Greenthumb: Superoptimizer Construction Framework

GreenThumb is an extensible framework for constructing superoptimizers. It is designed to be easily extended to a new target ISA using inheritance. GreenThumb is implemented in Racket. The top level directory contains ISA-independent files, which implement the superclasses to be extended. We have built superoptimizers for ARM, GreenArrays GA144 (GA), and a small subset of LLVM IR. Directories arm, GA, and llvm contain ISA-specific files for ARM, GA, and LLVM IR respectively.

References

• Greenthumb: Superoptimizer Construction Framework (CC'16) explains the framework overview.
• Scaling Up Superoptimization (ASPLOS'16) explains the search strategy provided by GreenThumb.

Software Prerequisites

• Racket: Download and install drracket version 6.7 from https://racket-lang.org/download/. Other versions of Racket may work, but they are not tested. Include the installed 'bin' directory, which contains racket, drracket, raco, and etc., to the environment path.
• Rosette: Download Rosette v1.1 and follow the instruction to install in Rosette's README file. Don't forget to put Z3 executable in rosette/bin, as GreenThumb depends on Z3 (but not CVC4).
• Python

Setting Up

git clone https://github.com/mangpo/greenthumb.git
cd greenthumb
make

path.rkt will be generated.

Running an Existing Superoptimizer

For example, we will walk through how one can run the ARM supertoptimizer.

Inputs

1. A code file, containing a straight-line ARM code. For example, see arm/programs/p14_floor_avg_o0.s.
2. A meta-data file, containing live-in and live-out information. The name of this file has to be the same as the code file appended with '.info', for example, arm/programs/p14_floor_avg_o0.s.info. The content of the meta-data file for ARM should look like the following:

0
0,1

The first line indicates live-out registers, and the second line indicates live-in registers. In this example, a live-out register is R0, and live-in registers are R0 and R1. Note that currently the tool only supports R registers. Live-in information is no longer used in GreenThumb 2.0.

Run

You can superoptimize one of the example programs or your own program by running

cd arm
racket optimize.rkt <search-type> <search-mode> -c <number-of-search-instances> \
  -t <time-limit-in-sec> <file_to_optimize>

• <search-type> can be --stoch (stochastic search), --sym (symbolic search), --enum (enumerative search) or --hybrid (cooperative search).
• When using --sym or --enum, <search-mode> is either
  • -l or --linear: optimizing by reducing cost incrementally,
  • -b or --binary: optimizing by binary searching on the cost, or
  • -p or --partial: optimizing by synthesizing small parts of the given code using context-aware window decomposition. Option --partial is recommended.
• When using --stoch, <search-mode> is either
  • -s or --synthesize: each search instance starts the search from a random program, or
  • -o or --optimize: each search instance starts the search from the original input program.

For example, to optimize arm/programs/p14_floor_avg_o0.s using cooperative search running all search techniques using eight search instances for an hour, run

racket optimize.rkt --hybrid -p -c 8 -t 3600 programs/p14_floor_avg_o0.s

Run racket optimize.rkt --help to see all supported arguments and what their default values are.

Outputs

An output directory containing driver-<id>.rkt files will be created. The default name of the output directory is output. Use -d flag to specify the output directory's name. Each driver-<id>.rkt file runs a search instance.

Each driver-<id>.rkt

• updates the shared file best.s, which always contains the current best program, if the search instance finds a better program than the current best.
• updates summary file, which contains the statistic of best programs found at different points of time.
• writes debug and error messages to driver-<id>.log

At the beginning, the search driver will report the numbers of search instances allocated for different search techniques.

SEARCH INSTANCES
----------------
stoch:	2 instances
sym:	2 instances
enum:	4 instances

ID 0-1: stoch (optimize)          << driver-0 and 1 run stochastic search.
ID 2-2: sym (window=L)            << driver-2 runs symbolic search.
ID 3-3: sym (window=2L)           << driver-3 runs symbolic search.
ID 4-4: enum (no-decomposition)   << driver-4 runs enumerative search (no window).
ID 5-6: enum (window=L)           << driver-5 and 6 run enumerative search.
ID 7-7: enum (window=2L)          << driver-7 run enumerative search.

X in (window=X) indicates the size of window used in the context-aware window decomposition.

Then, the search driver will report the status of the search every 10 seconds.

// This first part is the statistics from all stochastic search instances.
elapsed time:   74
mutate time:    0.080541	<< (time spent on mutating)/total
simulate time:  0.126095	<< (time spent on interpreting)/total
check time:     0.054595	<< (time spent on checking actual outputs against expected outputs)/total
validate time:  0.702649	<< (time spent on validating the correctness using solver)/total

iterations/s:      480.18
best cost:         5       << best cost despite the correctness
best correct cost: 5       << best performance cost considering the correctness
best correct time: 41      << time in seconds to find the best correct proposal

Mutate     Proposed  Accepted  Accepted/Proposed
opcode	   0.282484  0.058890  0.208473
operand	   0.182736  0.018227  0.099747
swap	   0.266824  0.092875  0.348078
inst	   0.118275  0.017906  0.151396
nop	       0.149678  0.015671  0.10470257303795003

accept-count:        0.047734  << rate of accepting mutated programs
accept-higher-count: 0.000457  << rate of accepting mutated programs with higher cost

// This second part summarizes the charateristics of the best program found so far.
// This section only appears if the superoptimizer found better program(s).
=============== SUMMARY ===============
cost:	3            << cost of the best program found so far
len:	3            << length of the best program found so far
time:	44           << time in seconds to find the best program
output/0/driver-7    << the best program is found by driver-7 (enumerative search in this example).

Press Ctrl-C to end the process early or wait until time is up. At the end, the search driver will print out the optimized program. Below is the output program when optimizing p14_floor_avg_o0.s.

OUTPUT
------
and r2, r0, r1
eor r3, r0, r1
add r0, r2, r3, asr 1

Reduce Memory Usage

To reduce the memory usage and the overhead of Racket translating program to bytecode, we can precompile our superoptimizer application to bytecode by running in command line:

raco make <list_of_flies_to_be_compiled>

For example, in ARM directory, run:

raco make test-search.rkt ../parallel-driver.rkt

More Documentation

• Extending GreenThumb to a New ISA
• Adding more instructions to an existing superoptimizer
• Special Objects for Program State
• Advanced Usage

Inquery and Bug Report

If you are interested in using GreenThumb, please feel free to contact mangpo [at] eecs.berkeley.edu.