8000 Branchless algorithm for RuntimeLong.toDouble. by sjrd · Pull Request #5204 · scala-js/scala-js · GitHub
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Branchless algorithm for RuntimeLong.toDouble. #5204

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Branchless algorithm for RuntimeLong.toDouble. #5204

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720015d
Branchless algorithm for RuntimeLong.toDouble.
sjrd Jun 23, 2025
2d4e635
Introduce code paths for Longs that are *signed* safe doubles.
sjrd Jun 23, 2025
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79 changes: 41 additions & 38 deletions javalib/src/main/scala/java/lang/Long.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -145,36 +145,44 @@ object Long {

// Must be called only with valid radix
private def toStringImpl(i: scala.Long, radix: Int): String = {
import js.JSNumberOps.enableJSNumberOps

val lo = i.toInt
val hi = (i >>> 32).toInt

if (lo >> 31 == hi) {
// It's a signed int32
import js.JSNumberOps.enableJSNumberOps
lo.toString(radix)
} else if (hi < 0) {
val neg = -i
"-" + toUnsignedStringInternalLarge(neg.toInt, (neg >>> 32).toInt, radix)
} else if (((hi ^ (hi >> 10)) & 0xffe00000) == 0) { // see RuntimeLong.isSignedSafeDouble
// (lo, hi) is small enough to be a Double, so toDouble is exact
i.toDouble.toString(radix)
} else {
toUnsignedStringInternalLarge(lo, hi, radix)
val abs = Math.abs(i)
val s = toUnsignedStringInternalLarge(abs.toInt, (abs >>> 32).toInt, radix)
if (hi < 0) "-" + s else s
}
}

// Must be called only with valid radix
private def toUnsignedStringImpl(i: scala.Long, radix: Int): String = {
import js.JSNumberOps.enableJSNumberOps

val lo = i.toInt
val hi = (i >>> 32).toInt

if (hi == 0) {
// It's an unsigned int32
import js.JSNumberOps.enableJSNumberOps
Integer.toUnsignedDouble(lo).toString(radix)
} else if ((hi & 0xffe00000) == 0) { // see RuntimeLong.isUnsignedSafeDouble
// (lo, hi) is small enough to be a Double, so toDouble is exact
i.toDouble.toString(radix)
} else {
toUnsignedStringInternalLarge(lo, hi, radix)
}
}

// Must be called only with valid radix and with (lo, hi) >= 2^30
// Must be called only with valid radix and with (lo, hi) >= 2^53
@inline // inlined twice: once in toStringImpl and once in toUnsignedStringImpl
private def toUnsignedStringInternalLarge(lo: Int, hi: Int, radix: Int): String = {
import js.JSNumberOps.enableJSNumberOps
import js.JSStringOps.enableJSStringOps
Expand All @@ -185,41 +193,36 @@ object Long {
}

val TwoPow32 = (1L << 32).toDouble
val approxNum =
Integer.toUnsignedDouble(hi) * TwoPow32 + Integer.toUnsignedDouble(lo)

if ((hi & 0xffe00000) == 0) { // see RuntimeLong.isUnsignedSafeDouble
// (lo, hi) is small enough to be a Double, so approxNum is exact
approxNum.toString(radix)
} else {
/* See RuntimeLong.toUnsignedString for a proof. Although that proof is
* done in terms of a fixed divisor of 10^9, it generalizes to any
* divisor that statisfies 2^12 < divisor <= 2^30 and
* ULong.MaxValue / divisor < 2^53, which is true for `radixPowLength`.
*/
/* See RuntimeLong.toUnsignedString for a proof. Although that proof is
* done in terms of a fixed divisor of 10^9, it generalizes to any
* divisor that statisfies 2^12 < divisor <= 2^30 and
* ULong.MaxValue / divisor < 2^53, which is true for `radixPowLength`.
*/

val radixInfo = StringRadixInfos(radix)
val divisor = radixInfo.radixPowLength
val divisorInv = radixInfo.radixPowLengthInverse
val paddingZeros = radixInfo.paddingZeros

// initial approximation of the quotient and remainder
var approxQuot = Math.floor(approxNum * divisorInv)
var approxRem = lo - divisor * unsignedSafeDoubleLo(approxQuot)

// correct the approximations
if (approxRem < 0) {
approxQuot -= 1.0
approxRem += divisor
} else if (approxRem >= divisor) {
approxQuot += 1.0
approxRem -= divisor
}
val radixInfo = StringRadixInfos(radix)
val divisor = radixInfo.radixPowLength
val divisorInv = radixInfo.radixPowLengthInverse
val paddingZeros = radixInfo.paddingZeros

// build the result string
val remStr = approxRem.toString(radix)
approxQuot.toString(radix) + paddingZeros.jsSubstring(remStr.length) + remStr
// initial approximation of the quotient and remainder
val approxNum =
Integer.toUnsignedDouble(hi) * TwoPow32 + Integer.toUnsignedDouble(lo)
var approxQuot = Math.floor(approxNum * divisorInv)
var approxRem = lo - divisor * unsignedSafeDoubleLo(approxQuot)

// correct the approximations
if (approxRem < 0) {
approxQuot -= 1.0
approxRem += divisor
} else if (approxRem >= divisor) {
approxQuot += 1.0
approxRem -= divisor
}

// build the result string
val remStr = approxRem.toString(radix)
approxQuot.toString(radix) + paddingZeros.jsSubstring(remStr.length) + remStr
}

def parseLong(s: String, radix: Int): scala.Long = {
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