Re: String comparison for fixed strings

[Per Jessen <per@computer.org>]

Glynn Clements wrote:

> In particular, branch instructions are dealt with by dedicated logic
> circuitry which does nothing but process branch instructions. This
> enables speculative execution to work handle branches even when the
> calculation of the branch condition hasn't completed.

Yep, I understand all that - it doesn't even have to be a particularly
modern CPU, even IBMs S390 architecture did this in the early '90s.
Speculative execution and branch prediction etc. have both been around
for a while, just not in the Intel world.

Regardless, I still wouldn't say "branches don't normally take any CPU
cycles", but maybe that's splitting hairs.

> The actual cost of a code cache miss varies depending upon the
> relative speed of the CPU and RAM, but 400 cycles is typical. You
> would need to have a lot of additional instructions before their cost
> outweighs that of a cache miss.

Very true, but given the size of L1 cache these days, you also have a
lot more leeway than you did e.g. in the 90s.  A typical memcmp() for
short strings is unrolled by default by gcc.  (at least I'm fairly
certain it does that).


/Per