riscv/rocket-chip
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Significant changes and fixes to BTB for superscalar fetch.

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- 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.
This commit is contained in:
Christopher Celio 2014-11-11 03:34:05 -08:00
parent 3be3cd7731
commit fea31d2167
4 changed files with 63 additions and 45 deletions
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69
rocket/src/main/scala/btb.scala
View File

@ -65,36 +65,40 @@ class BHT(nbht: Int) {
when (update) { history := Cat(taken, history(nbhtbits-1,1)) }
res
}
def update(addr: UInt, d: BHTResp, taken: Bool, mispredict: Bool): Unit = {
def update(addr: UInt, d: BHTResp, taken: Bool): Unit = {
val index = addr(nbhtbits+1,2) ^ d.history
table(index) := Cat(taken, (d.value(1) & d.value(0)) | ((d.value(1) | d.value(0)) & taken))
when (mispredict) { history := Cat(taken, d.history(nbhtbits-1,1)) }
history := Cat(taken, d.history(nbhtbits-1,1))
}
private val table = Mem(UInt(width = 2), nbht)
val history = Reg(UInt(width = nbhtbits))
}
// BTB update occurs during branch resolution.
// BTB update occurs during branch resolution (and only on a mispredict).
// - "pc" is what future fetch PCs will tag match against.
// - "br_pc" is the PC of the branch instruction.
// - "bridx" is the low-order PC bits of the predicted branch (after
// shifting off the lowest log(inst_bytes) bits off).
// - "resp.mask" provides a mask of valid instructions (instructions are
// masked off by the predicted taken branch).
class BTBUpdate extends Bundle with BTBParameters {
val prediction = Valid(new BTBResp)
val pc = UInt(width = vaddrBits)
val target = UInt(width = vaddrBits)
val returnAddr = UInt(width = vaddrBits)
val taken = Bool()
val isJump = Bool()
val isCall = Bool()
val isReturn = Bool()
val br_pc = UInt(width = vaddrBits)
val mispredict = Bool()
}
class RASUpdate extends Bundle with BTBParameters {
val isCall = Bool()
val isReturn = Bool()
val returnAddr = UInt(width = vaddrBits)
val prediction = Valid(new BTBResp)
}
// - "bridx" is the low-order PC bits of the predicted branch (after
// shifting off the lowest log(inst_bytes) bits off).
// - "resp.mask" provides a mask of valid instructions (instructions are
// masked off by the predicted taken branch).
class BTBResp extends Bundle with BTBParameters {
val taken = Bool()
val mask = Bits(width = params(FetchWidth))
@ -109,11 +113,15 @@ class BTBReq extends Bundle with BTBParameters {
}
// fully-associative branch target buffer
class BTB extends Module with BTBParameters {
// Higher-performance processors may cause BTB updates to occur out-of-order,
// which requires an extra CAM port for updates (to ensure no duplicates get
// placed in BTB).
class BTB(updates_out_of_order: Boolean = false) extends Module with BTBParameters {
val io = new Bundle {
val req = Valid(new BTBReq).flip
val resp = Valid(new BTBResp)
val update = Valid(new BTBUpdate).flip
val ras_update = Valid(new RASUpdate).flip
val invalidate = Bool(INPUT)
}
@ -158,8 +166,7 @@ class BTB extends Module with BTBParameters {
}
val updateHit = r_update.bits.prediction.valid
val updateValid = r_update.bits.mispredict || updateHit && Bool(nBHT > 0)
val updateTarget = updateValid && r_update.bits.mispredict && r_update.bits.taken
val updateTarget = r_update.bits.taken
val useUpdatePageHit = updatePageHit.orR
val doIdxPageRepl = updateTarget && !useUpdatePageHit
@ -179,16 +186,22 @@ class BTB extends Module with BTBParameters {
val pageReplEn = idxPageReplEn | tgtPageReplEn
idxPageRepl := UIntToOH(Counter(r_update.valid && doPageRepl, nPages)._1)
when (r_update.valid && !(updateValid && !updateTarget)) {
val nextRepl = Counter(!updateHit && updateValid, entries)._1
val waddr = Mux(updateHit, r_update.bits.prediction.bits.entry, nextRepl)
when (r_update.valid && updateTarget) {
assert(io.req.bits.addr === r_update.bits.target, "BTB request != I$ target")
val nextRepl = Counter(!updateHit, entries)._1
var waddr:UInt = null
if (!updates_out_of_order) {
waddr = Mux(updateHit, r_update.bits.prediction.bits.entry, nextRepl)
} else {
println(" BTB accepts out-of-order updates.")
waddr = Mux(updateHits.orR, OHToUInt(updateHits), nextRepl)
}
// invalidate entries if we stomp on pages they depend upon
idxValid := idxValid & ~Vec.tabulate(entries)(i => (pageReplEn & (idxPagesOH(i) | tgtPagesOH(i))).orR).toBits
idxValid(waddr) := updateValid
when (updateTarget) {
assert(io.req.bits.addr === r_update.bits.target, "BTB request != I$ target")
idxValid(waddr) := Bool(true)
idxs(waddr) := r_update.bits.pc
tgts(waddr) := update_target
idxPages(waddr) := idxPageUpdate
@ -200,7 +213,6 @@ class BTB extends Module with BTBParameters {
} else {
brIdx(waddr) := r_update.bits.br_pc >> log2Up(params(CoreInstBits)/8)
}
}
require(nPages % 2 == 0)
val idxWritesEven = (idxPageUpdateOH & Fill(nPages/2, UInt(1,2))).orR
@ -231,7 +243,9 @@ class BTB extends Module with BTBParameters {
io.resp.bits.mask := UInt(1)
} else {
// note: btb_resp is clock gated, so the mask is only relevant for the io.resp.valid case
io.resp.bits.mask := Cat((UInt(1) << brIdx(io.resp.bits.entry))-1, UInt(1))
val all_ones = UInt((1 << (params(FetchWidth)+1))-1)
io.resp.bits.mask := Mux(io.resp.bits.taken, Cat((UInt(1) << brIdx(io.resp.bits.entry))-1, UInt(1)),
all_ones)
}
if (nBHT > 0) {
@ -239,8 +253,7 @@ class BTB extends Module with BTBParameters {
val res = bht.get(io.req.bits.addr, io.req.valid && hits.orR && !Mux1H(hits, isJump))
val update_btb_hit = io.update.bits.prediction.valid
when (io.update.valid && update_btb_hit && !io.update.bits.isJump) {
bht.update(io.update.bits.pc, io.update.bits.prediction.bits.bht,
io.update.bits.taken, io.update.bits.mispredict)
bht.update(io.update.bits.pc, io.update.bits.prediction.bits.bht, io.update.bits.taken)
}
when (!res.value(0) && !Mux1H(hits, isJump)) { io.resp.bits.taken := false }
io.resp.bits.bht := res
@ -252,13 +265,13 @@ class BTB extends Module with BTBParameters {
when (!ras.isEmpty && doPeek) {
io.resp.bits.target := ras.peek
}
when (io.update.valid) {
when (io.update.bits.isCall) {
ras.push(io.update.bits.returnAddr)
when (io.ras_update.valid) {
when (io.ras_update.bits.isCall) {
ras.push(io.ras_update.bits.returnAddr)
when (doPeek) {
io.resp.bits.target := io.update.bits.returnAddr
io.resp.bits.target := io.ras_update.bits.returnAddr
}
}.elsewhen (io.update.bits.isReturn && io.update.bits.prediction.valid) {
}.elsewhen (io.ras_update.bits.isReturn && io.ras_update.bits.prediction.valid) {
ras.pop
}
}

