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Conditional Execution of Transpiler Passes Based on Per-Script Feature Detection

Rishipal Singh Bhatia

Abstract

As the ECMAScript specification evolves, industrial-scale JavaScript compilers face the challenge of supporting modern language syntax while maintaining compatibility for diverse execution environments. Traditionally, compilers solve this by running transpilation passes in a monolithic pipeline, where the transpilation passes are chosen to execute strictly based on a target language level. This results in significant computational waste, as compilers perform expensive Abstract Syntax Tree (AST) traversals to lower features that may not exist in the actual input source code. We present a compiler improvement that conditionally executes transpiler passes based on accurately tracking and dynamically maintaining the exact set of language features present in the compilation unit throughout the transpilation process. It is implemented in the production Google Closure Compiler. By populating and maintaining a FeatureSet at every JavaScript script-level, it dynamically skips running unnecessary lowering passes. We detail the architectural safeguards -- including strategic pass ordering and dynamic validation of the transpiled code for feature-correctness. Evaluation of this improvement on large-scale production monorepos produced a considerable reduction in compilation time and saved compute and memory usage.

Conditional Execution of Transpiler Passes Based on Per-Script Feature Detection

Abstract

As the ECMAScript specification evolves, industrial-scale JavaScript compilers face the challenge of supporting modern language syntax while maintaining compatibility for diverse execution environments. Traditionally, compilers solve this by running transpilation passes in a monolithic pipeline, where the transpilation passes are chosen to execute strictly based on a target language level. This results in significant computational waste, as compilers perform expensive Abstract Syntax Tree (AST) traversals to lower features that may not exist in the actual input source code. We present a compiler improvement that conditionally executes transpiler passes based on accurately tracking and dynamically maintaining the exact set of language features present in the compilation unit throughout the transpilation process. It is implemented in the production Google Closure Compiler. By populating and maintaining a FeatureSet at every JavaScript script-level, it dynamically skips running unnecessary lowering passes. We detail the architectural safeguards -- including strategic pass ordering and dynamic validation of the transpiled code for feature-correctness. Evaluation of this improvement on large-scale production monorepos produced a considerable reduction in compilation time and saved compute and memory usage.
Paper Structure (22 sections, 2 equations, 1 figure, 1 table)

This paper contains 22 sections, 2 equations, 1 figure, 1 table.

Table of Contents

  1. Introduction & Problem Statement
  2. Background: The Transpiler Pipeline
  3. Incremental Lowering & Pass Ordering
  4. The Cost of a Transpiler Pass
  5. Downward Monotonicity of Language Levels
  6. Improvement: Selective Transpilation
  7. Recording Input Features
  8. Mapping transpiler passes to their features
  9. Accurately Tracking the FeatureSet
  10. Synthetic Feature Propagation
  11. The Transpiled-Away Set
  12. Transpiler Pass Controlled FeatureSet
  13. Selective Execution Logic
  14. Architectural Safeguards
  15. Evaluation & Results
  16. ...and 7 more sections

Figures (1)

  • Figure 1: Selective Transpilation Logic: The compiler iterates through scripts for each pass, skipping scripts that lack the targeted feature $f_{source}$ according to their specific FeatureSet $\Phi(S)$.