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perf: do not allocate extra digit in bigint division #1852

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perf: do not allocate extra digit in bigint division #1852

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perf: do not allocate extra digit in bigint division
ChALkeR Nov 28, 2025
cf6cb61
nit
ChALkeR Dec 7, 2025
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15 changes: 9 additions & 6 deletions lib/Support/BigIntSupport.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1741,10 +1741,6 @@ multiply(MutableBigIntRef dst, ImmutableBigIntRef lhs, ImmutableBigIntRef rhs) {

namespace {
namespace div_rem {
static uint32_t getResultSize(ImmutableBigIntRef lhs, ImmutableBigIntRef rhs) {
return std::max(lhs.numDigits, rhs.numDigits) + 1;
}

static OperationStatus compute(
MutableBigIntRef quoc,
MutableBigIntRef rem,
Expand Down Expand Up @@ -1868,7 +1864,14 @@ static OperationStatus compute(
} // namespace

uint32_t divideResultSize(ImmutableBigIntRef lhs, ImmutableBigIntRef rhs) {
return div_rem::getResultSize(lhs, rhs);
if (isNegative(lhs) && compare(rhs, -1) == 0) {
// In this (and only this) case, we can end up with more digits than we started with
// Examples: -(2n**63n)/-1n, -(2n**127n)/-1n
// We avoid this in general case as it makes division much slower
return std::max(lhs.numDigits, rhs.numDigits) + 1;
}

return std::max(lhs.numDigits, rhs.numDigits);
}

OperationStatus
Expand All @@ -1879,7 +1882,7 @@ divide(MutableBigIntRef dst, ImmutableBigIntRef lhs, ImmutableBigIntRef rhs) {
}

uint32_t remainderResultSize(ImmutableBigIntRef lhs, ImmutableBigIntRef rhs) {
return div_rem::getResultSize(lhs, rhs);
return divideResultSize(lhs, rhs); // We currently expect them to be equivalent in div_rem::compute()
}

OperationStatus remainder(
Expand Down