ConditionProfile vs. BranchProfile

[Chris Seaton]

If you were to write

a = new BranchProfile();
b = new BranchProfile();

if (c) {
    a.enter();
    …
} else {
    b.enter();
    …
}

Then that’s almost the same thing as a ConditionProfile. So that’s useful on its own - it’s less code and less data than using two BranchProfiles.

As you say, ConditionProfile also introduces the branch counts and reorders the branches based on which is more likely. This works with the hardware branch predictor, because in most processors forward jumps are predicted not taken (or the other way around, I can’t remember but it’s in the Intel manual).

You ask if the hardware would predict this correctly anyway. Well the prediction cache is of course of limited size so it could run out. I would admit you may struggle to empirically prove that reordering branches like this makes a big difference on benchmarks. Maybe give it a go if you’re curious enough and tell us about the results?

Chris

> On 5 Jul 2016, at 13:58, Raffaello Giulietti <raffaello.giulietti at supsi.ch> wrote:
> 
> Hello,
> 
> while I think I understand the usefulness of Truffle's BranchProfile, I
> don't get what the point is with ConditionProfile.
> 
> I mean, except when the condition profile indicates a zero count on
> either the "then" or "else" parts, what is the real value of having such
> a profile? The only reason I can figure out is something related to code
> layout. I thought, however, that such optimizations were already
> addressed by the hardware branch prediction mechanics. Granted, the
> latter needs not necessarily be implemented on every CPU out there.
> 
> Any explanation is appreciated and welcome.
> 
> Greetings
> Raffaello