From dad3d504b156a8240ee828a3dfb777fe1d6aca54 Mon Sep 17 00:00:00 2001
From: Jonathan Woodruff <jdw57@cam.ac.uk>
Date: Mon, 11 Oct 2021 16:23:45 +0000
Subject: [PATCH] Take into account BTB predictions. nextPc only reflected
 things known in the Decode stage, but not before (i.e. BTB predictions). This
 meant that we were pretty much always redirecting JRs. I needed to pipe
 through whether a jump was predicted by the BTB or not, and then only stop
 predicting if there is no nextPc prediction and also no BTB prediction.  This
 should actually reasonably approximate SBC. Also, this one is not
 "redirecting", it's just flushing (flipping decode_epoch_local), trusting
 that we'll get a later redirection.

---
 .../RISCY_OOO/procs/RV64G_OOO/FetchStage.bsv    | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/src_Core/RISCY_OOO/procs/RV64G_OOO/FetchStage.bsv b/src_Core/RISCY_OOO/procs/RV64G_OOO/FetchStage.bsv
index b2af746..d26dc78 100644
--- a/src_Core/RISCY_OOO/procs/RV64G_OOO/FetchStage.bsv
+++ b/src_Core/RISCY_OOO/procs/RV64G_OOO/FetchStage.bsv
@@ -224,6 +224,7 @@ typedef struct {
   Dii_Parcel_Id dii_pid;
 `endif
   PcCompressed ppc;
+  Bool pred_jump;
   Bool decode_epoch;
   Epoch main_epoch;
   Instruction inst;
@@ -244,6 +245,7 @@ function InstrFromFetch3 fetch3_2_instC(Fetch3ToDecode in, Instruction inst, Bit
       ppc: fromMaybe(PcCompressed{lsb: in.pc.lsb + 2,
                                   idx: in.pc.idx + ((in.pc.lsb == -2) ? 1:0)}, // If we move to a new page, we will move to the next index in the compressed PC table.
                      in.ppc),
+      pred_jump: isValid(in.ppc),
       decode_epoch: in.decode_epoch,
       main_epoch: in.main_epoch,
       inst: inst,
@@ -660,8 +662,8 @@ module mkFetchStage(FetchStage);
       for (Integer i = 0; i < valueof(SupSize); i=i+1) begin
          if (decodeIn[i] matches tagged Valid .in)  begin
             let cause = in.cause;
-            let pc = decompressPc(in.pc);
-            let ppc = decompressPc(in.ppc);
+            CapMem pc = decompressPc(in.pc);
+            CapMem ppc = decompressPc(in.ppc);
             pcBlocks.rPort[i].remove(in.pc.idx);
             if (verbose)
                $display("Decode: %0d in = ", i, fshow (in));
@@ -744,19 +746,20 @@ module mkFetchStage(FetchStage);
                            // same reg: push
                            ras.ras[i].popPush(False, Valid (push_addr));
                         end
-                     end else if (!isValid(nextPc)) begin
-                        // A Jr will jump; if we don't have a record, we should wait to prevent wasted work.
-                        nextPc = Valid (setAddrUnsafe(nullCap,{2'b01,?})); // Invalid virtual address to prevent progress.
                      end
                   end
-
                   if(verbose) begin
                      $display("Branch prediction: ", fshow(dInst.iType), " ; ", fshow(pc), " ; ",
                               fshow(ppc), " ; ", fshow(pred_taken), " ; ", fshow(nextPc));
                   end
 
+                  // If we don't have a good guess about where we are going, don't proceed.
+                  if ((!isValid(nextPc)) && (!in.pred_jump)) begin
+                     // Invalid virtual address to ensure redirection.
+                     ppc = setAddrUnsafe(nullCap, {2'b01,?});
+                     decode_epoch_local = !decode_epoch_local;
                   // check previous mispred
-                  if (nextPc matches tagged Valid .decode_pred_next_pc &&& (decode_pred_next_pc != ppc)) begin
+                  end if (nextPc matches tagged Valid .decode_pred_next_pc &&& (decode_pred_next_pc != ppc)) begin
                      if (verbose) $display("%x: ppc and decodeppc :  %h %h", pc, ppc, decode_pred_next_pc);
                      decode_epoch_local = !decode_epoch_local;
                      redirectPc = Valid (decode_pred_next_pc); // record redirect next pc