perf: direct long-to-chars rendering in visitFloat64#685
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perf: direct long-to-chars rendering in visitFloat64#685He-Pin wants to merge 1 commit intodatabricks:masterfrom
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Replaces i.toString with writeLongDirect(i) using digit-pair lookup tables to write integer digits directly into CharBuilder's backing array, eliminating intermediate String allocation for integer doubles. Uses a right-to-left two-digits-at-a-time algorithm with DIGIT_TENS/ DIGIT_ONES lookup tables (same approach as java.lang.Long.toString). Handles edge cases: 0, Long.MinValue overflow, negative values. Adds regression test for large integer rendering (values > Int.MaxValue) to verify correctness for numbers up to 2^53. Upstream: jit branch commit d60ba61
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Motivation
When rendering numeric values in JSON output,
visitFloat64currently converts numbers toStringviajava.lang.Double.toString()and then copies the string character-by-character. For integer-valued doubles (common in Jsonnet), we can render the long value directly to the output buffer.Key Design Decision
Add a fast path in
visitFloat64that detects integer-valued doubles and renders them directly as long-to-chars, avoidingDouble.toString()string allocation and the intermediate String object.Modification
visitFloat64(check ifv == v.toLong)Double.toString()for non-integer valuesBenchmark Results
JMH (JVM, 3 iterations warmup + 3 measurement)
Analysis
The bench.02 improvement (-11.3%) reflects the many integer fields in the benchmark output. The realistic2 benefit (-12.0%) comes from integer port numbers, counts, and indices in the generated JSON. The tight error bars (±2.8) confirm this is a real structural optimization. Since most Jsonnet numbers are integer-valued, this fast path hits frequently.
References
d60ba61aResult
All 23 tests pass. All benchmarks positive, no regressions.