Intrinsics for Math.min and max

[Martin Grajcar]

On Wed, Apr 2, 2014 at 2:55 PM, Vitaly Davidovich <vitalyd at gmail.com> wrote:

> I think hotspot profiling records cumulative probability of a branch taken
> vs not taken - it doesn't record the pattern, right? This basically means
> that the best hotspot can do is determine if a jump is very likely taken or
> not, rather than being able to answer whether something is highly
> unpredictable.  You can have a branch with 50/50 chance of being taken or
> not, but the pattern is just as important for hardware branch prediction.
> For example, a loop runs for 100 iterations with first 50 taking the jump
> and last 50 not; branch predictor should do fine here even though frequency
> as recorded by hotspot will simply say 50/50 (correct me if I'm wrong
> though).
>
I'm afraid you're right.

> So it really seems that hotspot can only use profiling to determine very
> high probability of branches taken, but not able to say much else for other
> cases because it doesn't model prediction the same way as cpu does (nor
> should it).
>
Maybe  it should, but I'm afraid it's just too complicated and possibly not
publicly known. I guess hotspot could recognize some simple patterns, which
are sure to be recognized by the CPU as well.

Maybe considering the condition's origin might be a good
heuristic: Conditions dependent on array content are probably less
predictable than those depending let's say on the counter only.

> Given that, seems like code gen should prefer to emit jumps almost all the
> time.
>
I must disagree here. Even a 50% branch can be perfectly predictable, but
in case it isn't you lose too much. Just extrapolate the slowdown from the
first image on
http://stackoverflow.com/questions/19689214/strange-branching-performance to
50%. Not using cmov would lead to a slowdown factor 8.

> There's the other aspect to this which is that if hotspot emits jumps and
> profiling shows high branch misprediction, developer can possibly change
> their code to either remove branches or make them more predictable.  If
> cmov is emitted though, then there's nothing dev can do (unless you guys
> modify intrinsic for math/min to use profile info).
>
> Am I missing something in my reasoning?
>
I'm afraid there's no good way to remove branches in Java. In general
you'll get something notably slower than cmov.
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