How to select superinstructions for Ruby ZAKIROV Salikh*, CHIBA Shigeru*, and SHIBAYAMA Etsuya** * Tokyo Institute of Technology, dept. of Mathematical and Computing Sciences ** Tokyo University, Information Technology Center

Ruby Dynamic language Becoming popular recently Numeric benchmarks 100—1000 times slower than equivalent program in C Numeric benchmarks marked in red * 2

Interpreter optimization efforts Many techniques to optimize interpreter were proposed – Threaded interpretation – Stack top caching – Pipelining – Superinstructions Superinstructions – Merge code of operations executed in sequence 3 Focus of this presentation

Superinstructions (contrived example) PUSH: // put argument on stack stack[sp++] = *pc++; goto **pc++; ADD: // add two topmost values on stack sp--; stack[sp-1] += stack[sp]; goto **pc++; PUSH_ADD: // add to stack top stack[sp++] = *pc++; //goto **pc++; sp--; stack[sp-1] += stack[sp]; goto **pc++; PUSH_ADD: // add to stack top stack[sp-1] += *pc++; goto **pc++; Dispatch eliminated Optimizations applied 4

Superinstructions (effects) Effects 1.Reduce dispatch overhead a.Eliminate some jumps b.Provide more context for indirect branch predictorby replicating indirect jump instructions 2.Allow more optimizations within VM op 5

Good for reducing dispatch overhead Superinstructions help when: VM operations are small (~10 hwop/vmop) Dispatch overhead is high (~50%) Examples of successful use in prior research ANSI C interpreter: 2-3 times improvement (Proebsting 1995) Ocaml: more than 50% improvement (Piumarta 1998) Forth: 20-80% improvement (Ertl 2003) 6

Superinstructions help when: VM operations are small (~10 hwop/vmop) Dispatch overhead is high (~50%) Ruby does not fit well Hardware profiling data on Intel Core 2 Duo hardware ops per VM op Only 1-3% misprediction overhead on interpreter dispatch 7 BUT

Superinstructions for Ruby We experimentally evaluated effect of “naive” superinstructions on Ruby – Superinstructions are selected statically – Frequently occurring in training run combinations of length 2 selected as superinstructions – Training run uses the same benchmark – Superinstructions constructed by concatenating C source code, C compiler optimizations applied 8

Naive superinstructions effect on Ruby 9 Number of superinstructions used Normalized execution time Limited benefit Unpredictable effects 4 benchmarks

Branch mispredictions 10 Number of superinstructions used Normalized execution time 2 benchmarks: mandelbrot and spectral_norm

Branch mispredictions, reordered 11 Number of superinstructions used, reordered by execution time Normalized execution time 2 benchmarks: mandelbrot and spectral_norm

So why Ruby is slow? Profile of numeric benchmarks 12 Garbage collection takes significant time Boxed floating point values dominate allocation

Floating point value boxing 13 OPT_PLUS: VALUE a = *(sp-2); VALUE b = *(sp-1); /*... */ if (CLASS_OF(a) == Float && CLASS_OF(b) == Float) { sp--; *(sp-1) = NEW_FLOAT(DOUBLE_VALUE(a) + DOUBLE_VALUE(b)); } else { CALL(1/*argnum*/, PLUS, a); } goto **pc++; New “box” object is allocated on each operation Typical Ruby 1.9 VM operation

Proposal: use superinstructions for boxing optimization 2 operation per allocation instead of 1 14 OPT_MULT_OPT_PLUS: VALUE a = *(sp-3); VALUE b = *(sp-2); VALUE c = *(sp-1); /*... */ if (CLASS_OF(a) == Float && CLASS_OF(b) == Float && CLASS_OF(c) == Float) { sp-=2; *(sp-1) = NEW_FLOAT(DOUBLE_VALUE(a) + DOUBLE_VALUE(b)*DOUBLE_VALUE(c)); } else { CALL(1/*argnum*/, MULT/*method*/, b/*receiver*/); CALL(1/*argnum*/, PLUS/*method*/, a/*receiver*/); } goto **pc++; Boxing of intermediate result eliminated

Implementation 15 VM operations that handle floating point values directly: – opt_plus – opt_minus – opt_mult – opt_div – opt_mod We implemented all 25 combinations of length 2 – Based on Ruby – Using existing Ruby infrastructure for superinstructions with some modifications

Limitations Coding style-sensitive Not applicable to other types (e.g. Fixnum, Bignum, String) – Fixnum is already unboxed – Bignum and String cannot be unboxed Sequences of 3 arithmetic instructions or longer virtually non-existent – No occurrences in the benchmarks 16

Evaluation Methodology – median time of 30 runs Reduction in allocation 17

Results Up to 22% benefit on numeric benchmarks No slowdown on other benchmarks 18

Example: mandelbrot tweak 19 ITER.times do - tr = zrzr - zizi + cr + tr = cr + (zrzr - zizi) - ti = 2.0*zr*zi + ci + ti = ci + 2.0*zr*zi Slight modification produces 20% difference in performance – 4 of 9 arithmetic instructions get merged into 2 superinstructions – 24% reduction in float allocation Normalized execution time

Discussion of alternative approaches Faster GC would improve performance as well – Superinstructions still apply, but with reduced benefit Type inference – Would allow to specialize expressions and eliminate boxing – Interoperability with dynamic code is an issue Dynamic specialization – Topic for further research 20

Related work: Tagged values Use lower bits of pointers to trigger alternative handling Embed floating point value into higher bits Limited to 64-bit platforms, as Ruby uses double precision 64 bit floating point arithmetic – Our approach has same effect on 32 and 64 bit platforms Allows to eliminate majority of boxed floats Provides 28-35% benefit (on the same benchmarks) 21 * Sasada 2008

Related work: Lazy boxing Java-like language with generics over value- types Boxing needed to avoid duplication of template instantiation code for primitive types Lazy optimization works by allocating boxed objects in the stack frame, and moving to heap as needed Relies on static compiler analysis for escape path detection, and runtime checks 22 * Owen 2004

Related work:Superinstructions Superinstructions used for code compression – ANSI C hybrid compiler-interpreter – Trimedia code compression system Superinstructions chosen statically to minimize code size Superinstructions used to reduce dispatch overhead – Forth, Ocaml Superinstructions chosen dynamically 23 * Piumarta 1998 * Proebsting 1995 * Hoogerbrugge 1999 * Ertl 2003

Conclusion Naive approach to superinstructions does not produce substantial benefit for Ruby Floating point values boxing overhead is a problem of Ruby Superinstructions provide some help (up to 22%) Future work Eliminate float boxing further – Specializing computation loop 24