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Take into account BTB predictions.

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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.
This commit is contained in:
Jonathan Woodruff
2021-10-11 16:23:45 +00:00
parent 3636f6239a
commit dad3d504b1
1 changed files with 10 additions and 7 deletions
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17
src_Core/RISCY_OOO/procs/RV64G_OOO/FetchStage.bsv
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@@ -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