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[DRAFT] BigInteger.Divide perf optimization #89211

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[DRAFT] BigInteger.Divide perf optimization #89211

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a747ad9
avoid boundaries checks by using Unsafe
adamsitnik Jul 19, 2023
d4c0d4c
apply suggested optimization that removes one branch
adamsitnik Jul 19, 2023
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23 changes: 14 additions & 9 deletions src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.DivRem.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,8 @@

using System.Buffers;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace System.Numerics
{
Expand All @@ -16,14 +18,14 @@ public static void Divide(ReadOnlySpan<uint> left, uint right, Span<uint> quotie
// Executes the division for one big and one 32-bit integer.
// Thus, we've similar code than below, but there is no loop for
// processing the 32-bit integer, since it's a single element.

ref uint quotientPtr = ref MemoryMarshal.GetReference(quotient);
ulong carry = 0UL;

for (int i = left.Length - 1; i >= 0; i--)
{
ulong value = (carry << 32) | left[i];
ulong digit = value / right;
quotient[i] = (uint)digit;
Unsafe.Add(ref quotientPtr, i) = (uint)digit;
carry = value - digit * right;
}
remainder = (uint)carry;
Expand All @@ -35,13 +37,14 @@ public static void Divide(ReadOnlySpan<uint> left, uint right, Span<uint> quotie
Debug.Assert(quotient.Length == left.Length);

// Same as above, but only computing the quotient.

ref uint quotientPtr = ref MemoryMarshal.GetReference(quotient);
ulong carry = 0UL;

for (int i = left.Length - 1; i >= 0; i--)
{
ulong value = (carry << 32) | left[i];
ulong digit = value / right;
quotient[i] = (uint)digit;
Unsafe.Add(ref quotientPtr, i) = (uint)digit;
carry = value - digit * right;
}
}
Expand Down Expand Up @@ -199,11 +202,12 @@ private static uint AddDivisor(Span<uint> left, ReadOnlySpan<uint> right)

// Repairs the dividend, if the last subtract was too much

ref uint leftPtr = ref MemoryMarshal.GetReference(left);
ulong carry = 0UL;

for (int i = 0; i < right.Length; i++)
{
ref uint leftElement = ref left[i];
ref uint leftElement = ref Unsafe.Add(ref leftPtr, i);
ulong digit = (leftElement + carry) + right[i];
leftElement = unchecked((uint)digit);
carry = digit >> 32;
Expand All @@ -220,17 +224,18 @@ private static uint SubtractDivisor(Span<uint> left, ReadOnlySpan<uint> right, u
// Combines a subtract and a multiply operation, which is naturally
// more efficient than multiplying and then subtracting...

ref uint leftPtr = ref MemoryMarshal.GetReference(left);
ulong carry = 0UL;

for (int i = 0; i < right.Length; i++)
{
carry += right[i] * q;
uint digit = unchecked((uint)carry);
carry >>= 32;
ref uint leftElement = ref left[i];
if (leftElement < digit)
++carry;
leftElement = unchecked(leftElement - digit);
ref uint leftElement = ref Unsafe.Add(ref leftPtr, i);
ulong newDigit = unchecked((ulong)leftElement - digit);
carry += (newDigit >> 32) & 0x1; // This is the same as if (leftElement < digit) ++carry
leftElement = unchecked((uint)newDigit);
}

return (uint)carry;
Expand Down
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