riscv/rocket-chip
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Guarantee one-hotness of BTB entries

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This commit is contained in:
Andrew Waterman 2016-07-06 15:54:33 -07:00
parent c0e6ecebfc
commit 35a983275e
1 changed files with 20 additions and 17 deletions
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37
rocket/src/main/scala/btb.scala
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@ -147,6 +147,7 @@ class BTB(implicit p: Parameters) extends BtbModule {
val tgts = Reg(Vec(entries, UInt(width=matchBits))) val tgts = Reg(Vec(entries, UInt(width=matchBits)))
val tgtPages = Reg(Vec(entries, UInt(width=log2Up(nPages)))) val tgtPages = Reg(Vec(entries, UInt(width=log2Up(nPages))))
val pages = Reg(Vec(nPages, UInt(width=vaddrBits-matchBits))) val pages = Reg(Vec(nPages, UInt(width=vaddrBits-matchBits)))
val pageValid = Reg(init = UInt(0, nPages))
val idxPagesOH = idxPages.map(UIntToOH(_)(nPages-1,0)) val idxPagesOH = idxPages.map(UIntToOH(_)(nPages-1,0))
val tgtPagesOH = tgtPages.map(UIntToOH(_)(nPages-1,0)) val tgtPagesOH = tgtPages.map(UIntToOH(_)(nPages-1,0))
@ -157,12 +158,12 @@ class BTB(implicit p: Parameters) extends BtbModule {
private def page(addr: UInt) = addr >> matchBits private def page(addr: UInt) = addr >> matchBits
private def pageMatch(addr: UInt) = { private def pageMatch(addr: UInt) = {
val p = page(addr) val p = page(addr)
pages.map(_ === p).toBits pageValid & pages.map(_ === p).toBits
} }
private def tagMatch(addr: UInt, pgMatch: UInt) = { private def tagMatch(addr: UInt, pgMatch: UInt) = {
val idxMatch = idxs.map(_ === addr(matchBits-1,0)) val idxMatch = idxs.map(_ === addr(matchBits-1,0)).toBits
val idxPageMatch = idxPagesOH.map(_ & pgMatch).map(_.orR) val idxPageMatch = idxPagesOH.map(_ & pgMatch).map(_.orR).toBits
(idxPageMatch zip idxMatch) map { case (p, i) => p && i } idxMatch & idxPageMatch
} }
val r_btb_update = Pipe(io.btb_update) val r_btb_update = Pipe(io.btb_update)
@ -172,16 +173,20 @@ class BTB(implicit p: Parameters) extends BtbModule {
val hitsVec = tagMatch(io.req.bits.addr, pageHit) val hitsVec = tagMatch(io.req.bits.addr, pageHit)
val hits = hitsVec.toBits val hits = hitsVec.toBits
val updatePageHit = pageMatch(r_btb_update.bits.pc) val updatePageHit = pageMatch(r_btb_update.bits.pc)
val updateHits = tagMatch(r_btb_update.bits.pc, updatePageHit)
val updateHit = r_btb_update.bits.prediction.valid val updateHits = tagMatch(r_btb_update.bits.pc, updatePageHit)
val nextRepl = Reg(UInt(width = log2Ceil(entries))) val updateHit = if (updatesOutOfOrder) updateHits.orR else r_btb_update.bits.prediction.valid
when (r_btb_update.valid && !updateHit) { nextRepl := Mux(nextRepl === entries-1 && Bool(!isPow2(entries)), 0, nextRepl + 1) } val updateHitAddr = if (updatesOutOfOrder) OHToUInt(updateHits) else r_btb_update.bits.prediction.bits.entry
val nextPageRepl = Reg(UInt(width = log2Ceil(nPages)))
// 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 useUpdatePageHit = updatePageHit.orR val useUpdatePageHit = updatePageHit.orR
val usePageHit = pageHit.orR val usePageHit = pageHit.orR
val doIdxPageRepl = !useUpdatePageHit val doIdxPageRepl = !useUpdatePageHit
val nextPageRepl = Reg(UInt(width = log2Ceil(nPages)))
val idxPageRepl = Mux(usePageHit, Cat(pageHit(nPages-2,0), pageHit(nPages-1)), UIntToOH(nextPageRepl)) val idxPageRepl = Mux(usePageHit, Cat(pageHit(nPages-2,0), pageHit(nPages-1)), UIntToOH(nextPageRepl))
val idxPageUpdateOH = Mux(useUpdatePageHit, updatePageHit, idxPageRepl) val idxPageUpdateOH = Mux(useUpdatePageHit, updatePageHit, idxPageRepl)
val idxPageUpdate = OHToUInt(idxPageUpdateOH) val idxPageUpdate = OHToUInt(idxPageUpdateOH)
@ -199,18 +204,15 @@ class BTB(implicit p: Parameters) extends BtbModule {
nextPageRepl := Mux(next >= nPages, next(0), next) nextPageRepl := Mux(next >= nPages, next(0), next)
} }
when (r_btb_update.valid) { when (r_btb_update.valid || resetting) {
assert(io.req.bits.addr === r_btb_update.bits.target, "BTB request != I$ target") assert(resetting || io.req.bits.addr === r_btb_update.bits.target, "BTB request != I$ target")
val waddr = val waddr = Mux(updateHit && !resetting, updateHitAddr, nextRepl)
if (updatesOutOfOrder) Mux(updateHits.reduce(_|_), OHToUInt(updateHits), nextRepl) val mask = UIntToOH(waddr)
else Mux(updateHit, r_btb_update.bits.prediction.bits.entry, nextRepl) idxs(waddr) := Mux(resetting, Cat(r_btb_update.bits.pc >> log2Ceil(entries), nextRepl), r_btb_update.bits.pc)
idxs(waddr) := r_btb_update.bits.pc
tgts(waddr) := update_target tgts(waddr) := update_target
idxPages(waddr) := idxPageUpdate idxPages(waddr) := idxPageUpdate
tgtPages(waddr) := tgtPageUpdate tgtPages(waddr) := tgtPageUpdate
val mask = UIntToOH(waddr)
useRAS := Mux(r_btb_update.bits.isReturn, useRAS | mask, useRAS & ~mask) useRAS := Mux(r_btb_update.bits.isReturn, useRAS | mask, useRAS & ~mask)
isJump := Mux(r_btb_update.bits.isJump, isJump | mask, isJump & ~mask) isJump := Mux(r_btb_update.bits.isJump, isJump | mask, isJump & ~mask)
if (fetchWidth > 1) if (fetchWidth > 1)
@ -227,6 +229,7 @@ class BTB(implicit p: Parameters) extends BtbModule {
Mux(idxWritesEven, page(r_btb_update.bits.pc), page(update_target))) Mux(idxWritesEven, page(r_btb_update.bits.pc), page(update_target)))
writeBank(1, 2, Mux(idxWritesEven, tgtPageReplEn, idxPageReplEn), writeBank(1, 2, Mux(idxWritesEven, tgtPageReplEn, idxPageReplEn),
Mux(idxWritesEven, page(update_target), page(r_btb_update.bits.pc))) Mux(idxWritesEven, page(update_target), page(r_btb_update.bits.pc)))
pageValid := pageValid | tgtPageReplEn | idxPageReplEn
} }
io.resp.valid := hits.orR io.resp.valid := hits.orR