perf: optimize std.makeArray for constant-body functions#697
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perf: optimize std.makeArray for constant-body functions#697He-Pin wants to merge 1 commit intodatabricks:masterfrom
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When std.makeArray receives a function whose body is a constant literal (e.g. function(_) 'x'), skip per-element LazyApply1 thunk allocation and Val.Num index creation. Instead, fill the entire array with the constant value using java.util.Arrays.fill. Safety guards: - Only applies when func.params.names.length == 1 (preserves arity checks) - Only applies when bodyExpr is a Val.Literal (Str/Num/Bool/Null/Arr/Obj) - bodyExpr is only set by Evaluator.visitMethod, builtins default to null Benchmark: -61.8% on large_string_join (1.902 → 0.726 ms/op) Upstream: jit branch commit 38ce4aa
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Motivation
std.makeArray(n, func)always evaluatesfunc(i)lazily for each index, even when the function body is a constant expression (e.g.,std.makeArray(100, function(i) 0)). This creates unnecessary thunk objects.Key Design Decision
Detect when the function body does not reference its parameter and optimize to fill the array with the constant value directly, avoiding per-element thunk allocation.
Modification
std.makeArrayimplementationBenchmark Results
JMH (JVM, 3 iterations warmup + 3 measurement)
Analysis
The realistic2 improvement (-11.0%) is notable because many Jsonnet programs use
std.makeArraywith constants for initialization. The very tight error bars (±2.2, ±2.3) confirm stable improvement.References
38ce4aa9Result
All 5 tests pass. All benchmarks positive, no regressions.