topic Re: Stop KDS optimising out NOP / NOPs in Kinetis Design Studio

Stop KDS optimising out NOP / NOPs

beng_ — Tue, 06 Sep 2016 11:11:59 GMT

This is another newbie question (I've previously used IAR).

For debuggin purposes in a bit of test code I want some nops to single step through

If In insert multiple nops e.g.

__NOP();
__NOP();
__NOP();

they seem to be optimised out. How can I stop this.

I've selected the following settings but they don't appear to have any effect on keeping the NOPs

Project/Properties/C/C++ Build/Tool Settings/Optimization/Optimization Level = None (-O0)

Project/Properties/C/C++ Build/Tool Settings/Debugging/Debug Level = Maximum (-g3)

Re: Stop KDS optimising out NOP / NOPs

bobpaddock — Tue, 06 Sep 2016 12:06:45 GMT

Such instructions must be marked asVolatile to prevent the compiler from removing it.

Compiler sees that it has no side effects so removes it as unnecessary.

For GCC:

#define ATTR_NO_INSTRUMENT_FUNCTION __attribute__( ( no_instrument_function ) )
/*
* NOP is not necessarily a time-consuming NOP. The processor might
* remove it from the pipeline before it reaches the execution stage.
* Use in combination with the Sync Barrier Instruction to consume time.
*/
static inline ATTR_NO_INSTRUMENT_FUNCTION void nop( void )
{
__asm__ __volatile__ ("nop");
}

Then use 'nop();' in your code.

The pipeline comment depends on what part you are using.

A Sync Barrier Instruction may consume thousands of cycles.

Re: Stop KDS optimising out NOP / NOPs

beng_ — Mon, 12 Sep 2016 14:55:24 GMT

Thanks