Exactly how “fast” are modern CPUs?

Question

When I used to program embedded systems and early 8/16-bit PCs (6502, 68K, 8086) I had a pretty good handle on exacly how long (in nanoseconds or microseconds) each instruct

Answer 1

The kind of prediction you're asking for is hopeless.

If you want a rule of thumb, here are some rules of thumb:

• In the time it takes to get a word from level 2 cache, a processor can execute at least 10 instructions. So worry about memory access, not instruction counts---computation in registers is almost free.

• In the time it takes to get a word from RAM, a processor can execute thousands of instructions (this number varies by a couple of order of magnitude depending on the details of your hardware). Make sure this happens only on a cold cache; otherwise nothing else matters.

• If you're running on x86 CPUs, there aren't enough registers. Try not to have more than 5 live variables in your code at any moment. Or better yet, move to AMD64 (x86_64) and double the number of registers. With 16 registers, and parameters passed in registers, you can quit worrying about registers.

There was a time when every year I would ask an architect what rules of thumb I should use to predict the cost of the code my compilers generate. I've stopped, because the last time I received a useful answer was in 1999. (The answer was "make sure your loops fit in the reorder buffer". All those who know what a reorder buffer is may now raise your hands. Bonus points if you can discover the size of the reorder buffer on any computer you are currently using.)