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Skip the range check in compute_float after Clinger's range test under clang

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If the exponent passed that test, the mantissa failed it, so it is not zero and
the exponent is within range. GCC carries this into compute_float by itself.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Raúl Marín 2026-09-23 21:17:27 +02:00
parent 0900eedf2f
commit ef5797fa94
2 changed files with 16 additions and 2 deletions
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6
include/fast_float/decimal_to_binary.h
View File

@ -98,7 +98,9 @@ compute_error(int64_t q, uint64_t w) noexcept {
// packed. However, in some very rare cases, the computation will fail. In such
// cases, we return an adjusted_mantissa with a negative power of 2: the caller
// should recompute in such cases.
template <typename binary>
// known_in_range: the caller knows that q is within the range of powers of
// ten, so only w == 0 is checked.
template <typename binary, bool known_in_range = false>
fastfloat_really_inline FASTFLOAT_CONSTEXPR20 adjusted_mantissa
compute_float(int64_t q, uint64_t w) noexcept {
adjusted_mantissa answer;
@ -107,7 +109,7 @@ compute_float(int64_t q, uint64_t w) noexcept {
bool const q_out_of_range = uint64_t(q - binary::smallest_power_of_ten()) >
uint64_t(binary::largest_power_of_ten() -
binary::smallest_power_of_ten());
if ((w == 0) || q_out_of_range) {
if ((w == 0) || (!known_in_range && q_out_of_range)) {
bool const underflow = (w == 0) | (q < binary::smallest_power_of_ten());
answer.power2 = int32_t(!underflow) * binary::infinite_power();
answer.mantissa = 0;

12
include/fast_float/parse_number.h
View File

@ -445,8 +445,20 @@ from_chars_float_advanced(UC const *first, UC const *last, T &value,
return answer;
}
#ifdef __clang__
// If the exponent passed Clinger's range test, it is well within range. GCC
// carries this into compute_float by itself; clang needs the instantiation
// without the range check. The mantissa can still be zero when the rounding
// mode is not to nearest.
adjusted_mantissa am =
(binary_format<T>::min_exponent_fast_path() <= pns.exponent &&
pns.exponent <= binary_format<T>::max_exponent_fast_path())
? compute_float<binary_format<T>, true>(pns.exponent, pns.mantissa)
: compute_float<binary_format<T>>(pns.exponent, pns.mantissa);
#else
adjusted_mantissa am =
compute_float<binary_format<T>>(pns.exponent, pns.mantissa);
#endif
// Slow path B (rare): Eisel-Lemire could not resolve; digit_comp needs the
// integer/fraction spans. Route to the cold helper (clinger there is a
// dead-effect since it already failed here; the cold re-parse + digit_comp