BigInteger.MaxMagnitude

[PetSerAl]

https://github.com/dotnet/runtime/blob/53d1a6d8afa19e3c0cebdad3004325edf954d3d9/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs#L4095-L4114

Since BigInteger comparison could be costly, should not two comparisons (ax > ay and ax == ay) be replaced with single CompareTo call and switching on result?

return
  Abs(x).CompareTo(Abs(y)) switch {
    > 0 => x,
    0 => IsNegative(x) ? y : x,
    _ => y
  };

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[huoyaoyuan]

Creating Abs value for BigInteger tends to be much more costly as it allocates. Comparing BigIntegers is not so expensive for non-huge numbers, as they are vectorizable.

[PaulusParssinen]

Creating Abs value for BigInteger tends to be much more costly as it allocates.

The existing code already is taking absolute values of both operands. This should still be slight overall improvement, right?

[PetSerAl]

Creating Abs value for BigInteger tends to be much more costly as it allocates.

Abs does not allocate.

https://github.com/dotnet/runtime/blob/53d1a6d8afa19e3c0cebdad3004325edf954d3d9/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs#L709-L712

https://github.com/dotnet/runtime/blob/53d1a6d8afa19e3c0cebdad3004325edf954d3d9/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs#L2644-L2648

[huoyaoyuan]

Abs does not allocate.

Oh I forgot bits stores absolute value instead of raw value...never mind.

Yes, doing comparison only once does save.

[PetSerAl]

It seems Abs could also be improved with:

return new BigInteger(Math.Abs(value._sign), value._bits);

[karakasa]

We should also optimize Abs() though. Let me work on a PR later.

Method	Length	Mean	Error	StdDev	Ratio	Allocated	Alloc Ratio
Original_WorstPath	16	14.150 ns	0.0308 ns	0.0289 ns	1.00	-	NA
CompareTo_WorstPath	16	11.860 ns	0.0318 ns	0.0297 ns	0.84	-	NA
CompareToAndAbs_WorstPath	16	7.630 ns	0.0245 ns	0.0204 ns	0.54	-	NA
Original_WorstPath	256	82.871 ns	0.0887 ns	0.0740 ns	1.00	-	NA
CompareTo_WorstPath	256	49.875 ns	0.0941 ns	0.0834 ns	0.60	-	NA
CompareToAndAbs_WorstPath	256	36.596 ns	0.1276 ns	0.1066 ns	0.44	-	NA
Original_WorstPath	4096	990.591 ns	2.6575 ns	2.4858 ns	1.00	-	NA
CompareTo_WorstPath	4096	499.955 ns	1.6945 ns	1.5850 ns	0.50	-	NA
CompareToAndAbs_WorstPath	4096	495.232 ns	1.2101 ns	1.1320 ns	0.50	-	NA
Original_WorstPath	65536	15,411.929 ns	41.0962 ns	38.4414 ns	1.00	-	NA
CompareTo_WorstPath	65536	7,711.964 ns	24.9932 ns	23.3786 ns	0.50	-	NA
CompareToAndAbs_WorstPath	65536	7,702.426 ns	24.3057 ns	22.7355 ns	0.50	-	NA

code

```C# using BenchmarkDotNet.Attributes; using System.Numerics; namespace benchmark; [MemoryDiagnoser] [Config(typeof(AntiVirusFriendlyConfig))] public class BigNumberTest { [Params(16, 256, 4096, 65536)] public int Length { get; set; } public BigInteger A; public BigInteger B; [GlobalSetup] public void PrepareBigNumber() { var rng = new Random(42); var array = new byte[Length]; rng.NextBytes(array); A = new BigInteger(array); B = -new BigInteger(array); } [Benchmark(Baseline = true)] public BigInteger Original_WorstPath() { return BigInteger.MaxMagnitude(A, B); } [Benchmark] public BigInteger CompareTo_WorstPath() { return MaxMag1(A, B); } [Benchmark] public BigInteger CompareToAndAbs_WorstPath() { return MaxMag2(A, B); } private static BigInteger MaxMag1(BigInteger x, BigInteger y) { return BigInteger.Abs(x).CompareTo(BigInteger.Abs(y)) switch { > 0 => x, 0 => BigInteger.IsNegative(x) ? y : x, _ => y }; } private static BigInteger Abs(BigInteger bi) { return BigInteger.IsNegative(bi) ? -bi : bi; } private static BigInteger MaxMag2(BigInteger x, BigInteger y) { return Abs(x).CompareTo(Abs(y)) switch { > 0 => x, 0 => BigInteger.IsNegative(x) ? y : x, _ => y }; } } ```

[PetSerAl]

@karakasa WorstPath? In your code A and B share the same array. That means comparison can be done by comparing array reference. No need to compare array content.

[PetSerAl]

@karakasa But == have take advantage of that.

[karakasa]

@karakasa But == have take advantage of that.

You are right. 👍 Thank you for the correction. Benchmark has been updated.