Checked Load: Architectural support for JavaScript type-checking on mobile processors

Abstract

Dynamic languages such as Javascript are the de-facto standard for web applications. However, generating efficient code for dynamically-typed languages is a challenge, because it requires frequent dynamic type checks. Our analysis has shown that some programs spend upwards of 20% of dynamic instructions doing type checks, and 12.9% on average.In this paper we propose Checked Load, a lowcomplexity architectural extension that replaces softwarebased, dynamic type checking. Checked Load is comprised of four new ISA instructions that provide flexible and automatic type checks for memory operations, and whose implementation requires minimal hardware changes. We also propose hardware support for dynamic type prediction to reduce the cost of failed type checks. We show how to use Checked Load in the Nitro JavaScript just-in-time compiler (used in the Safari 5 browser). Speedups on a typical mobile processor range up to 44.6% (with a mean of 11.2%) in popular JavaScript benchmarks. While we have focused our work on JavaScript, Checked Load is sufficiently general to support other dynamically-typed languages, such as Python or Ruby.

Improving JavaScript performance by deconstructing the type system

“…The offset of the property is stored in a new hidden class ( 9 ). When the code reaches line 4 and tries to assign to property Bar, the property is not found in hidden class 9 . So a new hidden class (11 ) is created that stores the binding from Bar to the new function object (10 ).…”

“…For instance, recent works decrease overheads through program analysis, to statically infer a variable's type [14,16]. Another proposal extends the ISA to load and check the type with a single instruction [9]. Finally, ParaGuard [19] and ParaScript [18] propose hardware support for parallel execution of JavaScript programs.…”

Removing checks in dynamically typed languages through efficient profiling

“…Consequently, the proposed mechanism requires that objects are created aligned to cache lines. Note that this restriction is not costly [20] and both Nitro [26] and Mozilla JavaScript engines [19] already apply it. Moreover, a Class Cache request needs to specify the relative position that the property occupies inside the cache line.…”

“…Inline Caching technique was first introduced in the Smalltalk compilers [21] and it was later used in Self compilers [ [20]. Checked Load [20] introduces automatic checking of types.…”

“…Checked Load [20] introduces automatic checking of types. Basically, checks are not removed, but partially performed implicitly by a specialized hardware instead.…”