fix: UInt256: Divide, Lsh, Rsh, Exp, ExpMod, SubtractMod; Int256: Multiply - when in and out params are referenced to the same struct

[Abalioha]

fixes #32

[benaadams]

Could you add test; to ensure doesn't regress?

[Abalioha]

Added some tests - found much more bugs. I checked Nethermind - it is not affected

[benaadams]

Any reason to remove in here?

[Abalioha]

Yes, I made it intentionaly.
This will create a copy of a.
So when you use it like
UInt256.SubtractMod(a, in b, out a)
and inside the method you reach
if (SubtractUnderflow(a, b, out res))
this line will not override a (as a and res are the same) (and we need original value of a later)

I could just made a copy of a inside the method instead, but why do we need to pass it as reference and then just copy it?

Same situation you can find in
static void Exp(in UInt256 b, in UInt256 e, out UInt256 result)
where I removed a line UInt256 bs = b; and change signature to
public static void Exp(UInt256 b, in UInt256 e, out UInt256 result)

Same in ExpMod

[Abalioha]

Apple M1 Pro, .NET 7.0 : .NET 7.0.5 (7.0.523.17405), Arm64 RyuJIT AdvSIMD

Method	Mean	Error	StdDev	Ratio
SubtractMod_UInt256	5.878 ns	0.0035 ns	0.0029 ns	1.00
SubtractMod_UInt256_withoutIn	5.741 ns	0.0039 ns	0.0032 ns	0.98

[Abalioha]

And if I will change

public static void Multiply(in Int256 a, in Int256 b, out Int256 res)
{
    Int256 av = a, bv = b;
    if (a.Sign < 0)
    {
        a.Neg(out av);
    }
    if (b.Sign < 0)
    {
        b.Neg(out bv);
    }
    UInt256.Multiply(av._value, bv._value, out UInt256 ures);
    int aSign = a.Sign;
    int bSign = b.Sign;
    res = new Int256(ures);
...

to

public static void MultiplyWithoutIn(Int256 a, Int256 b, out Int256 res)
{
    int aSign = a.Sign;
    int bSign = b.Sign;

    if (aSign < 0) a.Neg(out a);
    if (bSign < 0) b.Neg(out b);
    UInt256.Multiply(a._value, b._value, out UInt256 ures);

    res = new Int256(ures);
...

(remove in and Int256 av = a, bv = b;)
this will give improvement too

Apple M1 Pro, .NET 7.0 : .NET 7.0.5 (7.0.523.17405), Arm64 RyuJIT AdvSIMD

Method	EnvironmentVariables	Mean	Ratio
Multiply_Int256	Empty	9.861 ns	1.00
Multiply_Int256_withoutIn	Empty	9.463 ns	0.96
Multiply_Int256	DOTNET_EnableHWIntrinsic=0	19.376 ns	1.96
Multiply_Int256_withoutIn	DOTNET_EnableHWIntrinsic=0	18.132 ns	1.84

[Abalioha]

last commit failed because this code gives different output in different OS

double x = ulong.MaxValue;
Console.WriteLine(x);
Console.WriteLine((ulong)x);

On Linux 22.04 (7.0.305) this will result in:

1.8446744073709552E+19                                                                                                                                                                                  
0     

And on MacOS (7.0.203)

1,8446744073709552E+19
18446744073709551615

Is it expected behaviour?

[LukaszRozmej]

last commit failed because this code gives different output in different OS

double x = ulong.MaxValue;
Console.WriteLine(x);
Console.WriteLine((ulong)x);

On Linux 22.04 (7.0.305) this will result in:

1.8446744073709552E+19                                                                                                                                                                                  
0     

And on MacOS (7.0.203)

1,8446744073709552E+19
18446744073709551615

Is it expected behaviour?

That sounds very weird