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Simplify Pow2Montgomery GPU executor branching

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Marek Lemańczyk committed 2025-10-22 01:17:35 +02:00
1 parent 865a88fac0
commit b08175dadf
1 file changed
+26 -23
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PerfectNumbers.Core/ULongExtensions.Gpu.cs
+26 -23
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@@ -135,43 +135,46 @@ public static partial class ULongExtensions
{
ulong currentBit = (exponent >> index) & 1UL;
ulong squared = MontgomeryMultiply(result, result, modulus, nPrime);
result = currentBit == 0UL ? squared : result;
index -= (int)(currentBit ^ 1UL);
if (currentBit == 0UL)
{
continue;
}
bool processWindow = currentBit != 0UL;
int windowStart = index - windowSize + 1;
// if (windowStart < 0)
result = processWindow ? result : squared;
index -= (int)(currentBit ^ 1UL);
int windowStartCandidate = index - windowSize + 1;
// if (windowStartCandidate < 0)
// {
// windowStart = 0;
// windowStartCandidate = 0;
// }
// Clamp the negative offset without branching so the GPU loop stays divergence-free.
// This still handles windows that would otherwise extend past the most significant bit.
int negativeMask = windowStart >> 31;
windowStart &= ~negativeMask;
int negativeMask = windowStartCandidate >> 31;
windowStartCandidate &= ~negativeMask;
windowStart = GetNextSetBitIndexGpu(exponent, windowStart);
int windowLength = index - windowStart + 1;
int windowStart = processWindow ? GetNextSetBitIndexGpu(exponent, windowStartCandidate) : windowStartCandidate;
int windowLength = processWindow ? index - windowStart + 1 : 0;
for (int square = 0; square < windowLength; square++)
{
result = MontgomeryMultiply(result, result, modulus, nPrime);
}
ulong mask = (1UL << windowLength) - 1UL;
ulong windowValue = (exponent >> windowStart) & mask;
ulong multiplier = ComputeMontgomeryOddPowerGpu(windowValue, divisor, modulus, nPrime);
result = MontgomeryMultiply(result, multiplier, modulus, nPrime);
result = processWindow
? MontgomeryMultiply(
result,
ComputeMontgomeryOddPowerGpu(
(exponent >> windowStart) & ((1UL << windowLength) - 1UL),
divisor,
modulus,
nPrime),
modulus,
nPrime)
: result;
index = windowStart - 1;
index = processWindow ? windowStart - 1 : index;
}
if (!keepMontgomery)
{
result = MontgomeryMultiply(result, 1UL, modulus, nPrime);
}
result = keepMontgomery
? result
: MontgomeryMultiply(result, 1UL, modulus, nPrime);
return result;
}