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rocket-chip/rocket/src/main/scala/dpath_alu.scala
Andrew Waterman 52d6b0b1a5 Improve ALU QoR
Rejigger muxes; share XOR gates between ADD/SUB, XOR, and BEQ.
2016-01-20 17:42:31 -08:00

97 lines
2.8 KiB
Scala

// See LICENSE for license details.
package rocket
import Chisel._
import cde.{Parameters, Field}
import Instructions._
object ALU
{
val SZ_ALU_FN = 4
val FN_X = BitPat("b????")
val FN_ADD = UInt(0)
val FN_SL = UInt(1)
val FN_SEQ = UInt(2)
val FN_SNE = UInt(3)
val FN_XOR = UInt(4)
val FN_SR = UInt(5)
val FN_OR = UInt(6)
val FN_AND = UInt(7)
val FN_SUB = UInt(10)
val FN_SRA = UInt(11)
val FN_SLT = UInt(12)
val FN_SGE = UInt(13)
val FN_SLTU = UInt(14)
val FN_SGEU = UInt(15)
val FN_DIV = FN_XOR
val FN_DIVU = FN_SR
val FN_REM = FN_OR
val FN_REMU = FN_AND
val FN_MUL = FN_ADD
val FN_MULH = FN_SL
val FN_MULHSU = FN_SLT
val FN_MULHU = FN_SLTU
def isMulFN(fn: UInt, cmp: UInt) = fn(1,0) === cmp(1,0)
def isSub(cmd: UInt) = cmd(3)
def isCmp(cmd: UInt) = cmd === FN_SEQ || cmd === FN_SNE || cmd >= FN_SLT
def cmpUnsigned(cmd: UInt) = cmd(1)
def cmpInverted(cmd: UInt) = cmd(0)
def cmpEq(cmd: UInt) = !cmd(3)
}
import ALU._
class ALU(implicit p: Parameters) extends CoreModule()(p) {
val io = new Bundle {
val dw = Bits(INPUT, SZ_DW)
val fn = Bits(INPUT, SZ_ALU_FN)
val in2 = UInt(INPUT, xLen)
val in1 = UInt(INPUT, xLen)
val out = UInt(OUTPUT, xLen)
val adder_out = UInt(OUTPUT, xLen)
val cmp_out = Bool(OUTPUT)
}
// ADD, SUB
val in2_inv = Mux(isSub(io.fn), ~io.in2, io.in2)
val in1_xor_in2 = io.in1 ^ in2_inv
io.adder_out := io.in1 + in2_inv + isSub(io.fn)
// SLT, SLTU
io.cmp_out := cmpInverted(io.fn) ^
Mux(cmpEq(io.fn), in1_xor_in2 === UInt(0),
Mux(io.in1(xLen-1) === io.in2(xLen-1), io.adder_out(xLen-1),
Mux(cmpUnsigned(io.fn), io.in2(xLen-1), io.in1(xLen-1))))
// SLL, SRL, SRA
val (shamt, shin_r) =
if (xLen == 32) (io.in2(4,0), io.in1)
else {
require(xLen == 64)
val shin_hi_32 = Fill(32, isSub(io.fn) && io.in1(31))
val shin_hi = Mux(io.dw === DW_64, io.in1(63,32), shin_hi_32)
val shamt = Cat(io.in2(5) & (io.dw === DW_64), io.in2(4,0))
(shamt, Cat(shin_hi, io.in1(31,0)))
}
val shin = Mux(io.fn === FN_SR || io.fn === FN_SRA, shin_r, Reverse(shin_r))
val shout_r = (Cat(isSub(io.fn) & shin(xLen-1), shin).toSInt >> shamt)(xLen-1,0)
val shout_l = Reverse(shout_r)
val shout = Mux(io.fn === FN_SR || io.fn === FN_SRA, shout_r, UInt(0)) |
Mux(io.fn === FN_SL, shout_l, UInt(0))
// AND, OR, XOR
val logic = Mux(io.fn === FN_XOR || io.fn === FN_OR, in1_xor_in2, UInt(0)) |
Mux(io.fn === FN_OR || io.fn === FN_AND, io.in1 & io.in2, UInt(0))
val shift_logic = (isCmp(io.fn) && io.cmp_out) | logic | shout
val out = Mux(io.fn === FN_ADD || io.fn === FN_SUB, io.adder_out, shift_logic)
io.out := out
if (xLen > 32) {
require(xLen == 64)
when (io.dw === DW_32) { io.out := Cat(Fill(32, out(31)), out(31,0)) }
}
}