Significant changes and fixes to BTB for superscalar fetch.
- BTBUpdate only occurs on mispredicts now. - RASUpdate broken out from BTBUpdate (allows RASUpdate to be performed in Decode). - Added optional 2nd CAM port to BTB for updates (for when updates to the BTB may occur out-of-order). - Fixed resp.mask bit logic.
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3be3cd7731
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@ -65,36 +65,40 @@ class BHT(nbht: Int) {
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when (update) { history := Cat(taken, history(nbhtbits-1,1)) }
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res
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}
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def update(addr: UInt, d: BHTResp, taken: Bool, mispredict: Bool): Unit = {
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def update(addr: UInt, d: BHTResp, taken: Bool): Unit = {
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val index = addr(nbhtbits+1,2) ^ d.history
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table(index) := Cat(taken, (d.value(1) & d.value(0)) | ((d.value(1) | d.value(0)) & taken))
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when (mispredict) { history := Cat(taken, d.history(nbhtbits-1,1)) }
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history := Cat(taken, d.history(nbhtbits-1,1))
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}
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private val table = Mem(UInt(width = 2), nbht)
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val history = Reg(UInt(width = nbhtbits))
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}
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// BTB update occurs during branch resolution.
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// BTB update occurs during branch resolution (and only on a mispredict).
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// - "pc" is what future fetch PCs will tag match against.
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// - "br_pc" is the PC of the branch instruction.
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// - "bridx" is the low-order PC bits of the predicted branch (after
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// shifting off the lowest log(inst_bytes) bits off).
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// - "resp.mask" provides a mask of valid instructions (instructions are
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// masked off by the predicted taken branch).
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class BTBUpdate extends Bundle with BTBParameters {
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val prediction = Valid(new BTBResp)
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val pc = UInt(width = vaddrBits)
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val target = UInt(width = vaddrBits)
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val returnAddr = UInt(width = vaddrBits)
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val taken = Bool()
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val isJump = Bool()
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val isCall = Bool()
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val isReturn = Bool()
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val br_pc = UInt(width = vaddrBits)
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val mispredict = Bool()
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}
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class RASUpdate extends Bundle with BTBParameters {
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val isCall = Bool()
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val isReturn = Bool()
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val returnAddr = UInt(width = vaddrBits)
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val prediction = Valid(new BTBResp)
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}
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// - "bridx" is the low-order PC bits of the predicted branch (after
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// shifting off the lowest log(inst_bytes) bits off).
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// - "resp.mask" provides a mask of valid instructions (instructions are
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// masked off by the predicted taken branch).
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class BTBResp extends Bundle with BTBParameters {
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val taken = Bool()
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val mask = Bits(width = params(FetchWidth))
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@ -109,11 +113,15 @@ class BTBReq extends Bundle with BTBParameters {
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}
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// fully-associative branch target buffer
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class BTB extends Module with BTBParameters {
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// Higher-performance processors may cause BTB updates to occur out-of-order,
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// which requires an extra CAM port for updates (to ensure no duplicates get
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// placed in BTB).
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class BTB(updates_out_of_order: Boolean = false) extends Module with BTBParameters {
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val io = new Bundle {
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val req = Valid(new BTBReq).flip
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val resp = Valid(new BTBResp)
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val update = Valid(new BTBUpdate).flip
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val ras_update = Valid(new RASUpdate).flip
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val invalidate = Bool(INPUT)
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}
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@ -158,8 +166,7 @@ class BTB extends Module with BTBParameters {
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}
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val updateHit = r_update.bits.prediction.valid
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val updateValid = r_update.bits.mispredict || updateHit && Bool(nBHT > 0)
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val updateTarget = updateValid && r_update.bits.mispredict && r_update.bits.taken
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val updateTarget = r_update.bits.taken
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val useUpdatePageHit = updatePageHit.orR
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val doIdxPageRepl = updateTarget && !useUpdatePageHit
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@ -179,27 +186,32 @@ class BTB extends Module with BTBParameters {
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val pageReplEn = idxPageReplEn | tgtPageReplEn
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idxPageRepl := UIntToOH(Counter(r_update.valid && doPageRepl, nPages)._1)
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when (r_update.valid && !(updateValid && !updateTarget)) {
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val nextRepl = Counter(!updateHit && updateValid, entries)._1
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val waddr = Mux(updateHit, r_update.bits.prediction.bits.entry, nextRepl)
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when (r_update.valid && updateTarget) {
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assert(io.req.bits.addr === r_update.bits.target, "BTB request != I$ target")
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val nextRepl = Counter(!updateHit, entries)._1
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var waddr:UInt = null
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if (!updates_out_of_order) {
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waddr = Mux(updateHit, r_update.bits.prediction.bits.entry, nextRepl)
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} else {
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println(" BTB accepts out-of-order updates.")
