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Some small timing optimistations in the direction predictor, and also a new optional smaller configuration for the tournament predictor.

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Jonathan Woodruff
2025-01-21 10:08:31 +00:00
parent 6f30f2b703
commit 6ffde560b5
2 changed files with 35 additions and 13 deletions
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38
src_Core/RISCY_OOO/procs/lib/TourPred.bsv
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@@ -52,11 +52,17 @@ export mkTourPred;
export PCIndexSz;
export PCIndex;
// 4KB tournament predictor
typedef 12 TourGlobalHistSz;
typedef 10 TourLocalHistSz;
typedef 10 PCIndexSz;
`ifdef SMALL_BRANCH_PREDICTOR
// 512B tournament predictor
typedef 9 TourGlobalHistSz;
typedef 7 TourLocalHistSz;
typedef 7 PCIndexSz;
`else
// 4KB tournament predictor
typedef 12 TourGlobalHistSz;
typedef 10 TourLocalHistSz;
typedef 10 PCIndexSz;
`endif
typedef Bit#(TourGlobalHistSz) TourGlobalHist;
typedef Bit#(TourLocalHistSz) TourLocalHist;
@@ -94,6 +100,7 @@ module mkTourPred(DirPredictor#(TourTrainInfo));
// Lookup PC
Reg#(Addr) pc_reg <- mkRegU;
Reg#(Vector#(SupSize, PCIndex)) pcIndex <- mkRegU;
// EHR to record predict results in this cycle
Ehr#(TAdd#(1, SupSize), SupCnt) predCnt <- mkEhr(0);
@@ -109,6 +116,7 @@ module mkTourPred(DirPredictor#(TourTrainInfo));
boundedPlus(cnt, (taken) ? -1 : 1);
TourGlobalHist curGHist = gHistReg.history; // global history: MSB is the latest branch
Reg#(TourGlobalHist) prevGHist <- mkRegU;
Reg#(Vector#(SupSize, Bool)) globalTakenVec <- mkRegU;
Reg#(Vector#(SupSize, Bool)) useLocalVec <- mkRegU;
@@ -116,9 +124,8 @@ module mkTourPred(DirPredictor#(TourTrainInfo));
for(Integer i = 0; i < valueof(SupSize); i = i+1) begin
predIfc[i] = (interface DirPred;
method ActionValue#(DirPredResult#(TourTrainInfo)) pred;
PCIndex pcIndex = getPCIndex(offsetPc(pc_reg, i));
// get local history & prediction
TourLocalHist localHist = localHistTab.sub(pcIndex);
TourLocalHist localHist = localHistTab.sub(pcIndex[i]);
Bool localTaken = isTaken(localBht.sub(localHist));
// get the global history
@@ -126,6 +133,10 @@ module mkTourPred(DirPredictor#(TourTrainInfo));
// otherwise this branch should be on wrong path
// because all inst in same cycle are fetched consecutively
// get global prediction
// XXX This isn't true anymore...
// The current pipeline does not keep fetched bundles
// together at decode; you might get decoded next
// to an instruction from the next bundle.
Bool globalTaken = globalTakenVec[predCnt[i]];
// make choice
@@ -142,11 +153,11 @@ module mkTourPred(DirPredictor#(TourTrainInfo));
return DirPredResult {
taken: taken,
train: TourTrainInfo {
globalHist: curGHist >> predCnt[i],
globalHist: prevGHist >> predCnt[i],
localHist: localHist,
globalTaken: globalTaken,
localTaken: localTaken,
pcIndex: pcIndex
pcIndex: pcIndex[i]
}
};
endmethod
@@ -155,8 +166,8 @@ module mkTourPred(DirPredictor#(TourTrainInfo));
(* fire_when_enabled, no_implicit_conditions *)
rule canonGlobalHist;
// Update history for next cycle's lookup.
gHistReg.addHistory(predRes[valueof(SupSize)], predCnt[valueof(SupSize)]);
// Buffer useLocalVec
// Reproduce next history; this would ideally be done in GlobalBrHistReg to avoid duplicating logic.
TourGlobalHist nHist = truncate({predRes[valueof(SupSize)], curGHist} >> predCnt[valueof(SupSize)]);
function Bool globalTakenLookup (Integer i) = isTaken(globalBht.sub(nHist >> i));
@@ -166,9 +177,14 @@ module mkTourPred(DirPredictor#(TourTrainInfo));
// Reset counters and prediction.
predRes[valueof(SupSize)] <= 0;
predCnt[valueof(SupSize)] <= 0;
prevGHist <= nHist;
endrule
method nextPc = pc_reg._write;
method Action nextPc(Addr pc);
pc_reg <= pc;
function PCIndex genPcIndex(Integer i) = getPCIndex(offsetPc(pc, i));
pcIndex <= genWith(genPcIndex); // Call getPCIndex with 0-SupSize.
endmethod
interface pred = predIfc;

10
src_Core/RISCY_OOO/procs/lib/TourPredSecure.bsv
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@@ -50,11 +50,17 @@ export mkTourPredSecure;
// logic. It is guaranteed outside that no prediction or update can happen when
// flushing.
typedef 10 PCIndexSz;
`ifdef SMALL_BRANCH_PREDICTOR
typedef 7 PCIndexSz;
typedef 6 TabIndexSz;
`else
typedef 10 PCIndexSz;
typedef 9 TabIndexSz;
`endif
typedef Bit#(PCIndexSz) PCIndex;
// We group several sat counters/local hists together in order to flush faster
typedef 9 TabIndexSz;
typedef Bit#(TabIndexSz) TabIndex;
// vector of local hists