Plru

Introduction

The Pseudo Least Recently Used algorithm can be used for example in cache to select efficiently a line for eviction.

It is used for example in the VexiiRiscV core or in spinal.lib.bus.tilelink.coherent.Cache.

io.context.state need to be handled externally.
When you want to specify a access to a entry, you can use the io.update interface to get the new state value.
plru.io.evict.id tells you the id of the next block to be evicted
plru.io.update.id lets you update what you recently used

PLRU Code:

 val io = new Bundle {
  val context = new Bundle {
    // user -> plru, specify the current state
    val state = Plru.State(entries)
    // user -> plru, allow to specify preferred entries to remove. Each bit
    // set mean : "i would prefer that way to not to be selected by PLRU"
    val valids = withEntriesValid generate Bits(entries bits)
  }
  val evict = new Bundle {
    // PLRU -> user, Tells you the least recently used entry for the
    // given context provided above
    val id =  UInt(log2Up(entries) bits)
  }
  val update = new Bundle {
    // user -> PLRU specify which entry the user want to mark as
    // most recently used
    val id = UInt(log2Up(entries) bits)
    // PLRU -> user specfy what should then be the new value of the PLRU status
    val state = Plru.State(entries)
  }
}

Example usage in a cache:

val plru = new Area {
   // Define a Mem, to track the state of each set
   val ram = Mem.fill(nSets)(Plru.State(wayCount))
   val write = ram.writePort
   val fromLoad, fromStore = cloneOf(write)
   write.valid := fromLoad.valid || fromStore.valid
   write.payload := fromLoad.valid.mux(fromLoad.payload, fromStore.payload)
}

Get the ID of the way to evict from:

val replacedWay = plru.io.evict.id

Update recently used way:

plru.update.id := refillWay