riscv/rocket-chip
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Always verify BTB result; don't bother flushing it

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This improves CPI for things like

    lbu t0, (t0)
    j foo
    addi t0, t0, 1

where the addi would stall, causing j's misprediction check to fail,
flushing the pipeline.
This commit is contained in:
Andrew Waterman
2016-06-22 13:47:15 -07:00
parent 4c31248917
commit 7f88a00a38
2 changed files with 3 additions and 3 deletions
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4
rocket/src/main/scala/rocket.scala
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@ -343,13 +343,13 @@ class Rocket(implicit p: Parameters) extends CoreModule()(p) {
Mux(mem_ctrl.jal, ImmGen(IMM_UJ, mem_reg_inst), SInt(4)))
val mem_int_wdata = Mux(mem_ctrl.jalr, mem_br_target, mem_reg_wdata.toSInt).toUInt
val mem_npc = (Mux(mem_ctrl.jalr, encodeVirtualAddress(mem_reg_wdata, mem_reg_wdata).toSInt, mem_br_target) & SInt(-2)).toUInt
val mem_wrong_npc = mem_npc =/= ex_reg_pc || !ex_reg_valid
val mem_wrong_npc = Mux(ex_reg_valid, mem_npc =/= ex_reg_pc, Mux(io.imem.resp.valid, mem_npc =/= id_pc, Bool(true)))
val mem_npc_misaligned = mem_npc(1)
val mem_cfi = mem_ctrl.branch || mem_ctrl.jalr || mem_ctrl.jal
val mem_cfi_taken = (mem_ctrl.branch && mem_br_taken) || mem_ctrl.jalr || mem_ctrl.jal
val mem_misprediction =
if (p(BtbKey).nEntries == 0) mem_cfi_taken
else mem_cfi && mem_wrong_npc
else mem_wrong_npc
val want_take_pc_mem = mem_reg_valid && (mem_misprediction || mem_reg_flush_pipe)
take_pc_mem := want_take_pc_mem && !mem_npc_misaligned