perf: foldl string concat O(n) StringBuilder optimization#665
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Detect the pattern std.foldl(function(acc, elem) acc + elem, arr, stringInit) at runtime by inspecting the function's body AST. When the body is a BinaryOp(OP_+, ValidId(param0), ValidId(param1)) and the initial value is a string, use a StringBuilder for O(n) instead of O(n²) concatenation. Changes: - Val.Func: add bodyExpr hook (default null) for AST inspection - Evaluator.visitMethod: override bodyExpr to expose function body - ArrayModule.Foldl: add tryStringBuilderFoldl fast path that detects the concatenation pattern and builds the result in a single pass This addresses the 88x gap with jrsonnet on the foldl string concat benchmark by converting from quadratic string copying to linear StringBuilder appending. Upstream: jit branch commit 2d3e56d
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Motivation
std.foldlwith string concatenation has O(n²) complexity because each+creates a new string. This detects the foldl+string-concat pattern and uses a StringBuilder for O(n) complexity.Key Design Decision
Detection at runtime in the foldl hot loop rather than static analysis. When foldl accumulator is a string and the function body performs string concatenation, we switch to a StringBuilder-based fast path that avoids quadratic string copying.
Modification
Added
tryStringBuilderFoldlinArrayModule.scalathat detects the string concat pattern and uses StringBuilder. Falls back to standard foldl for non-string cases.Benchmark Results
JMH Regression Suite (1 fork, 3 warmup, 1 measurement)
All other benchmarks within noise margin.
Scala Native Hyperfine (
-N -w4 -m20)Analysis
This is the single largest performance improvement in the entire optimization suite. The O(n²) → O(n) complexity change produces dramatic speedups that scale with input size. On native, this makes sjsonnet nearly 80x faster than jrsonnet (Rust) for this workload.
References
Upstream jit branch exploration at he-pin/sjsonnet@jit
Result
Massive reduction in foldl string concatenation time. bench.04 goes from 507ms to 6.9ms on native (73.9x speedup). sjsonnet is now 79.9x faster than jrsonnet for this benchmark.