perf: specialize decimal digit scanning

[pelletier]

Splitting #1088: this PR carries one optimization technique so its impact and review surface stay isolated.

What

scanDigitsWithUnderscores pays an indirect isInRange function call per byte. Decimal runs — every integer, float fraction, and exponent, the overwhelmingly common case — now scan through scanDecimalDigits, a specialized loop with the range test inlined and the same underscore validation (and error message). Hex, octal, and binary literals keep the generic scanner.

Impact

Benchmarked on a dedicated linux/amd64 spot VM (t2d), go1.26.4, interleaved A/B vs the base of this PR, benchstat over 10 samples per side:

• sec/op geomean: -0.90%
• UnmarshalDataset/canada: -13.09%
• UnmarshalDataset/code: -6.49%
• Unmarshal/SimpleDocument/map: -2.97%
• RealWorldContainerdConfig: +1.59%
• Marshal/HugoFrontMatter: +3.18%

Full benchstat (sec/op, allocs/op)

sec/op

UnmarshalDataset/config  10.43m ± 3%  10.28m ±  1%  -1.37% (p=0.009 n=10)
UnmarshalDataset/canada  23.59m ± 5%  20.50m ± 10%  -13.09% (p=0.000 n=10)
UnmarshalDataset/citm_catalog  15.12m ± 5%  15.12m ±  6%  ~ (p=0.853 n=10)
UnmarshalDataset/twitter  3.629m ± 4%  3.582m ±  5%  ~ (p=0.052 n=10)
UnmarshalDataset/code  34.00m ± 3%  31.80m ±  4%  -6.49% (p=0.000 n=10)
UnmarshalDataset/example  77.31µ ± 2%  77.09µ ±  3%  ~ (p=0.190 n=10)
Unmarshal/SimpleDocument/struct  335.8n ± 1%  333.4n ±  3%  ~ (p=0.239 n=10)
Unmarshal/SimpleDocument/map  425.4n ± 1%  412.7n ±  2%  -2.97% (p=0.001 n=10)
Unmarshal/ReferenceFile/struct  39.52µ ± 3%  39.60µ ±  1%  ~ (p=0.631 n=10)
Unmarshal/ReferenceFile/map  31.50µ ± 2%  31.75µ ±  2%  ~ (p=0.342 n=10)
Unmarshal/HugoFrontMatter  5.656µ ± 2%  5.718µ ±  3%  +1.11% (p=0.045 n=10)
Marshal/SimpleDocument/struct  398.1n ± 3%  392.2n ±  4%  ~ (p=0.118 n=10)
Marshal/SimpleDocument/map  596.4n ± 1%  602.4n ±  5%  +1.01% (p=0.027 n=10)
Marshal/ReferenceFile/struct  20.70µ ± 1%  20.29µ ±  3%  ~ (p=0.172 n=10)
Marshal/ReferenceFile/map  39.85µ ± 4%  40.29µ ±  4%  ~ (p=0.353 n=10)
Marshal/HugoFrontMatter  8.030µ ± 3%  8.286µ ±  6%  +3.18% (p=0.022 n=10)
RealWorldContainerdConfig  39.96µ ± 2%  40.60µ ±  2%  +1.59% (p=0.035 n=10)
RealWorldViperRead  8.342µ ± 1%  8.244µ ±  2%  ~ (p=0.190 n=10)
RealWorldViperWrite  12.37µ ± 5%  12.58µ ±  2%  ~ (p=0.684 n=10)
RealWorldHugoFrontMatterBatch  95.47µ ± 2%  95.55µ ±  2%  ~ (p=0.971 n=10)
RealWorldGitleaksRules  65.12µ ± 2%  65.23µ ±  1%  ~ (p=0.912 n=10)
RealWorldPyproject  15.20µ ± 2%  15.20µ ±  3%  ~ (p=1.000 n=10)
RealWorldGolangciStrict  11.83µ ± 3%  11.88µ ±  1%  ~ (p=0.469 n=10)
geomean  46.79µ  46.36µ  -0.90%

allocs/op

UnmarshalDataset/config  70.19k ± 0%  70.19k ± 0%  ~ (p=1.000 n=10) ¹
UnmarshalDataset/canada  223.2k ± 0%  223.2k ± 0%  ~ (p=1.000 n=10) ¹
UnmarshalDataset/citm_catalog  49.98k ± 0%  49.98k ± 0%  ~ (p=1.000 n=10) ¹
UnmarshalDataset/twitter  15.78k ± 0%  15.78k ± 0%  ~ (p=1.000 n=10) ¹
UnmarshalDataset/code  129.5k ± 0%  129.5k ± 0%  ~ (p=1.000 n=10) ¹
UnmarshalDataset/example  399.0 ± 0%  399.0 ± 0%  ~ (p=1.000 n=10) ¹
Unmarshal/SimpleDocument/struct  2.000 ± 0%  2.000 ± 0%  ~ (p=1.000 n=10) ¹
Unmarshal/SimpleDocument/map  5.000 ± 0%  5.000 ± 0%  ~ (p=1.000 n=10) ¹
Unmarshal/ReferenceFile/struct  83.00 ± 0%  83.00 ± 0%  ~ (p=1.000 n=10) ¹
Unmarshal/ReferenceFile/map  194.0 ± 0%  194.0 ± 0%  ~ (p=1.000 n=10) ¹
Unmarshal/HugoFrontMatter  54.00 ± 0%  54.00 ± 0%  ~ (p=1.000 n=10) ¹
Marshal/SimpleDocument/struct  4.000 ± 0%  4.000 ± 0%  ~ (p=1.000 n=10) ¹
Marshal/SimpleDocument/map  4.000 ± 0%  4.000 ± 0%  ~ (p=1.000 n=10) ¹
Marshal/ReferenceFile/struct  4.000 ± 0%  4.000 ± 0%  ~ (p=1.000 n=10) ¹
Marshal/ReferenceFile/map  91.00 ± 0%  91.00 ± 0%  ~ (p=1.000 n=10) ¹
Marshal/HugoFrontMatter  20.00 ± 0%  20.00 ± 0%  ~ (p=1.000 n=10) ¹
RealWorldContainerdConfig  144.0 ± 0%  144.0 ± 0%  ~ (p=1.000 n=10) ¹
RealWorldViperRead  65.00 ± 0%  65.00 ± 0%  ~ (p=1.000 n=10) ¹
RealWorldViperWrite  33.00 ± 0%  33.00 ± 0%  ~ (p=1.000 n=10) ¹
RealWorldHugoFrontMatterBatch  820.0 ± 0%  820.0 ± 0%  ~ (p=1.000 n=10) ¹
RealWorldGitleaksRules  259.0 ± 0%  259.0 ± 0%  ~ (p=1.000 n=10) ¹
RealWorldPyproject  142.0 ± 0%  142.0 ± 0%  ~ (p=1.000 n=10) ¹
RealWorldGolangciStrict  48.00 ± 0%  48.00 ± 0%  ~ (p=1.000 n=10) ¹
geomean  211.1  211.1  +0.00%
¹ all samples are equal

🤖 Generated with Claude Code