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EXP-052: 2x unroll of char loop_parse_if_eight_digits (ported from ffc EXP-044)

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Clang/AArch64-gated 16-digit-per-iteration unroll of the fraction SWAR loop;
eliminates the back-edge for typical 17-digit [0,1] mantissas. GCC keeps the
auto-unrolled simple loop.

ARM Graviton4 (canonical fast_float MB/s vs EXP-050):
  Clang: random +2.8% (1365.7 from 1328.8), mesh +1.7%, canada +0.5%
  GCC:   unchanged (#else path)
Correctness: 14/14 pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
fcostaoliveira 2026-06-01 00:56:52 +01:00
parent 8db9edbc18
commit ee84946702
1 changed files with 29 additions and 0 deletions
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29
include/fast_float/ascii_number.h
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@ -259,6 +259,34 @@ fastfloat_really_inline FASTFLOAT_CONSTEXPR20 void
loop_parse_if_eight_digits(char const *&p, char const *const pend,
uint64_t &i) {
// optimizes better than parse_if_eight_digits_unrolled() for UC = char.
#if defined(__aarch64__) && defined(__clang__)
// 2x unroll (ported from ffc EXP-044): on Clang/AArch64, consuming 16 digits
// per iteration eliminates the loop back-edge for typical fractions (e.g. the
// 17-digit mantissas of uniform [0,1] inputs) and keeps the SWAR constants
// resident. GCC auto-unrolls the simple loop well, so it keeps the original.
while ((pend - p) >= 16) {
uint64_t val1 = read8_to_u64(p);
if (!is_made_of_eight_digits_fast(val1)) {
break;
}
uint64_t val2 = read8_to_u64(p + 8);
if (!is_made_of_eight_digits_fast(val2)) {
i = i * 100000000 + parse_eight_digits_unrolled(val1);
p += 8;
return; // val2 is not a full digit block; caller's byte loop handles rest
}
i = (i * 100000000 + parse_eight_digits_unrolled(val1)) * 100000000 +
parse_eight_digits_unrolled(val2); // in rare cases overflows, that's ok
p += 16;
}
if ((pend - p) >= 8) {
uint64_t val = read8_to_u64(p);
if (is_made_of_eight_digits_fast(val)) {
i = i * 100000000 + parse_eight_digits_unrolled(val);
p += 8;
}
}
#else
while ((std::distance(p, pend) >= 8) &&
is_made_of_eight_digits_fast(read8_to_u64(p))) {
i = i * 100000000 +
@ -266,6 +294,7 @@ loop_parse_if_eight_digits(char const *&p, char const *const pend,
p)); // in rare cases, this will overflow, but that's ok
p += 8;
}
#endif
}
enum class parse_error {