LoadQueue: opt writeback select timing

We used to select writeback inst and update writebacked bit
at the same cycle. However, it is too long to finish in one cycle.

Now we select writeback inst and gen wbSelectedMask in cycle 1,
then we use RegNext(wbSelectedMask) and writeback bit to select
inst in the next cycle.

src/main/scala/xiangshan/mem/lsqueue/LoadQueue.scala

@@ -294,8 +294,12 @@ class LoadQueue extends XSModule
 
   // Stage 0
   // Generate writeback indexes
+  val wbSelectedMask = RegInit(VecInit(Seq.fill(LoadPipelineWidth)(0.U(LoadQueueSize.W))))
+  val selected = (0 until LoadPipelineWidth).map(i => {
+    wbSelectedMask(i).asUInt
+  }).reduce(_ | _)
   val loadWbSelVec = VecInit((0 until LoadQueueSize).map(i => {
-    allocated(i) && !writebacked(i) && datavalid(i)
+    allocated(i) && !writebacked(i) && datavalid(i) && !selected(i)
   })).asUInt() // use uint instead vec to reduce verilog lines
   val loadEvenSelVec = VecInit((0 until LoadQueueSize/2).map(i => {loadWbSelVec(2*i)}))
   val loadOddSelVec = VecInit((0 until LoadQueueSize/2).map(i => {loadWbSelVec(2*i+1)}))
@@ -314,18 +318,19 @@ class LoadQueue extends XSModule
   (0 until LoadPipelineWidth).map(i => {
     val canGo = io.ldout(i).fire() || !loadWbSelV(i)
     val valid = loadWbSelVGen(i)
-    // store selected index in pipeline reg
     loadWbSel(i) := RegEnable(loadWbSelGen(i), valid && canGo)
-    // Mark them as writebacked, so they will not be selected in the next cycle
-    when(valid && canGo){
-      writebacked(loadWbSelGen(i)) := true.B
-    }
-    // update loadWbSelValidReg
     when(io.ldout(i).fire()){
+      // Mark them as writebacked, so they will not be selected in the next cycle
+      writebacked(loadWbSel(i)) := true.B
+      // update loadWbSelValidReg
       loadWbSelV(i) := false.B
+      // store selected index in pipeline reg
+      wbSelectedMask(i) := 0.U
     }
     when(valid && canGo){
       loadWbSelV(i) := true.B
+      // store selected index in pipeline reg
+      wbSelectedMask(i) := UIntToOH(loadWbSelGen(i))
     }
   })