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xo is a superoptimizer, a tool for generating optimal assembly programs.


xo requires that BuDDy, a package for manipulating binary decision diagrams, is installed on your system.

To install xo, do the following:


This only applies if you've fetched the code from the development repository, not if you've obtained it by downloading a release version.

./configure CPPFLAGS='-I/path/to/bdd/include' LDFLAGS='-L/path/to/bdd/lib'

Passing CPPFLAGS and LDFLAGS is only required if you've installed BuDDy in a non-standard location.


Build the package.

make check

Run the test suite. This is optional, but recommended.

make install

Install the package on your system.


xo has four forms:

xo [-m M] [-qv] PROGRAM

In its standard form, xo generates programs and tests them for equivalence with the given program.

By default, xo will generate programs up to 5 instructions in length, test them by running them on test vectors (which may yield false positives), and by modeling them as binary decision diagrams, (which should yield correct output, barring any bugs in the implementation), and print those which pass both tests on stdout.

The -m option sets the maximum length of the generated programs.

The -v option increases verbosity. Passing it once causes xo to also print programs which pass the probabilistic test, as well as some diagnostic information, on stderr. Passing it twice causes xo to also print every generated program on stderr, which impacts performance considerably.

The -q option decreases verbosity, and is currently useless.

The syntax used for the assembly programs is detailed below.

xo -L

In this form, xo prints a list of the supported instructions, along with their arity, on stdout, and exits.

xo -H

In this form, xo prints brief usage instructions on stdout and exits.

xo -V

In this form, xo prints its version number on stdout and exits.


xo supports a small subset of the x86 instruction set. Intel syntax is used, with some modifications:

  • Registers are named r0--r7.
  • Statements are terminated by semicolons.
  • Immediate values are not supported.
  • Effective addresses are not supported.


add r0,r0; sbb r1,r1; sub r1,r0; adc r0,r1;


xo is a prototype, and has numerous shortcomings:

  • Support for many important instructions, such as mul and lea, as well as shift and rotate instructions, is missing.
  • Immediate values are not supported.
  • Multiple output registers are not supported. The output register is assumed to be the last register written to.
  • Only 32-bit arithmetic is simulated.
  • The equivalence test is unnecessarily slow, as values for the input program are not cached.
  • The code for the generator is a mess.


xo is free and unencumbered software released into the public domain. See UNLICENSE for details.


xo was created by Jesper Särnesjö, as part of a degree project at KTH.


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