How to get the Index of Max element in UInt Vec , Chisel

Question

I'm trying to get the index of the Max element in a UInt vector. My code looks like this

val pwr = Vec.tabulate(N) {i => energyMeters(i).io.pwr}
val maxPwr = pwr.indexOf(pwr.max)

However this code generate compilation error:

No implicit Ordering Defined for Chisel.UInt.
val maxPwr = pwr.indexOf(pwr.max)
                             ^     

I understand that I probably need to implement the max function , can someone give an example how this should be done ?

Edit: I also tried this:

val pwr = Vec.tabulate(N) {i => energyMeters(i).io.pwr}
val maxPwr = pwr reduceLeft {(x,y) => Mux(x > y,x,y)}
val maxPwridx = pwr.indexOf(maxPwr)

But it fails on elaboration , when I tried to cast maxPwridx to UInt. I've ended up with this workaround:

val pwr = Vec.tabulate(N) {i => energyMeters(i).io.pwr}
val maxPwr = pwr reduceLeft {(x,y) => Mux(x > y,x,y)}
val maxPwridx = pwr.indexWhere((x : UInt => x === maxPwr))

Answer 1

Chisel's Vec extends Scala's Seq. This means that a Vec has both dynamic access hardware methods that will allow you to generate hardware to search for something in a Vec (e.g., indexWhere, onlyIndexWhere, lastIndexWhere) as well as all the methods available to normal Scala sequences (e.g., indexOf).

For the purposes of doing hardware operations, you want to use the former (as you found in your last edit---which looks great!) as opposed to the latter.

To get some handle on this, the screenshot below shows the Chisel 3.3.0-RC1 API documentation for VecLike, filtered to excluded inherited methods. Notable here are indexWhere, onlyIndexWhere, lastIndexWhere, exists, forall, and contains:

And the documentation for Vec. The only interesting method here would be reduceTree: