TRACE_BUFFER
     // Turn off the micro trace buffer so we don't fill it up in the infinite
     // loop below.
diff --git a/shared-bindings/alarm/time/TimeAlarm.c b/shared-bindings/alarm/time/TimeAlarm.c
@@ -14,7 +14,7 @@
 #include "shared-bindings/time/__init__.h"
 
 #if MICROPY_LONGINT_IMPL != MICROPY_LONGINT_IMPL_NONE
-mp_obj_t MP_WEAK rtc_get_time_source_time(void) {
+NORETURN mp_obj_t MP_WEAK rtc_get_time_source_time(void) {
     mp_raise_RuntimeError(MP_ERROR_TEXT("RTC is not supported on this board"));
 }
 #endif
diff --git a/shared-bindings/microcontroller/__init__.h b/shared-bindings/microcontroller/__init__.h
@@ -20,7 +20,7 @@ extern void common_hal_mcu_disable_interrupts(void);
 extern void common_hal_mcu_enable_interrupts(void);
 
 extern void common_hal_mcu_on_next_reset(mcu_runmode_t runmode);
-extern void common_hal_mcu_reset(void);
+NORETURN extern void common_hal_mcu_reset(void);
 
 extern const mp_obj_dict_t mcu_pin_globals;
 
diff --git a/shared-bindings/storage/__init__.h b/shared-bindings/storage/__init__.h
@@ -16,7 +16,7 @@ void common_hal_storage_umount_path(const char *path);
 void common_hal_storage_umount_object(mp_obj_t vfs_obj);
 void common_hal_storage_remount(const char *path, bool readonly, bool disable_concurrent_write_protection);
 mp_obj_t common_hal_storage_getmount(const char *path);
-void common_hal_storage_erase_filesystem(bool extended);
+NORETURN void common_hal_storage_erase_filesystem(bool extended);
 
 bool common_hal_storage_disable_usb_drive(void);
 bool common_hal_storage_enable_usb_drive(void);
diff --git a/shared-bindings/time/__init__.c b/shared-bindings/time/__init__.c
@@ -172,7 +172,7 @@ MP_DEFINE_CONST_FUN_OBJ_0(time_not_implemented_obj, time_not_implemented);
 #endif
 
 #if MICROPY_LONGINT_IMPL != MICROPY_LONGINT_IMPL_NONE
-mp_obj_t MP_WEAK rtc_get_time_source_time(void) {
+NORETURN mp_obj_t MP_WEAK rtc_get_time_source_time(void) {
     mp_raise_RuntimeError(MP_ERROR_TEXT("RTC is not supported on this board"));
 }
 
diff --git a/shared-bindings/util.h b/shared-bindings/util.h
@@ -9,6 +9,6 @@
 #include "py/mpprint.h"
 #include "py/runtime.h"
 
-void raise_deinited_error(void);
+NORETURN void raise_deinited_error(void);
 void properties_print_helper(const mp_print_t *print, mp_obj_t self_in, const mp_arg_t *properties, size_t n_properties);
 void properties_construct_helper(mp_obj_t self_in, const mp_arg_t *args, const mp_arg_val_t *vals, size_t n_properties);

Original file line number	Diff line number	Diff line change
`@@ -7,6 +7,6 @@`
`7`	`7`	`#include "shared-bindings/alarm/touch/TouchAlarm.h"`
`8`	`8`	`#include "shared-bindings/microcontroller/__init__.h"`
`9`	`9`
`10`		`-void common_hal_alarm_touch_touchalarm_construct(alarm_touch_touchalarm_obj_t self, const mcu_pin_obj_t pin) {`
	`10`	`+NORETURN void common_hal_alarm_touch_touchalarm_construct(alarm_touch_touchalarm_obj_t self, const mcu_pin_obj_t pin) {`
`11`	`11`	`mp_raise_NotImplementedError_varg(MP_ERROR_TEXT("%q"), MP_QSTR_TouchAlarm);`
`12`	`12`	`}`
Original file line number	Diff line number	Diff line change
`@@ -14,7 +14,7 @@`
`14`	`14`	`#include "shared-bindings/time/__init__.h"`
`15`	`15`
`16`	`16`	`#if MICROPY_LONGINT_IMPL != MICROPY_LONGINT_IMPL_NONE`
`17`		`-mp_obj_t MP_WEAK rtc_get_time_source_time(void) {`
	`17`	`+NORETURN mp_obj_t MP_WEAK rtc_get_time_source_time(void) {`
`18`	`18`	`mp_raise_RuntimeError(MP_ERROR_TEXT("RTC is not supported on this board"));`
`19`	`19`	`}`
`20`	`20`	`#endif`
Original file line number	Diff line number	Diff line change
`@@ -172,7 +172,7 @@ MP_DEFINE_CONST_FUN_OBJ_0(time_not_implemented_obj, time_not_implemented);`
`172`	`172`	`#endif`
`173`	`173`
`174`	`174`	`#if MICROPY_LONGINT_IMPL != MICROPY_LONGINT_IMPL_NONE`
`175`		`-mp_obj_t MP_WEAK rtc_get_time_source_time(void) {`
	`175`	`+NORETURN mp_obj_t MP_WEAK rtc_get_time_source_time(void) {`
`176`	`176`	`mp_raise_RuntimeError(MP_ERROR_TEXT("RTC is not supported on this board"));`
`177`	`177`	`}`
`178`	`178`