Command-line Options

The herbie command has several subcommands and allows multiple options that influence its search procedure and the types of solutions it finds. These options apply both to the report generator and the one-off command-line tool.

Herbie commands

Herbie can be run both interactively and in batch mode, and can generate output intended either for the command line or the web. We call these different ways of running Herbie different tools. Herbie provides four tools:

herbie web
Use Herbie through your browser. herbie web starts a web server for running Herbie on your local machine, and directs a browser to visit that server.
herbie shell
Starts a command-line interactive shell for using Herbie. Enter an FPCore expression and Herbie will print its more-accurate version.
herbie improve input output
Runs Herbie on the expressions in the input file or directory, and outputs the result to output, which will be a single file of FPCore outputs.
herbie report input output
Runs Herbie on the expressions in the input file or directory, and produces a directory of HTML web pages that describe Herbie's output, how it derived that output, and additional charts and information about the improvement process. These pages can be viewed in any browser (though with a quirk for Chrome).

We recommend using the web tools, web and report, since HTML allows Herbie to give you more information about how and why it improved a floating-point expression's accuracy. Particularly useful are the graphs it produces of error versus input, which can help you understand whether Herbie's improvements matter for your user cases.

For any tool, you can run herbie tool --help to see a listing of all available command-line options. This listing will include unsupported options not listed on this page.

Upgrading from Herbie 1.0

Herbie 1.0 used a different command line syntax, without multiple tools. Translate like so:

The new syntax somewhat changes Herbie's behavior, such as by using the input expression as the output if Herbie times out. It also makes it easier to write Herbie's output to a file without using command-line redirection. The old syntax still works but is deprecated and will be removed in the next release.

General options

These options can be set on any tool. Pass them after the tool name but before other arguments, such as: 

herbie improve --timeout 60 in.fpcore out.fpcore

Arguments cannot be put anywhere else.

--seed S
The random seed, which changes the randomly-selected points that Herbie evaluates candidate expressions on. The format of the seed is that used by the Racket vector->pseudo-random-generator! function; in practice, just use a seed produced by an earlier run. This option can be used to make Herbie's results reproducible or to compare two different runs.
--num-iters N
The number of improvements Herbie attempts to make to the program. The default, 2, suffices for most programs and helps keep Herbie fast. If this is set very high, Herbie may run out of things to do and terminate before the given number of iterations, but in practice iterations beyond the first few rarely lead to lower error. This option can be increased to 3 or 4 to check that there aren't further improvements that Herbie could seek out.
--num-points N
The number of randomly-selected points used to evaluate candidate expressions. The default, 256, gives good behavior for most programs. The more points sampled, the slower Herbie is. This option can be increased to 512 or 1024 if Herbie gives very inconsistent results between runs with different seeds.
--timeout T
The timeout to use per-example, in seconds. A fractional number of seconds can be given.
--threads N, for improve and reports
Enables multi-threaded operation. By default, no threads are used. A number can be passed to this option to use that many threads, or yes can be passed to tell Herbie to use all but one of the hardware threads.

Web shell options

The web tool runs Herbie as a web server, and connects to it from your browser. It has additional options to control this server.

--port N
The port to run the Herbie server on. The default port is 8000.
--save-session dir
Save all the reports for expressions enterred into the web shell to this directory. The directory is also used as a cache of already-computed expressions.
--log file
Write a web access log to this file. The file is formatted similarly to Apache logs. If Herbie crashes for some reason, this log will not contain a traceback.
--quiet
When set, a browser is not started to point to the server main page, and a smaller banner is printed to the command line.

Search options

These options influence the fine properties of Herbie's search, most importantly the types of transformations that Herbie uses to find candidate programs. These options offer very fine-grained control over Herbie's output, and are only recommended for advanced uses of Herbie.