9
rocket/src/main/scala/ctrl.scala
View File

@ -652,14 +652,17 @@ class Control extends Module
Mux(replay_wb, PC_WB, // replay
PC_MEM)))
io.imem.btb_update.valid := (mem_reg_branch || io.imem.btb_update.bits.isJump) && !take_pc_wb
io.imem.btb_update.valid := take_pc_mem && !take_pc_wb
io.imem.btb_update.bits.prediction.valid := mem_reg_btb_hit
io.imem.btb_update.bits.prediction.bits := mem_reg_btb_resp
io.imem.btb_update.bits.taken := mem_reg_branch && io.dpath.mem_br_taken || io.imem.btb_update.bits.isJump
io.imem.btb_update.bits.mispredict := take_pc_mem
io.imem.btb_update.bits.isJump := mem_reg_jal || mem_reg_jalr
io.imem.btb_update.bits.isCall := mem_reg_wen && io.dpath.mem_waddr(0)
io.imem.btb_update.bits.isReturn := mem_reg_jalr && io.dpath.mem_rs1_ra
io.imem.ras_update.valid := io.imem.btb_update.bits.isJump && !take_pc_wb
io.imem.ras_update.bits.isCall := mem_reg_wen && io.dpath.mem_waddr(0)
io.imem.ras_update.bits.isReturn := mem_reg_jalr && io.dpath.mem_rs1_ra
io.imem.ras_update.bits.prediction.valid := mem_reg_btb_hit
io.imem.ras_update.bits.prediction.bits := mem_reg_btb_resp
io.imem.req.valid := take_pc
val bypassDst = Array(id_raddr1, id_raddr2)

2
rocket/src/main/scala/dpath.scala
View File

@ -286,8 +286,8 @@ class Datapath extends Module
wb_reg_pc)).toUInt // PC_WB
io.imem.btb_update.bits.pc := mem_reg_pc
io.imem.btb_update.bits.target := io.imem.req.bits.pc
io.imem.btb_update.bits.returnAddr := mem_int_wdata
io.imem.btb_update.bits.br_pc := mem_reg_pc
io.imem.ras_update.bits.returnAddr := mem_int_wdata
// for hazard/bypass opportunity detection
io.ctrl.ex_waddr := ex_reg_inst(11,7)

6
rocket/src/main/scala/icache.scala
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@ -33,18 +33,19 @@ class CPUFrontendIO extends CoreBundle {
val resp = Decoupled(new FrontendResp).flip
val btb_resp = Valid(new BTBResp).flip
val btb_update = Valid(new BTBUpdate)
val ras_update = Valid(new RASUpdate)
val ptw = new TLBPTWIO().flip
val invalidate = Bool(OUTPUT)
}
class Frontend extends FrontendModule
class Frontend(btb_updates_out_of_order: Boolean = false) extends FrontendModule
{
val io = new Bundle {
val cpu = new CPUFrontendIO().flip
val mem = new UncachedTileLinkIO
}
val btb = Module(new BTB)
val btb = Module(new BTB(btb_updates_out_of_order))
val icache = Module(new ICache)
val tlb = Module(new TLB(params(NITLBEntries)))
@ -88,6 +89,7 @@ class Frontend extends FrontendModule
btb.io.req.valid := !stall && !icmiss
btb.io.req.bits.addr := s1_pc & SInt(-coreInstBytes)
btb.io.update := io.cpu.btb_update
btb.io.ras_update := io.cpu.ras_update
btb.io.invalidate := io.cpu.invalidate || io.cpu.ptw.invalidate
tlb.io.ptw <> io.cpu.ptw