<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <title>A memoization routine for GNU Make functions — Luke Shumaker</title> <link rel="stylesheet" href="assets/style.css"> <link rel="alternate" type="application/atom+xml" href="./index.atom" name="web log entries"/> </head> <body> <header><a href="/">Luke Shumaker</a> » <a href=/blog>blog</a> » make-memoize</header> <article> <h1 id="a-memoization-routine-for-gnu-make-functions">A memoization routine for GNU Make functions</h1> <p>I'm a big fan of <a href="https://www.gnu.org/software/make/">GNU Make</a>. I'm pretty knowledgeable about it, and was pretty active on the help-make mailing list for a while. Something that many experienced make-ers know of is John Graham-Cumming's "GNU Make Standard Library", or <a href="http://gmsl.sourceforge.net/">GMSL</a>.</p> <p>I don't like to use it, as I'm capable of defining macros myself as I need them instead of pulling in a 3rd party dependency (and generally like to stay away from the kind of Makefile that would lean heavily on something like GMSL).</p> <p>However, one really neat thing that GMSL offers is a way to memoize expensive functions (such as those that shell out). I was considering pulling in GMSL for one of my projects, almost just for the <code>memoize</code> function.</p> <p>However, John's <code>memoize</code> has a couple short-comings that made it unsuitable for my needs.</p> <ul> <li>Only allows functions that take one argument.</li> <li>Considers empty values to be unset; for my needs, an empty string is a valid value that should be cached.</li> </ul> <p>So, I implemented my own, more flexible memoization routine for Make.</p> <pre><code># This definition of `rest` is equivalent to that in GMSL rest = $(wordlist 2,$(words $1),$1) # How to use: Define 2 variables (the type you would pass to $(call): # `_<var>NAME</var>_main` and `_<var>NAME</var>_hash`. Now, `_<var>NAME</var>_main` is the function getting # memoized, and _<var>NAME</var>_hash is a function that hashes the function arguments # into a string suitable for a variable name. # # Then, define the final function like: # # <var>NAME</var> = $(foreach func,<var>NAME</var>,$(memoized)) _main = $(_$(func)_main) _hash = __memoized_$(_$(func)_hash) memoized = $(if $($(_hash)),,$(eval $(_hash) := _ $(_main)))$(call rest,$($(_hash)))</code></pre> <p>However, I later removed it from the Makefile, as I <a href="https://projects.parabola.nu/~lukeshu/maven-dist.git/commit/?id=fec5a7281b3824cb952aa0bb76bbbaa41eaafdf9">re-implemented</a> the bits that it memoized in a more efficient way, such that memoization was no longer needed, and the whole thing was faster.</p> <p>Later, I realized that my memoized routine could have been improved by replacing <code>func</code> with <code>$0</code>, which would simplify how the final function is declared:</p> <pre><code># This definition of `rest` is equivalent to that in GMSL rest = $(wordlist 2,$(words $1),$1) # How to use: # # _<var>NAME</var>_main = <var>your main function to be memoized</var> # _<var>NAME</var>_hash = <var>your hash function for parameters</var> # <var>NAME</var> = $(memoized) # # The output of your hash function should be a string following # the same rules that variable names follow. _main = $(_$0_main) _hash = __memoized_$(_$0_hash) memoized = $(if $($(_hash)),,$(eval $(_hash) := _ $(_main)))$(call rest,$($(_hash)))</pre> <p></code></p> <p>Now, I'm pretty sure that should work, but I have only actually tested the first version.</p> <h2 id="tldr">TL;DR</h2> <p>Avoid doing things in Make that would make you lean on complex solutions like an external memoize function.</p> <p>However, if you do end up needing a more flexible memoize routine, I wrote one that you can use.</p> </article> <footer> <p>The content of this page is Copyright © 2014 <a href="mailto:lukeshu@sbcglobal.net">Luke Shumaker</a>.</p> <p>This page is licensed under the <a href="http://www.wtfpl.net/txt/copying/">WTFPL-2</a> license.</p> </footer> </body> </html>