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fix fpu underflow rounding (#343)

This commit is contained in:
Dolu1990 2023-05-16 16:50:38 +01:00
parent ba6dcb1789
commit b81029f619
1 changed files with 11 additions and 4 deletions
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15
src/main/scala/vexriscv/ip/fpu/FpuCore.scala
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@ -1547,15 +1547,22 @@ case class FpuCore( portCount : Int, p : FpuParameter) extends Component{
val expBase = muxDouble[UInt](input.format)(exponentF64Subnormal + 1)(exponentF32Subnormal + 1) val expBase = muxDouble[UInt](input.format)(exponentF64Subnormal + 1)(exponentF32Subnormal + 1)
val expDif = expBase -^ input.value.exponent val expDif = expBase -^ input.value.exponent
val expSubnormal = !input.value.special && !expDif.msb val expSubnormal = !input.value.special && !expDif.msb
var discardCount = (expSubnormal ? expDif.resize(log2Up(p.internalMantissaSize) bits) | U(0)) var discardCount = (expSubnormal ? expDif | U(0))
if (p.withDouble) when(input.format === FpuFormat.FLOAT) { if (p.withDouble) when(input.format === FpuFormat.FLOAT) {
discardCount \= discardCount + 29 discardCount \= discardCount + 29
} }
val exactMask = (List(True) ++ (0 until p.internalMantissaSize + 1).map(_ < discardCount)).asBits.asUInt val discardCountTrunk = discardCount.resize(log2Up(p.internalMantissaSize) bits)
val roundAdjusted = (True ## (manAggregate >> 1)) (discardCount) ## ((manAggregate & exactMask) =/= 0) val exactMask = (List(True) ++ (0 until p.internalMantissaSize + 1).map(_ < discardCountTrunk)).asBits.asUInt
val roundAdjusted = (True ## (manAggregate >> 1)) (discardCountTrunk) ## ((manAggregate & exactMask) =/= 0)
val rneBit = CombInit((U"01" ## (manAggregate >> 2))(discardCountTrunk))
when(discardCount >= widthOf(manAggregate)){
rneBit := False
roundAdjusted(1) := False
exactMask := exactMask.maxValue
}
val mantissaIncrement = !input.value.special && input.roundMode.mux( val mantissaIncrement = !input.value.special && input.roundMode.mux(
FpuRoundMode.RNE -> (roundAdjusted(1) && (roundAdjusted(0) || (U"01" ## (manAggregate >> 2)) (discardCount))), FpuRoundMode.RNE -> (roundAdjusted(1) && (roundAdjusted(0) || rneBit)),
FpuRoundMode.RTZ -> False, FpuRoundMode.RTZ -> False,
FpuRoundMode.RDN -> (roundAdjusted =/= 0 && input.value.sign), FpuRoundMode.RDN -> (roundAdjusted =/= 0 && input.value.sign),
FpuRoundMode.RUP -> (roundAdjusted =/= 0 && !input.value.sign), FpuRoundMode.RUP -> (roundAdjusted =/= 0 && !input.value.sign),