The Challenges and Opportunities of Compiling Ruby for Faster Performance

While there is indeed a promising fast compiler for Ruby, namely MacRuby (in which I am a core contributor), the reality is that the Ruby ecosystem lacks a widely-used compiler. This article delves into the complexities and opportunities associated with compiling Ruby to improve performance.

The Core Problems: Ruby's Flexibility and Open Classes

One of the primary reasons for the absence of a universally accepted Ruby compiler is the language's remarkable flexibility and open classes. Unlike statically typed languages, Ruby allows for dynamic modification of classes at runtime, making it challenging for compilers to determine the exact semantics of Ruby code accurately.

Take, for instance, MacRuby's approach to precompiling arithmetic operators into LLVM bitcode. MacRuby must first check whether users have modified the Numeric class and its arithmetic operators. If the operators have been redefined, the compiler must follow the slower path, relying on dynamic dispatch based on the updated operator definitions. However, if the Numeric class remains unmodified, the arithmetic operations can be efficiently compiled into an LLVM addition operation.

This corner case might seem trivial, but in practice, it occurs often enough to significantly impact performance. The flexibility of Ruby's open classes not only complicates the compilation process but also limits the extent to which code can be optimized.

Dependence on Metaprogramming and Runtime Behavior

Ruby programmers heavily rely on metaprogramming and runtime behavior, which further hinders the development of a fast compiler. Functions like eval and send are prime examples. Evaluating an arbitrary string always necessitates the presence of a Ruby interpreter, which means there is no straightforward way to avoid this process.

To encourage better practices, MacRuby users are advised to avoid eval whenever possible and instead use blocks with more predictable semantics. However, many Ruby libraries and frameworks depend on functionality such as eval and send, making it difficult to fully optimize these code paths.

Lack of Homogeneity in Ruby Implementations

A significant hurdle in compiling Ruby is the lack of homogeneity among different Ruby interpreters. Developers prefer the canonical 1.8/1.9 interpreter, and compiling Ruby to produce a version that is fully compatible with this interpreter is a complex task.

Efforts to compile Ruby code have often faced small but significant incompatibilities with the canonical interpreter. However, the RubySpecs project provides a framework to categorize and resolve such incompatibilities, enabling developers to create more compatible versions of Ruby code.

In summary, while the challenges of compiling Ruby are significant, they are not insurmountable. Efforts like MacRuby demonstrate that it is possible to improve performance through targeted optimization. However, the ultimate solution will require a broad consensus and consistent effort from both the Ruby community and compiler developers.

By addressing these challenges, we can accelerate the adoption of compiled Ruby and unlock the full potential of this powerful language for performance-critical applications.