perf: optimize parseInt with 4-digits-at-a-time parsing#897
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Inspired by jsoniter-scala's SWAR digit parsing technique, process 4 decimal digits at a time instead of 1. This reduces the number of loop iterations and charAt calls by 4x for the common case. The optimization validates all 4 digits before combining them, falling through to the scalar loop if any non-digit is found. Uses Double accumulator throughout to handle numbers beyond Long range. Benchmark results (parseInt): - Before: jrsonnet 1.97x faster - After: jrsonnet 1.27x faster (36% gap reduction) Reference: plokhotnyuk/jsoniter-scala@df92601
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Motivation: The bulk numeric array parser (previous commit) used Double.parseDouble(data.substring()) for each element, creating a substring allocation and invoking the full double parser even for simple integers. Modification: - Parse simple integers directly using 4-digits-at-a-time technique (inspired by jsoniter-scala/PR databricks#897), avoiding substring allocation and Double.parseDouble overhead entirely - Use Val.cachedNum for values 0-255, reusing pre-allocated instances instead of creating new Val.Num objects - Float/exponent numbers still fall back to Double.parseDouble Result: Native A/B (member): 7.1ms → 5.9ms (-17.4%). Combined with bulk parser: member gap vs jrsonnet 1.97x → 1.42x. Also improves base64_byte_array: 11.3ms → 10.4ms (-7.9%).
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Motivation
The
std.parseIntfunction was 1.97x slower than jrsonnet (Rust implementation) on Scala Native. The main bottleneck was the per-character digit parsing loop.Key Design Decision
Inspired by jsoniter-scala's SWAR digit parsing technique, process 4 decimal digits at a time instead of 1. This reduces the number of loop iterations and
charAtcalls by 4x for the common case.Modification
Changed
sjsonnet/src/sjsonnet/stdlib/StringModule.scala:Doubleaccumulator throughout to handle numbers beyond Long rangeBenchmark Results
Scala Native vs jrsonnet (hyperfine)
JMH (JVM)
Baseline JMH benchmarks are stable; the change benefits all platforms.
Analysis
The 4-digits-at-a-time approach reduces loop overhead by processing 4 digits per iteration instead of 1. For the benchmark input "-123949595" (9 digits), this means 2 iterations instead of 9.
The optimization validates all 4 digits before combining them, falling through to the scalar loop if any non-digit is found. This ensures correct error reporting for invalid inputs.
References
Result
./mill __.test)./mill __.reformat)