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perf(operator): lookup table for Math.pow(10, n) in MathFunctions.round #3

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perf(operator): lookup table for Math.pow(10, n) in MathFunctions.round #3

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21 changes: 19 additions & 2 deletions core/trino-main/src/main/java/io/trino/operator/scalar/MathFunctions.java
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Original file line number Diff line number Diff line change
Expand Up @@ -84,6 +84,15 @@ public final class MathFunctions
private static final Int128[] DECIMAL_HALF_UNSCALED_FOR_SCALE;
private static final Int128[] DECIMAL_ALMOST_HALF_UNSCALED_FOR_SCALE;

// Lookup table of 10^n as double for small non-negative n. Values are bit-exact matches of Math.pow(10, n)
// for n in [0, 18]; see Pow10 verification in the session notes. Hot-path round(double, long) and
// roundReal(long, long) use this to avoid calling Math.pow for the typical decimals range.
private static final double[] POWERS_OF_TEN_DOUBLE = new double[] {
1.0, 10.0, 100.0, 1000.0, 10000.0, 100000.0, 1000000.0, 10000000.0, 100000000.0, 1000000000.0,
10000000000.0, 100000000000.0, 1000000000000.0, 10000000000000.0, 100000000000000.0,
1000000000000000.0, 10000000000000000.0, 100000000000000000.0, 1000000000000000000.0
};

static {
DECIMAL_HALF_UNSCALED_FOR_SCALE = new Int128[Decimals.MAX_PRECISION];
DECIMAL_ALMOST_HALF_UNSCALED_FOR_SCALE = new Int128[Decimals.MAX_PRECISION];
Expand All @@ -102,6 +111,14 @@ public final class MathFunctions
}
}

private static double powerOfTen(long decimals)
{
if (decimals >= 0 && decimals < POWERS_OF_TEN_DOUBLE.length) {
return POWERS_OF_TEN_DOUBLE[(int) decimals];
}
return Math.pow(10, decimals);
}

private MathFunctions() {}

@Description("Absolute value")
Expand Down Expand Up @@ -899,7 +916,7 @@ public static double round(@SqlType(StandardTypes.DOUBLE) double num, @SqlType(S
return num;
}

double factor = Math.pow(10, decimals);
double factor = powerOfTen(decimals);
int sign = (num < 0) ? -1 : 1;
double rescaled = sign * num * factor;
long rescaledRound = Math.round(rescaled);
Expand All @@ -926,7 +943,7 @@ public static long roundReal(@SqlType(StandardTypes.REAL) long num, @SqlType(Sta
return num;
}

double factor = Math.pow(10, decimals);
double factor = powerOfTen(decimals);
int sign = (numInFloat < 0) ? -1 : 1;
double result;
double rescaled = sign * numInFloat * factor;
Expand Down