Sorry, if this has been asked - but I couldn't find an answer. If I have a chunk of assembly code - how can I tell how many CPU clocks it takes to get through it?


Thanks.
Eric.

use RDTSC instruction

Don't know what operating system you use, but note that RDTSC can be privileged instruction on some operating systems. I've read somewhere that it is privileged on some Linux systems. As for win32, I'm not aware of any version which makes it privileged though.

Got any code example commeted enough to be understandable to begineers? I think that this should be extremely useful for code optimization performance increase results.

Making it privileged would be pretty silly, but it's technically possible.

Note that if you time with RDTSC, you'll run into trouble on AMD64 dualcore machines if you don't limit thread affinity to one CPU. Results also tend to vary a fair amount, boosting thread+process priority and looping the code a fair amount of times helps.

Also keep in mind that for machines with power-saving features, you need to "warm up the CPU" before you start counting.

So ideally, you should study min,max,average,median runtimes of the code snippet, not just "total_cycles / total_iterations".

http://en.wikipedia.org/wiki/RDTSC

Issues:

- Need to detect CPU before executing

- Multitasking / Multi-CPU (This problem can be very well bypassed using
DOS - only 1 CPU active, only 1 task )