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Simplify timing optimisation by including in the root of the original

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loop.
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Jonathan Woodruff
2022-01-27 17:13:11 +00:00
parent 4cfd652b0d
commit 1ee961bb83
1 changed files with 13 additions and 36 deletions
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49
src_Core/RISCY_OOO/procs/RV64G_OOO/FetchStage.bsv
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@@ -235,14 +235,6 @@ typedef struct {
Bool mispred_first_half;
} InstrFromFetch3 deriving(Bits, Eq, FShow);
typedef struct {
CapMem pc;
CapMem ppc;
DecodeResult result;
DirPredResult#(DirPredTrainInfo) dir_pred;
Maybe#(CapMem) dir_ppc;
} PreDecode deriving(Bits, Eq, FShow);
function InstrFromFetch3 fetch3_2_instC(Fetch3ToDecode in, Instruction inst, Bit#(32) orig_inst) =
InstrFromFetch3 {
pc: in.pc,
@@ -670,40 +662,25 @@ module mkFetchStage(FetchStage);
// Note that only 1 redirection may happen in a cycle
Maybe#(IType) redirectInst = Invalid;
`endif
// A loop to prepare values for the main decode loop.
// These allow us to perform direction prediction with minimal dependencies
// between instructions to avoid a critical path.
function t valid (Maybe#(t) v) = v.Valid;
Vector#(SupSize, PreDecode) pd = ?;
Bool fetch_branch_misprediction = False;
Bool likely_epoch_change = False;
for (Integer i = 0; i < valueof(SupSize); i=i+1) begin
pd[i].pc = decompressPc(valid(decodeIn[i]).pc);
pd[i].ppc = decompressPc(valid(decodeIn[i]).ppc);
pd[i].result = decode(valid(decodeIn[i]).inst, getFlags(pd[i].pc)==1);
// Estimate when we won't use later instructions in the bundle to avoid
// poluting the global history. Depending on the full "local_epoch" logic
// in the main loop is too slow timing-wise.
if(pd[i].result.dInst.iType == Br && !fetch_branch_misprediction) begin
pd[i].dir_pred <- dirPred.pred[i].pred;
fetch_branch_misprediction = (pd[i].dir_pred.taken != valid(decodeIn[i]).pred_jump);
CapMem pc = decompressPc(validValue(decodeIn[i]).pc);
CapMem ppc = decompressPc(validValue(decodeIn[i]).ppc);
let decode_result = decode(validValue(decodeIn[i]).inst, getFlags(pc)==1); // Decode 32b inst, or 32b expansion of 16b inst
let dInst = decode_result.dInst;
let regs = decode_result.regs;
DirPredResult#(DirPredTrainInfo) dir_pred = DirPredResult{taken: False, train: ?};
if(decode_result.dInst.iType == Br && !likely_epoch_change) begin
dir_pred <- dirPred.pred[i].pred;
likely_epoch_change = (dir_pred.taken != validValue(decodeIn[i]).pred_jump);
end
pd[i].dir_ppc = decodeBrPred(pd[i].pc, pd[i].result.dInst, pd[i].dir_pred.taken, (valid(decodeIn[i]).inst_kind == Inst_32b));
end
for (Integer i = 0; i < valueof(SupSize); i=i+1) begin
Maybe#(CapMem) dir_ppc = decodeBrPred(pc, decode_result.dInst, dir_pred.taken, (validValue(decodeIn[i]).inst_kind == Inst_32b));
if (decodeIn[i] matches tagged Valid .in) begin
let cause = in.cause;
PreDecode p = pd[i];
CapMem pc = p.pc;
CapMem ppc = p.ppc;
pcBlocks.rPort[i].remove(in.pc.idx);
if (verbose)
$display("Decode: %0d in = ", i, fshow (in));
let decode_result = p.result; // Decode 32b inst, or 32b expansion of 16b inst
let dInst = decode_result.dInst;
let regs = decode_result.regs;
// do decode and branch prediction
// Drop here if does not match the decode_epoch.
if (in.decode_epoch == decode_epoch_local && in.mispred_first_half) begin
@@ -728,7 +705,7 @@ module mkFetchStage(FetchStage);
// update predicted next pc
if (!isValid(cause)) begin
// direction predict
Maybe#(CapMem) nextPc = p.dir_ppc;
Maybe#(CapMem) nextPc = dir_ppc;
// return address stack link reg is x1 or x5
function Bool linkedR(Maybe#(ArchRIndx) register);
Bool res = False;
@@ -807,7 +784,7 @@ module mkFetchStage(FetchStage);
`endif
ppc: ppc,
main_epoch: in.main_epoch,
dpTrain: p.dir_pred.train,
dpTrain: dir_pred.train,
inst: in.inst,
dInst: dInst,
orig_inst: in.orig_inst,