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[rocket] Allow zapping of BTB entries 

This is necessary to guarantee forward progress with RVC, since if the
BTB keeps mispredicting, the processor might never successfully
fetch both halves of a misaligned instruction.
This commit is contained in:
Andrew Waterman 2016-07-29 15:01:05 -07:00
parent 8e0392f24b
commit 0d3d9fca25
1 changed files with 12 additions and 16 deletions
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28
rocket/src/main/scala/btb.scala
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@ -91,6 +91,7 @@ class BTBUpdate(implicit p: Parameters) extends BtbBundle()(p) {
val pc = UInt(width = vaddrBits)
val target = UInt(width = vaddrBits)
val taken = Bool()
val isValid = Bool()
val isJump = Bool()
val isReturn = Bool()
val br_pc = UInt(width = vaddrBits)
@ -151,7 +152,8 @@ class BTB(implicit p: Parameters) extends BtbModule {
val idxPagesOH = idxPages.map(UIntToOH(_)(nPages-1,0))
val tgtPagesOH = tgtPages.map(UIntToOH(_)(nPages-1,0))
val useRAS = Reg(UInt(width = entries))
val isValid = Reg(init = UInt(0, entries))
val isReturn = Reg(UInt(width = entries))
val isJump = Reg(UInt(width = entries))
val brIdx = Reg(Vec(entries, UInt(width=log2Up(fetchWidth))))
@ -163,7 +165,7 @@ class BTB(implicit p: Parameters) extends BtbModule {
private def tagMatch(addr: UInt, pgMatch: UInt) = {
val idxMatch = idxs.map(_ === addr(matchBits-1, log2Up(coreInstBytes))).toBits
val idxPageMatch = idxPagesOH.map(_ & pgMatch).map(_.orR).toBits
idxMatch & idxPageMatch
idxMatch & idxPageMatch & isValid
}
val r_btb_update = Pipe(io.btb_update)
@ -177,11 +179,7 @@ class BTB(implicit p: Parameters) extends BtbModule {
val updateHits = tagMatch(r_btb_update.bits.pc, updatePageHit)
val updateHit = if (updatesOutOfOrder) updateHits.orR else r_btb_update.bits.prediction.valid
val updateHitAddr = if (updatesOutOfOrder) OHToUInt(updateHits) else r_btb_update.bits.prediction.bits.entry
// guarantee one-hotness of idx after reset
val resetting = Reg(init = Bool(true))
val (nextRepl, wrap) = Counter(resetting || (r_btb_update.valid && !updateHit), entries)
when (wrap) { resetting := false }
val nextRepl = Counter(r_btb_update.valid && !updateHit, entries)._1
val useUpdatePageHit = updatePageHit.orR
val usePageHit = pageHit.orR
@ -204,17 +202,15 @@ class BTB(implicit p: Parameters) extends BtbModule {
nextPageRepl := Mux(next >= nPages, next(0), next)
}
when (r_btb_update.valid || resetting) {
assert(resetting || io.req.bits.addr === r_btb_update.bits.target, "BTB request != I$ target")
val waddr = Mux(updateHit && !resetting, updateHitAddr, nextRepl)
when (r_btb_update.valid) {
val waddr = Mux(updateHit, updateHitAddr, nextRepl)
val mask = UIntToOH(waddr)
val newIdx = r_btb_update.bits.pc(matchBits-1, log2Up(coreInstBytes))
idxs(waddr) := Mux(resetting, Cat(newIdx >> log2Ceil(entries), nextRepl), newIdx)
idxs(waddr) := r_btb_update.bits.pc(matchBits-1, log2Up(coreInstBytes))
tgts(waddr) := update_target(matchBits-1, log2Up(coreInstBytes))
idxPages(waddr) := idxPageUpdate
tgtPages(waddr) := tgtPageUpdate
useRAS := Mux(r_btb_update.bits.isReturn, useRAS | mask, useRAS & ~mask)
isValid := Mux(r_btb_update.bits.isValid, isValid | mask, isValid & ~mask)
isReturn := Mux(r_btb_update.bits.isReturn, isReturn | mask, isReturn & ~mask)
isJump := Mux(r_btb_update.bits.isJump, isJump | mask, isJump & ~mask)
if (fetchWidth > 1)
brIdx(waddr) := r_btb_update.bits.br_pc >> log2Up(coreInstBytes)
@ -234,7 +230,7 @@ class BTB(implicit p: Parameters) extends BtbModule {
}
io.resp.valid := hits.orR
io.resp.bits.taken := io.resp.valid
io.resp.bits.taken := true
io.resp.bits.target := Cat(Mux1H(Mux1H(hitsVec, tgtPagesOH), pages), Mux1H(hitsVec, tgts) << log2Up(coreInstBytes))
io.resp.bits.entry := OHToUInt(hits)
io.resp.bits.bridx := Mux1H(hitsVec, brIdx)
@ -254,7 +250,7 @@ class BTB(implicit p: Parameters) extends BtbModule {
if (nRAS > 0) {
val ras = new RAS(nRAS)
val doPeek = (hits & useRAS).orR
val doPeek = (hits & isReturn).orR
when (!ras.isEmpty && doPeek) {
io.resp.bits.target := ras.peek
}