riscv/rocket-chip
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fix bug in BTB

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a BTB update followed by a taken branch could cause incorrect control flow.
This commit is contained in:
Andrew Waterman 2012-02-15 21:36:08 -08:00
parent fc5ba769da
commit 1b5e39e7fc
1 changed files with 8 additions and 6 deletions
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14
rocket/src/main/scala/dpath_util.scala
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@ -23,22 +23,22 @@ class rocketDpathBTB(entries: Int) extends Component
{ {
val io = new ioDpathBTB(); val io = new ioDpathBTB();
val do_update = io.wen || io.clr
val expected_tag = Mux(do_update, io.correct_pc, io.current_pc)
val repl_way = LFSR16(io.wen)(log2up(entries)-1,0) // TODO: pseudo-LRU val repl_way = LFSR16(io.wen)(log2up(entries)-1,0) // TODO: pseudo-LRU
var hit_reduction = Bool(false) var hit_reduction = Bool(false)
val hit = Wire() { Bool() } val hit = Wire() { Bool() }
val update = Wire() { Bool() }
var update_reduction = Bool(false)
val mux = (new Mux1H(entries)) { Bits(width = VADDR_BITS) } val mux = (new Mux1H(entries)) { Bits(width = VADDR_BITS) }
for (i <- 0 until entries) { for (i <- 0 until entries) {
val tag = Reg() { UFix() } val tag = Reg() { UFix() }
val target = Reg() { UFix() } val target = Reg() { UFix() }
val valid = Reg(resetVal = Bool(false)) val valid = Reg(resetVal = Bool(false))
val my_hit = valid && tag === expected_tag val my_hit = valid && tag === io.current_pc
val my_clr = io.clr && my_hit || io.invalidate val my_update = valid && tag === io.correct_pc
val my_wen = io.wen && (my_hit || !hit && UFix(i) === repl_way) val my_clr = io.clr && my_update || io.invalidate
val my_wen = io.wen && (my_update || !update && UFix(i) === repl_way)
valid := !my_clr && (valid || my_wen) valid := !my_clr && (valid || my_wen)
when (my_wen) { when (my_wen) {
@ -47,10 +47,12 @@ class rocketDpathBTB(entries: Int) extends Component
} }
hit_reduction = hit_reduction || my_hit hit_reduction = hit_reduction || my_hit
update_reduction = update_reduction || my_update
mux.io.sel(i) := my_hit mux.io.sel(i) := my_hit
mux.io.in(i) := target mux.io.in(i) := target
} }
hit := hit_reduction hit := hit_reduction
update := update_reduction
io.hit := hit io.hit := hit
io.target := mux.io.out.toUFix io.target := mux.io.out.toUFix