Speed up decimal integer parsing with SWAR - #161019
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Use SIMD-within-a-register to process 8 ASCII digits at once in from_str_radix when radix == 10 and the result is guaranteed not to overflow. Falls back to the existing per-digit loop for the remaining 0-7 digits. The fast path uses two helper functions: - is_8digits: branch-free check that all 8 bytes are b'0'..=b'9' - parse_8digits: 3 multiplications to pack 8 digits into a u64 Benchmark on 16-20 digit decimal strings (5000 iterations, stage 1): bench_u64_from_str_radix_10_long 98818 -> 73194 ns (-25.9%) bench_i64_from_str_radix_10_long 149705 -> 120089 ns (-19.8%) Also add LONG_ASCII_NUMBERS and from_str_radix_long_bench macro to exercise the fast path with strings that trigger 2+ SWAR iterations.
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Align trailing `//` comments vertically in the new benchmark data constant. rustfmt in nightly runs during the tidy CI job flags the misaligned comments.
tgross35
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| if radix == 10 { | ||
| while let [a, b, c, d, e, f, g, h, rest @ ..] = digits { | ||
| let chunk = u64::from_le_bytes([*a, *b, *c, *d, *e, *f, *g, *h]); | ||
| if !is_8digits(chunk) { | ||
| return Err(PIE { kind: InvalidDigit }); | ||
| } | ||
| let parsed = parse_8digits(chunk) as $int_ty; | ||
| result = result * (100_000_000u32 as $int_ty); | ||
| if is_positive { | ||
| result = result + parsed; | ||
| } else { | ||
| result = result - parsed; | ||
| } | ||
| digits = rest; | ||
| } | ||
| } |
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There should probably be a 32- and (maybe) 16-bit version so this doesn't wind up slower on those platforms
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Speed up from_str_radix for base 10 with SWAR
Closes #87249
Process 8 decimal digits at a time using SIMD-within-a-register
instead of one digit per loop iteration. Only applies when radix
is 10 and can_not_overflow guarantees the result fits, so the
fast path needs no overflow checks.
The SWAR technique was suggested by @bormand and @Alexhuszagh in
the issue thread. Two helpers:
is_8digits(v: u64) -> bool — branch-free check that all 8
bytes are b'0'..=b'9'
parse_8digits(v: u64) -> u64 — 3 multiplications to pack 8
digits into a single value
Remaining <8 digits fall through to the existing per-digit loop.
Benchmark (stage 1, x86_64, 16-20 digit strings × 5000):
bench_u64_from_str_radix_10_long 98818 → 73194 ns (−25.9%)
bench_i64_from_str_radix_10_long 149705 → 120089 ns (−19.8%)
The mixed-input benchmark barely moves because only 2 of 19 inputs
are long enough to trigger the fast path. Added LONG_ASCII_NUMBERS
and from_str_radix_long_bench! to cover it properly.