perf: extend comprehension scope reuse to builtins, select, lookup#783
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stephenamar-db merged 1 commit intodatabricks:masterfrom Apr 21, 2026
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Motivation: The mutable scope reuse path in comprehension evaluation avoids O(n) ValScope allocations (Arrays.copyOf per iteration), but was gated by isNonCapturingBody which only covered ValidId, BinaryOp, UnaryOp, And, Or, IfElse, and Literal. Common patterns like [std.length(x) for x in arr] or [x.field for x in arr] fell through to the per-iteration scope allocation path. Modification: Extend isNonCapturingBody to cover ApplyBuiltin0-4, Select, and Lookup with recursive argument checks. These produce eager values via visitExpr and don't create closures that capture the comprehension scope. The same expression types are already handled by isInvariantExpr. Result: More comprehension patterns benefit from mutable scope reuse, eliminating per-iteration ValScope allocation + Arrays.copyOf overhead.
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Summary
isNonCapturingBodyto recognize more expression types that produce eager values without capturing the comprehensionValScope[std.length(x) for x in arr]or[x.field for x in arr], avoiding per-iterationValScopeallocation viascope.extendSimple()+Arrays.copyOfNew expression types covered
ApplyBuiltin0ApplyBuiltin1isNonCapturingBody(a1)ApplyBuiltin2ApplyBuiltin3ApplyBuiltin4SelectisNonCapturingBody(value)LookupSafety argument
The mutable scope reuse path evaluates the body with
visitExpr(eager). The returnedValdoesn't capture the comprehension scope. InternalvisitAsLazycalls within builtins receive non-capturing sub-expressions, sovisitAsLazyreturns eagerly viatryEagerEvalor direct binding lookup — noLazyExprcreation, no stale scope capture.NOT extended to:
Apply0-3(user-defined function calls may create closures that capture scope),Function(createsVal.Funccapturing scope),ObjBody.MemberList(createsVal.Objwith lazy fields capturing scope).Test plan
./mill 'sjsonnet.jvm[3.3.7]'.test— all tests pass./mill 'sjsonnet.jvm[3.3.7]'.compile— compiles cleanisInvariantExpr(line 478+) already covers the same expression types, confirming this pattern is established