I am porting some code from an M3 to an M4 which uses 3 NOPs to provide a very short delay between serial output clock changes. The M3 instruction set defines the time for a NOP as 1 cycle. I notice that NOPs in the M4 do not necessarily delay any time at all. I am aware that I will need to disable compiler optimisation but I'm looking for a low level command that will give me reliable, repeatable times. In practice in this particular case the serial is used very occasionally and could be very slow but I'd still like to know the best way to obtain cycle level delays.
If you need such very short, but deterministic "at least" delays, maybe you could consider using other instructions than nop which have deterministic nonzero latency.
The Cortex-M4 NOP as described is not necessarily time consuming.
You could replace it to, say and reg, reg, or something coarsely equivalent to a nop in the context. Alternatively, when toggling GPIO, you could also repeat the I/O instructions themselves to enforce the minimal length of a state (such as if your GPIO writing instruction takes at least 5ns, repeat it five times to get at least 25ns). This could even work well within C if you were inserting nops in a C program (just repeat the writes to the port, if it's volatile as it should be, the compiler wouldn't remove the repeated accesses).
Of course this only applies to very short delays, otherwise for short delays, like mentioned by others, busy loops waiting for some timing source would work much better (they take at least the clocks required to sample the timing source, set up the target, and go through once the wait loop).