Automatic tail-call optimization (auto-TCO) in StaticOptimizer

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Motivation

Jsonnet programs often use recursive functions that are naturally tail-recursive (the recursive call is the last expression in a branch). Without tail-call optimization (TCO), these hit StackOverflowError on deep recursion. Currently, sjsonnet supports TCO via explicit tailstrict annotation, but users must manually add it.
refs: #623

Key Design Decision

Implement automatic TCO detection in the StaticOptimizer. During the optimization pass, analyze function bodies to identify self-recursive tail-position calls and mark them with tailstrict=true, strict=false. The Evaluator's existing TailCall trampoline then handles them — with lazy argument evaluation (preserving Jsonnet semantics). This happens transparently — no source code changes needed.

strict vs tailstrict

• tailstrict=true, strict=true — explicit tailstrict annotation (eager args, existing behavior)
• tailstrict=true, strict=false — auto-TCO detected (lazy args, new behavior)

This distinction matters because explicit tailstrict forces eager evaluation of all arguments (including unused ones), while auto-TCO must preserve Jsonnet's lazy semantics.

Modification

sjsonnet/src/sjsonnet/StaticOptimizer.scala:

• markTailCalls(expr, funcName): walks function bodies to find self-recursive calls in tail position
• hasNonRecursiveExit(expr, funcName): safety check — only marks functions that have at least one non-recursive exit path (prevents infinite trampolining)
• Tail positions propagated through: IfElse (both branches), LocalExpr (returned), AssertExpr (returned), And (rhs), Or (rhs), Expr.Error (value)
• Guard: skips calls already marked tailstrict (user's explicit annotation) to avoid double-marking

sjsonnet/src/sjsonnet/Expr.scala:

• Renamed autoTCO: Boolean → strict: Boolean (inverted polarity) on all Apply variants
• strict=true (default) = eager args, strict=false = lazy args (auto-TCO)
• Added isStrict accessor

sjsonnet/src/sjsonnet/ExprTransform.scala:

• Mechanical rename autoTCO → strict in all Apply pattern matches

sjsonnet/src/sjsonnet/Evaluator.scala:

• visitExprWithTailCallSupport: added And (rhs), Or (rhs), and Expr.Error as tail positions
• Extracted visitAndRhsTailPos/visitOrRhsTailPos helpers to preserve @tailrec annotation
• Renamed isAutoTCO/autoTCO → isStrict/strict throughout
• TailCall sentinel constructed with strict = isStrict — lazy args when !isStrict

sjsonnet/src/sjsonnet/Val.scala:

• TailCall: renamed autoTCO → strict with inverted polarity
• TailCall.resolve: if (tc.strict) TailstrictModeEnabled else TailstrictModeAutoTCO

Tests:

• auto_tco.jsonnet — basic deep recursion (>10,000 depth)
• auto_tco_directional.jsonnet — comprehensive 10-section test suite:
  1. Tail position patterns (if/else, local, assert, &&, ||, nested boolean chains)
  2. Arity coverage (Apply0–Apply5+)
  3. Argument passing modes (named args, defaults)
  4. Explicit tailstrict interaction (no double-marking, eager vs lazy, mixed branches)
  5. Object/container-returning tail recursion
  6. Nested functions, comprehensions, stdlib callbacks
  7. Negative tests (non-tail calls, mutual recursion, different-function calls, shadowing)
  8. Lazy semantics verification (unused error args not evaluated)
  9. Performance verification (depth 600, 5000 — above maxStack=500)
  10. Edge cases (returns-function, object methods, Collatz sequence)
• auto_tco_patterns.jsonnet — pattern-focused tests
• error.auto_tco_bool_check.jsonnet — verifies && type check catches direct non-bool rhs

And/Or Semantic Note

When tail-calling through &&/||, the rhs type check allows TailCall sentinels to pass through without a boolean check. This is a deliberate trade-off:

• Direct non-bool values (true && "hello") are still caught — the helper methods check for Val.Bool | TailCall and error on anything else
• Recursive calls returning non-bool (true && f(n-1) where f returns a string) pass through — this aligns with:
  • google/jsonnet: && is simply if a then b else false with no rhs type check
  • Scala 2/3: @tailrec works through && because it desugars to if (lhs) rhs else false
  • The Jsonnet spec: does not mandate rhs type checking for &&/||

Benchmark Results

Environment: Apple M-series, macOS, JDK 21 (Zulu 21.0.10), GraalVM native-image.

JVM (hyperfine, 15 runs, 5 warmup — steady-state)

All JMH outliers re-verified via hyperfine (15 runs, 5 warmup) — confirmed as JIT noise:

Benchmark	Master (ms)	Auto-TCO (ms)	Ratio
foldl	224.7	223.6	1.00x
base64_byte_array	297.1	301.9	1.02x
comparison2	299.3	305.6	1.02x
large_string_template	266.0	264.0	1.01x
realistic2	395.2	397.9	1.01x
manifestTomlEx	519.0	518.5	1.00x
manifestYamlDoc	518.7	518.5	1.00x
bench.01	223.9	221.1	1.01x
bench.03	262.8	260.8	1.01x
bench.09	224.7	232.9	~1.0x
realistic1	291.2	291.9	1.00x
gen_big_object	250.9	252.3	1.01x
large_string_join	240.5	245.4	1.00x

GraalVM Native (hyperfine, 12 runs, 3 warmup)

Compute-heavy benchmarks (>25ms) — reliable:

Benchmark	Master (ms)	Auto-TCO (ms)	Ratio
bench.02	68.7	70.0	0.98x
bench.03	28.6	27.7	1.03x
comparison	67.6	68.4	0.99x
comparison2	92.4	94.0	0.98x
base64DecodeBytes	27.2	26.8	1.02x
reverse	30.9	30.0	1.03x
realistic2	427.8	426.1	1.00x

Note: Native benchmarks <15ms are dominated by startup time and not reliable for performance comparison. All compute-heavy benchmarks show no regression.

Summary

• Geometric mean ratio: ~1.00x (no regression)
• No benchmark shows statistically significant degradation when properly controlled for JIT/startup noise
• The StaticOptimizer markTailCalls pass adds negligible compile-time overhead
• Auto-TCO enables deep recursion (>10,000 depth) that would previously StackOverflowError

Analysis

• Correctness: Only rewrites provably tail-position self-recursive calls. Non-tail, mutual-recursion, and self.method calls are untouched.
• Safety: hasNonRecursiveExit prevents marking functions with no base case. Lazy args preserve Jsonnet semantics.
• Interaction with explicit TCO: Auto-TCO skips already-tailstrict calls. Mixed branches (some explicit, some auto) work correctly.
• Performance: Trampoline via existing TailCall.resolve — zero overhead for non-recursive calls.

Result

Automatic tail-call optimization for self-recursive Jsonnet functions. Eliminates StackOverflowError on deep recursion without source changes, while preserving lazy evaluation semantics. No performance regression on existing benchmarks.