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Parse the integer part with one subtraction per digit

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As in the fraction tail and the exponent: clang computed the digit twice,
once for the test and once for the value.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Raúl Marín 2026-09-24 19:09:51 +02:00
parent 8218446e2f
commit 784cd670c3
1 changed files with 19 additions and 14 deletions
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33
include/fast_float/ascii_number.h
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@ -475,27 +475,32 @@ parse_number_string_impl(UC const *p, UC const *pend,
// Straight-line unroll of the integer-part scan: most integer parts are
// 1-5 digits, so peeling the first iterations eliminates the loop back-edge
// for the common case. Semantics are identical to the original `while` loop:
// i = 10*i + digit, advancing p.
if ((p != pend) && is_integer(*p)) {
i = uint64_t(*p - UC('0'));
// i = 10*i + int_digit, advancing p. Each int_digit is tested and used
// through one subtraction, as in the fraction tail (clang otherwise computes
// it twice).
uint64_t int_digit;
if ((p != pend) && (int_digit = uint64_t(*p) - uint64_t(UC('0'))) <= 9) {
i = int_digit;
++p;
if ((p != pend) && is_integer(*p)) {
i = 10 * i + uint64_t(*p - UC('0'));
if ((p != pend) && (int_digit = uint64_t(*p) - uint64_t(UC('0'))) <= 9) {
i = 10 * i + int_digit;
++p;
if ((p != pend) && is_integer(*p)) {
i = 10 * i + uint64_t(*p - UC('0'));
if ((p != pend) && (int_digit = uint64_t(*p) - uint64_t(UC('0'))) <= 9) {
i = 10 * i + int_digit;
++p;
if ((p != pend) && is_integer(*p)) {
i = 10 * i + uint64_t(*p - UC('0'));
if ((p != pend) &&
(int_digit = uint64_t(*p) - uint64_t(UC('0'))) <= 9) {
i = 10 * i + int_digit;
++p;
if ((p != pend) && is_integer(*p)) {
i = 10 * i + uint64_t(*p - UC('0'));
if ((p != pend) &&
(int_digit = uint64_t(*p) - uint64_t(UC('0'))) <= 9) {
i = 10 * i + int_digit;
++p;
while ((p != pend) && is_integer(*p)) {
while ((p != pend) &&
(int_digit = uint64_t(*p) - uint64_t(UC('0'))) <= 9) {
// a multiplication by 10 is cheaper than an arbitrary integer
// multiplication
i = 10 * i +
uint64_t(*p - UC('0')); // might overflow, handled later
i = 10 * i + int_digit; // might overflow, handled later
++p;
}
}