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Merge pull request #398 from redis-performance/exp062-063-combo

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Enable the 4-digit SWAR follow-up on GCC + skip the rounding-mode probe for long mantissas
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Daniel Lemire 2026-07-13 15:13:16 -04:00 committed by GitHub
commit f6df0f2917
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2 changed files with 20 additions and 17 deletions
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8
include/fast_float/ascii_number.h
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@ -268,10 +268,9 @@ loop_parse_if_eight_digits(char const *&p, char const *const pend,
} }
// Consume a remaining 4-7 digit run in a single SWAR step instead of // Consume a remaining 4-7 digit run in a single SWAR step instead of
// byte-by-byte (reuses the existing 4-digit helpers). The parsed result is // byte-by-byte (reuses the existing 4-digit helpers). The parsed result is
// identical either way. Gated to clang: on gcc the extra 4-digit check // identical either way. Historically gated to clang because gcc regressed on
// regresses inputs whose remainder is shorter than 4 digits (it becomes pure // short remainders, but that verdict predates the span-elision restructure;
// overhead there); clang does not show that. // with the leaner hot path the 4-digit step now wins on gcc as well.
#if defined(__clang__)
if ((pend - p) >= 4) { if ((pend - p) >= 4) {
uint32_t const val4 = read4_to_u32(p); uint32_t const val4 = read4_to_u32(p);
if (is_made_of_four_digits_fast(val4)) { if (is_made_of_four_digits_fast(val4)) {
@ -280,7 +279,6 @@ loop_parse_if_eight_digits(char const *&p, char const *const pend,
p += 4; p += 4;
} }
} }
#endif
} }
enum class parse_error { enum class parse_error {

29
include/fast_float/parse_number.h
View File

@ -198,7 +198,14 @@ clinger_fast_path_impl(uint64_t mantissa, int64_t exponent, bool is_negative,
// We proceed optimistically, assuming that detail::rounds_to_nearest() // We proceed optimistically, assuming that detail::rounds_to_nearest()
// returns true. // returns true.
if (binary_format<T>::min_exponent_fast_path() <= exponent && if (binary_format<T>::min_exponent_fast_path() <= exponent &&
exponent <= binary_format<T>::max_exponent_fast_path()) { exponent <= binary_format<T>::max_exponent_fast_path() &&
mantissa <= binary_format<T>::max_mantissa_fast_path()) {
// The mantissa bound above is a necessary condition for BOTH branches
// below: the rounding-mode-dependent branch checks the tighter
// max_mantissa_fast_path(exponent) <= max_mantissa_fast_path(). Testing
// it before detail::rounds_to_nearest() spares long-mantissa inputs
// (which can never take the fast path) the volatile-float probe.
//
// Unfortunately, the conventional Clinger's fast path is only possible // Unfortunately, the conventional Clinger's fast path is only possible
// when the system rounds to the nearest float. // when the system rounds to the nearest float.
// //
@ -209,18 +216,16 @@ clinger_fast_path_impl(uint64_t mantissa, int64_t exponent, bool is_negative,
if (!cpp20_and_in_constexpr() && detail::rounds_to_nearest()) { if (!cpp20_and_in_constexpr() && detail::rounds_to_nearest()) {
// We have that fegetround() == FE_TONEAREST. // We have that fegetround() == FE_TONEAREST.
// Next is Clinger's fast path. // Next is Clinger's fast path.
if (mantissa <= binary_format<T>::max_mantissa_fast_path()) { value = T(mantissa);
value = T(mantissa); if (exponent < 0) {
if (exponent < 0) { value = value / binary_format<T>::exact_power_of_ten(-exponent);
value = value / binary_format<T>::exact_power_of_ten(-exponent); } else {
} else { value = value * binary_format<T>::exact_power_of_ten(exponent);
value = value * binary_format<T>::exact_power_of_ten(exponent);
}
if (is_negative) {
value = -value;
}
return true;
} }
if (is_negative) {
value = -value;
}
return true;
} else { } else {
// We do not have that fegetround() == FE_TONEAREST. // We do not have that fegetround() == FE_TONEAREST.
// Next is a modified Clinger's fast path, inspired by Jakub Jelínek's // Next is a modified Clinger's fast path, inspired by Jakub Jelínek's