Each option can be turned off with the -o X or --disable X command-line flag, and turned on with the +o X or --enable X. The defaults are the recommended options; turning a default-on option off typically results in less-accurate results, while turning a default-off option on typically results in more-complex and more-surprising output expressions.

precision:double
This option, on by default, tells Herbie to treat its input as double-precision calculations. If turned off, Herbie treats its input as a single-precision calculation.
setup:simplify
This option, on by default, simplifies the expression before passing it to Herbie. If turned off, Herbie will not simplify input programs before improving them. You will want to turn off this option if simplifying the program will create a lot of error, say if the association of operations is cleverly chosen.
setup:early-exit
This option, off by default, causes Herbie to exit without modifying the input program if it determines that the input program has less than 0.1 bits of error. You will want to turn this option on if you are running Herbie on a large corpus of programs that you do not believe to be inaccurate.
generate:rr
This option, on by default, uses Herbie's recursive rewriting algorithm to generate candidate programs. If turned off, Herbie will use a non-recursive rewriting algorithm, which will substantially limit the candidates Herbie finds. You will rarely want to turn this option off.
generate:taylor
This option, on by default, uses series expansion to produce new candidates during the main improvement loop. If turned off, Herbie will not use series expansion in the main improvement loop. You will want to turn this option off if you want to avoid series-expansion-based rewrites, such as if you need to preserve the equivalence of the input and output expressions as real-number formulas.
generate:simplify
This option, on by default, simplifies candidates during the main improvement loop. If turned off, candidates will not be simplified, which typically results in much less accurate expressions, since simplification is often necessary for cancelling terms. You will rarely want to turn this option off.
reduce:regimes
This option, on by default, uses Herbie's regime inference algorithm to branch between several program candidates. If turned off, branches will not be inferred and the output program will be straight-line code (if the input was). You will want to turn this option off if your programming environment makes branches very expensive, such as in some cases of GPU programming.
reduce:taylor
This option, on by default, uses a final set of series expansions after all improvements have been made. This sometimes improves accuracy further. If turned off, this final series expansion pass will not be done. You will want to turn this option off if you want to avoid series-expansion-based rewrites, such as if you need to preserve the equivalence of the input and output expressions as real-number formulas.
reduce:simplify
This option, on by default, uses a final simplification pass after all improvements have been made. This sometimes improves accuracy further. If turned off, this final simplification pass will not be done. You will rarely want to turn this option off.
reduce:avg-error
This option, on by default, causes Herbie to output the candidate with the best average error over the chosen inputs. If turned off, Herbie will choose the candidate with the least maximum error instead. This usually produces programs with worse overall accuracy. You may want to turn this option off if worst-case accuracy is more important to you than overall accuracy.
rules:arithmetic
This option, on by default, allows Herbie to use basic arithmetic facts during its search. If turned off, Herbie will not be able to use those facts. You will rarely want to turn this option off.
rules:polynomials
This option, on by default, allows Herbie to use facts about polynomials during its search. If turned off, Herbie will not be able to use those facts. You will rarely want to turn this option off.
rules:fractions
This option, on by default, allows Herbie to use facts about fractions during its search. If turned off, Herbie will not be able to use those facts. You will rarely want to turn this option off.
rules:exponents
This option, on by default, allows Herbie to use facts about exponents during its search. If turned off, Herbie will not be able to use those facts. You may want to turn this option off in the rare case that you do not want exponents or logarithms used in Herbie's output expressions, which may be the case when code runtime is more important than accuracy.
rules:trigonometry
This option, on by default, allows Herbie to use basic trigonometry facts during its search. If turned off, Herbie will not be able to use those facts. You may want to turn this option off in the rare case that you do not want trigonometric functions used in Herbie's output expressions, which may be the case when code runtime is more important than accuracy.
rules:numerics
This option, off by default, allows Herbie to use special numerical functions. If turned off, Herbie will not be able to use these functions. You will want to turn this option on if these functions (currently expm1, log1p, and hypot) are available in your language.