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Round to even in a branchless fashion.

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This commit is contained in:
Amaury Séchet 2023-07-10 12:00:09 +00:00
parent 6a390f63e9
commit 1eb42b8569
1 changed files with 4 additions and 10 deletions
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14
include/fast_float/decimal_to_binary.h
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@ -128,7 +128,8 @@ adjusted_mantissa compute_float(int64_t q, uint64_t w) noexcept {
// is easily predicted. Which is best is data specific.
int upperbit = int(product.high >> 63);
answer.mantissa = product.high >> (upperbit + 64 - binary::mantissa_explicit_bits() - 3);
int shift = upperbit + 64 - binary::mantissa_explicit_bits() - 3;
answer.mantissa = product.high >> shift;
answer.power2 = int32_t(detail::power(int32_t(q)) + upperbit - lz - binary::minimum_exponent());
if (answer.power2 <= 0) { // we have a subnormal?
@ -159,15 +160,8 @@ adjusted_mantissa compute_float(int64_t q, uint64_t w) noexcept {
// usually, we round *up*, but if we fall right in between and and we have an
// even basis, we need to round down
// We are only concerned with the cases where 5**q fits in single 64-bit word.
if ((product.low <= 1) && (q >= binary::min_exponent_round_to_even()) && (q <= binary::max_exponent_round_to_even()) &&
((answer.mantissa & 3) == 1) ) { // we may fall between two floats!
// To be in-between two floats we need that in doing
// answer.mantissa = product.high >> (upperbit + 64 - binary::mantissa_explicit_bits() - 3);
// ... we dropped out only zeroes. But if this happened, then we can go back!!!
if((answer.mantissa << (upperbit + 64 - binary::mantissa_explicit_bits() - 3)) == product.high) {
answer.mantissa &= ~uint64_t(1); // flip it so that we do not round up
}
}
uint64_t mask = (4ULL << shift) - 1;
answer.mantissa ^= (product.low == 0) & ((product.high & mask) == (1ULL << shift));
answer.mantissa += (answer.mantissa & 1); // round up
answer.mantissa >>= 1;