reduced latency

bs/TagEngine.bs

@@ -23,8 +23,10 @@ mkTagEngine =
     do
         let reifiedNumTags = fromInteger |> valueOf numTags
 
+        -- we really only use inUseVec after emptying out our `resetTagCounter` buffer
+        -- perhaps rename this variable to better communicate that
         inUseVec :: Vector numTags (Reg Bool)
-        inUseVec <- replicateM |> mkReg False
+        inUseVec <- replicateM |> mkReg True
 
         -- Since Bluespec doesn't allow us to initialize FIFOs with values at
         -- reset, we can pretend as if the buffer within our tagFIFO is populated
@@ -32,11 +34,12 @@ mkTagEngine =
         -- by having our tag engine effectively return the value of a decrementing
         -- counter initialized to (numTags - 1) for the first n tag requests made 
         -- to TagEngine where `n := numTags`.
+        -- Maybe resetTagCounter isn't the greatest name?
         resetTagCounter :: (Reg (Maybe UIntLog2N(numTags)))
         resetTagCounter <- mkReg |> Just |> reifiedNumTags - 1
 
-        retireQueue ::  FIFOF UIntLog2N(numTags)
-        retireQueue <- mkSizedFIFOF reifiedNumTags
+        retireTag ::  Wire UIntLog2N(numTags)
+        retireTag <- mkWire
 
         freeQueue ::  FIFOF UIntLog2N(numTags)
         freeQueue <- mkSizedFIFOF reifiedNumTags
@@ -56,11 +59,9 @@ mkTagEngine =
                 --    2. the `requestTag` method handles the case where there are some tags to retire.
                 "retire_tags": when True ==>
                     do
-                        let tag = retireQueue.first
-                        $display "firing retire_tags" (fshow tag)
-                        retireQueue.deq
-                        freeQueue.enq tag
-                        (select inUseVec tag) := False
+                        $display "firing retire_tags" (fshow retireTag)
+                        freeQueue.enq retireTag
+                        (select inUseVec retireTag) := False
 
         return $
           interface TagEngine
@@ -71,11 +72,9 @@ mkTagEngine =
                     case resetTagCounter of
                         Just 0 -> do
                             resetTagCounter := Nothing
-                            (select inUseVec 0) := True
                             return 0
                         Just tag -> do
                             resetTagCounter := Just |> tag - 1
-                            (select inUseVec tag) := True
                             return tag
                         Nothing -> do
                             let tag = freeQueue.first
@@ -86,14 +85,18 @@ mkTagEngine =
             -- so it is advisable that the caller of `retireTag` only attempt to retire valid tags.
             -- Internally, the tagEngine will keep a correct and consistent state since TagEngine
             -- validates tags before attempting to retire them.
+            -- Also worth noting the TagEngin will ignore attempts to retire a tag you just 
+            -- requested in the same cycle. This is enforce with `tag > tagCounter` below.
             retireTag :: UIntLog2N(numTags) -> Action
             retireTag tag =
                 do
                     let
                         tagValid = tag < reifiedNumTags
-                        tagInUse = readReg (select inUseVec tag)
+                        tagInUse = case resetTagCounter of
+                            Just tagCounter -> tag > tagCounter
+                            Nothing -> readReg (select inUseVec tag)
                     if (tagValid && tagInUse)
                         then do
-                            retireQueue.enq tag
+                            retireTag := tag
                         else do
                             action {}