Add InitLocals instruction for bulk local initialization

[antoniosarosi]

Summary

• Adds a new InitLocals(n) bytecode instruction that pushes n null values onto the stack in one shot, replacing N consecutive LoadConst(null) instructions emitted at the start of every function for local variable pre-allocation.
• Uses Vec::resize for the VM handler instead of per-element push, avoiding N dispatch cycles through the main match loop.
• Reduces bytecode size across all 33 codegen snapshots (e.g., 7 instructions → 1 for the common baml.llm.call_llm_function prologue).

Test plan

• All 139 baml_compiler_emit tests pass
• All 375 baml_tests pass (33 codegen snapshots auto-updated)
• All 24 bex_vm tests pass
• cargo fmt --check clean
• cargo clippy clean on affected crates

Summary by CodeRabbit

• Refactor
  • Consolidated local variable pre-allocation into a single bytecode initialization step instead of multiple per-slot null pushes.
• New Features
  • VM and bytecode now recognize an explicit local-initialization instruction; debugger output and coloring updated accordingly.
• Tests
  • Updated emitted-code expectations and test harness to reflect the consolidated local initialization.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
beps	Ready	Preview, Comment	Feb 17, 2026 8:20pm
promptfiddle	Ready	Preview, Comment	Feb 17, 2026 8:20pm

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📝 Walkthrough

Walkthrough

Introduce a new InitLocals(usize) bytecode instruction to pre-allocate N local slots; replace emitted per-slot LoadConst(Value::Null) sequences with a single InitLocals(n). Compiler emit, VM types, VM executor, test harness, and tests updated to emit, represent, execute, and validate the instruction. (44 words)

Changes

Cohort / File(s)	Summary
Compiler Emission baml_language/crates/baml_compiler_emit/src/emit.rs	Replaced per-slot LoadConst(Value::Null) emissions with a single InitLocals(slots_to_allocate) emission for local pre-allocation.
Compiler Tests baml_language/crates/baml_compiler_emit/tests/classes.rs, baml_language/crates/baml_compiler_emit/tests/for_loops.rs, baml_language/crates/baml_compiler_emit/tests/if_else.rs, baml_language/crates/baml_compiler_emit/tests/maps.rs, baml_language/crates/baml_compiler_emit/tests/match_expr.rs, baml_language/crates/baml_compiler_emit/tests/operators.rs, baml_language/crates/baml_compiler_emit/tests/watch.rs, baml_language/crates/baml_compiler_emit/tests/while_loops.rs	Updated expected MIR/bytecode in tests to use InitLocals(n) instead of sequences of LoadConst(Value::Null); only the initialization prelude changed.
Test Framework baml_language/crates/baml_tests/src/vm.rs, baml_language/crates/baml_tests/src/codegen.rs	Added InitLocals(usize) to the test Instruction enum and mapping from runtime instructions to test instructions.
VM Execution baml_language/crates/bex_vm/src/vm.rs	Implemented execution for InitLocals(count) by extending the evaluation stack with count null-initialized slots.
VM Debug Support baml_language/crates/bex_vm/src/debug.rs	Display handling added for InitLocals(_) and assigned debugger color Green.
VM Types / Bytecode baml_language/crates/bex_vm_types/src/bytecode.rs	Added public InitLocals(usize) instruction variant and Display formatting as INIT_LOCALS {n}.

Sequence Diagram(s)

mermaid
sequenceDiagram
participant Compiler as Compiler (emit)
participant Bytecode as Bytecode (Instruction::InitLocals)
participant VM as VM Executor
participant Tests as Test Harness
Note over Compiler,Bytecode: Emit consolidated local pre-allocation
Compiler->>Bytecode: emit InitLocals(n)
Bytecode->>Tests: included in expected instruction stream
Tests->>VM: load bytecode (contains InitLocals)
VM->>VM: execute InitLocals -> expand eval stack by n (Null)
VM->>VM: continue executing subsequent instructions

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• VM cleanup: remove dead code, refactor watch graph, fix GC tracing #3108: Modifies VM bytecode/instruction set and related instruction handling; likely related to instruction-set changes.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Add InitLocals instruction for bulk local initialization' accurately describes the main change: introducing a new instruction that replaces multiple LoadConst(null) operations with a single bulk initialization operation.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch antonio/vm-locals-init

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[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 15 untouched benchmarks
⏩ 84 skipped benchmarks¹

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_{Comparing antonio/vm-locals-init (a65666a) with canary (9215d51)}

Footnotes

1. 84 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[coderabbitai]

Actionable comments posted: 1

[github-actions]

Binary size checks passed

✅ 7 passed

	Artifact	Platform	Gzip	Baseline	Delta	Status
✅	bridge_cffi	Linux	4.0 MB	4.0 MB	+393 B (+0.0%)	OK
✅	bridge_cffi-stripped	Linux	2.1 MB	2.1 MB	+98 B (+0.0%)	OK
✅	bridge_cffi	macOS	3.3 MB	3.3 MB	+340 B (+0.0%)	OK
✅	bridge_cffi-stripped	macOS	1.7 MB	1.7 MB	+207 B (+0.0%)	OK
✅	bridge_cffi	Windows	3.3 MB	3.3 MB	+539 B (+0.0%)	OK
✅	bridge_cffi-stripped	Windows	1.8 MB	1.8 MB	+817 B (+0.0%)	OK
✅	bridge_wasm	WASM	1.3 MB	1.3 MB	+223 B (+0.0%)	OK

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Generated by cargo size-gate · workflow run