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waddr = Mux(updateHits.orR, OHToUInt(updateHits), nextRepl)
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}
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// invalidate entries if we stomp on pages they depend upon
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idxValid := idxValid & ~Vec.tabulate(entries)(i => (pageReplEn & (idxPagesOH(i) | tgtPagesOH(i))).orR).toBits
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idxValid(waddr) := updateValid
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when (updateTarget) {
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assert(io.req.bits.addr === r_update.bits.target, "BTB request != I$ target")
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idxs(waddr) := r_update.bits.pc
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tgts(waddr) := update_target
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idxPages(waddr) := idxPageUpdate
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tgtPages(waddr) := tgtPageUpdate
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useRAS(waddr) := r_update.bits.isReturn
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isJump(waddr) := r_update.bits.isJump
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if (params(FetchWidth) == 1) {
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brIdx(waddr) := UInt(0)
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} else {
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brIdx(waddr) := r_update.bits.br_pc >> log2Up(params(CoreInstBits)/8)
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}
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idxValid(waddr) := Bool(true)
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idxs(waddr) := r_update.bits.pc
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tgts(waddr) := update_target
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idxPages(waddr) := idxPageUpdate
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tgtPages(waddr) := tgtPageUpdate
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useRAS(waddr) := r_update.bits.isReturn
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isJump(waddr) := r_update.bits.isJump
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if (params(FetchWidth) == 1) {
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brIdx(waddr) := UInt(0)
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} else {
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brIdx(waddr) := r_update.bits.br_pc >> log2Up(params(CoreInstBits)/8)
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}
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require(nPages % 2 == 0)
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@ -231,7 +243,9 @@ class BTB extends Module with BTBParameters {
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io.resp.bits.mask := UInt(1)
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} else {
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// note: btb_resp is clock gated, so the mask is only relevant for the io.resp.valid case
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io.resp.bits.mask := Cat((UInt(1) << brIdx(io.resp.bits.entry))-1, UInt(1))
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val all_ones = UInt((1 << (params(FetchWidth)+1))-1)
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io.resp.bits.mask := Mux(io.resp.bits.taken, Cat((UInt(1) << brIdx(io.resp.bits.entry))-1, UInt(1)),
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all_ones)
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}
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if (nBHT > 0) {
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@ -239,8 +253,7 @@ class BTB extends Module with BTBParameters {
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val res = bht.get(io.req.bits.addr, io.req.valid && hits.orR && !Mux1H(hits, isJump))
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val update_btb_hit = io.update.bits.prediction.valid
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when (io.update.valid && update_btb_hit && !io.update.bits.isJump) {
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bht.update(io.update.bits.pc, io.update.bits.prediction.bits.bht,
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io.update.bits.taken, io.update.bits.mispredict)
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bht.update(io.update.bits.pc, io.update.bits.prediction.bits.bht, io.update.bits.taken)
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}
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when (!res.value(0) && !Mux1H(hits, isJump)) { io.resp.bits.taken := false }
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io.resp.bits.bht := res
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@ -252,13 +265,13 @@ class BTB extends Module with BTBParameters {
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when (!ras.isEmpty && doPeek) {
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io.resp.bits.target := ras.peek
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}
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when (io.update.valid) {
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when (io.update.bits.isCall) {
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ras.push(io.update.bits.returnAddr)
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when (io.ras_update.valid) {
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when (io.ras_update.bits.isCall) {
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ras.push(io.ras_update.bits.returnAddr)
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when (doPeek) {
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io.resp.bits.target := io.update.bits.returnAddr
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io.resp.bits.target := io.ras_update.bits.returnAddr
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}
