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EXP-050: straight-line nested-if integer-part scan (ported from ffc EXP-026/028)

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Peel the first 5 iterations of the integer-part digit loop into nested ifs,
eliminating the loop back-edge for the common 1-5 digit integer case. Identical
semantics (i = 10*i + digit). Biggest win on inputs with multi-digit integer
parts (mesh 3D coordinates).

ARM Graviton4 (canonical MB/s, vs upstream 7790aa6 baseline):
  GCC:   random +0.05%, canada +4.0%, mesh +34.3% (c/f 55.7->41.4)
  Clang: random +4.9%,  canada +2.8%, mesh +5.1%
Correctness: 14/14 core+supplemental pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
fcostaoliveira 2026-06-01 00:48:45 +01:00
parent 7790aa6231
commit 8db9edbc18
1 changed files with 29 additions and 6 deletions
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35
include/fast_float/ascii_number.h
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@ -354,13 +354,36 @@ parse_number_string(UC const *p, UC const *pend,
uint64_t i = 0; // an unsigned int avoids signed overflows (which are bad)
while ((p != pend) && is_integer(*p)) {
// a multiplication by 10 is cheaper than an arbitrary integer
// multiplication
i = 10 * i +
uint64_t(*p -
UC('0')); // might overflow, we will handle the overflow later
// 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 (ported from ffc EXP-026/028). 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'));
++p;
if ((p != pend) && is_integer(*p)) {
i = 10 * i + uint64_t(*p - UC('0'));
++p;
if ((p != pend) && is_integer(*p)) {
i = 10 * i + uint64_t(*p - UC('0'));
++p;
if ((p != pend) && is_integer(*p)) {
i = 10 * i + uint64_t(*p - UC('0'));
++p;
if ((p != pend) && is_integer(*p)) {
i = 10 * i + uint64_t(*p - UC('0'));
++p;
while ((p != pend) && is_integer(*p)) {
// a multiplication by 10 is cheaper than an arbitrary integer
// multiplication
i = 10 * i +
uint64_t(*p - UC('0')); // might overflow, handled later
++p;
}
}
}
}
}
}
UC const *const end_of_integer_part = p;
int64_t digit_count = int64_t(end_of_integer_part - start_digits);