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}.elsewhen (io.update.bits.isReturn && io.update.bits.prediction.valid) {
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}.elsewhen (io.ras_update.bits.isReturn && io.ras_update.bits.prediction.valid) {
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ras.pop
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}
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}
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@ -652,15 +652,18 @@ class Control extends Module
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Mux(replay_wb, PC_WB, // replay
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PC_MEM)))
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io.imem.btb_update.valid := (mem_reg_branch || io.imem.btb_update.bits.isJump) && !take_pc_wb
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io.imem.btb_update.valid := take_pc_mem && !take_pc_wb
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io.imem.btb_update.bits.prediction.valid := mem_reg_btb_hit
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io.imem.btb_update.bits.prediction.bits := mem_reg_btb_resp
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io.imem.btb_update.bits.taken := mem_reg_branch && io.dpath.mem_br_taken || io.imem.btb_update.bits.isJump
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io.imem.btb_update.bits.mispredict := take_pc_mem
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io.imem.btb_update.bits.isJump := mem_reg_jal || mem_reg_jalr
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io.imem.btb_update.bits.isCall := mem_reg_wen && io.dpath.mem_waddr(0)
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io.imem.btb_update.bits.isReturn := mem_reg_jalr && io.dpath.mem_rs1_ra
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io.imem.req.valid := take_pc
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io.imem.ras_update.valid := io.imem.btb_update.bits.isJump && !take_pc_wb
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io.imem.ras_update.bits.isCall := mem_reg_wen && io.dpath.mem_waddr(0)
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io.imem.ras_update.bits.isReturn := mem_reg_jalr && io.dpath.mem_rs1_ra
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io.imem.ras_update.bits.prediction.valid := mem_reg_btb_hit
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io.imem.ras_update.bits.prediction.bits := mem_reg_btb_resp
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io.imem.req.valid := take_pc
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val bypassDst = Array(id_raddr1, id_raddr2)
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val bypassSrc = Array.fill(NBYP)((Bool(true), UInt(0)))
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@ -286,8 +286,8 @@ class Datapath extends Module
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wb_reg_pc)).toUInt // PC_WB
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io.imem.btb_update.bits.pc := mem_reg_pc
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io.imem.btb_update.bits.target := io.imem.req.bits.pc
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io.imem.btb_update.bits.returnAddr := mem_int_wdata
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io.imem.btb_update.bits.br_pc := mem_reg_pc
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io.imem.ras_update.bits.returnAddr := mem_int_wdata
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// for hazard/bypass opportunity detection
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io.ctrl.ex_waddr := ex_reg_inst(11,7)
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@ -33,18 +33,19 @@ class CPUFrontendIO extends CoreBundle {
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val resp = Decoupled(new FrontendResp).flip
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val btb_resp = Valid(new BTBResp).flip
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val btb_update = Valid(new BTBUpdate)
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val ras_update = Valid(new RASUpdate)
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val ptw = new TLBPTWIO().flip
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val invalidate = Bool(OUTPUT)
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}
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class Frontend extends FrontendModule
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class Frontend(btb_updates_out_of_order: Boolean = false) extends FrontendModule
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{
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val io = new Bundle {
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val cpu = new CPUFrontendIO().flip
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val mem = new UncachedTileLinkIO
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}
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val btb = Module(new BTB)
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val btb = Module(new BTB(btb_updates_out_of_order))
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val icache = Module(new ICache)
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val tlb = Module(new TLB(params(NITLBEntries)))
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@ -88,6 +89,7 @@ class Frontend extends FrontendModule
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btb.io.req.valid := !stall && !icmiss
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btb.io.req.bits.addr := s1_pc & SInt(-coreInstBytes)
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btb.io.update := io.cpu.btb_update
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btb.io.ras_update := io.cpu.ras_update
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btb.io.invalidate := io.cpu.invalidate || io.cpu.ptw.invalidate
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tlb.io.ptw <> io.cpu.ptw
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