How to quickly navigate an unfamiliar makefile

The other day, I was working with an unfamiliar build and I needed to get familiar with it in a hurry. In this case, I was dealing with a makefile generated by the Perl utility h2xs, but the trick I’ll show you here works any time you need to find your way around a new build system, whether it’s something you just downloaded or an internal project you just transferred to.

What I wanted to do was add a few object files to the link command. Here’s the build log, with the link command highlighted:

gcc -c  -I. -D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -O2 -g   -DVERSION=\"0.01\" -DXS_VERSION=\"0.01\" -fPIC "-I/usr/lib/perl/5.10/CORE"   mylib.c
rm -f blib/arch/auto/mylib/mylib.so
gcc  -shared -O2 -g -L/usr/local/lib mylib.o   -o blib/arch/auto/mylib/mylib.so   \
                \
          
chmod 755 blib/arch/auto/mylib/mylib.so

Should be easy, right? I just needed to find that command in the makefile and make my changes. Wrong. Read on to see how annotation helped solve this problem.

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Subbuilds: build avoidance done right

I’ve heard it said that the best programmer is a lazy programmer. I’ve always taken that to mean that the best programmers avoid unnecessary work, by working smarter and not harder; and that they focus on building only those features that are really required now, not allowing speculative work to distract them.

I wouldn’t presume to call myself a great programmer, but I definitely hate doing unnecessary work. That’s why the concept of build avoidance is so intriguing. If you’ve spent any time on the build speed problem, you’ve probably come across this term. Unfortunately it’s been conflated with the single technique implemented by tools like ccache and ClearCase winkins. I say “unfortunate” for two reasons: first, those tools don’t really work all that well, at least not for individual developers; and second, the technique they employ is not really build avoidance at all, but rather object reuse. But by co-opting the term build avoidance and associating it with such lackluster results, many people have become dismissive of build avoidance.

Subbuilds are a more literal, and more effective, approach to build avoidance: reduce build time by building only the stuff required for your active component. Don’t waste time building the stuff that’s not related to what you’re working on now. It seems so obvious I’m almost embarrassed to be explaining it. But the payoff is anything but embarrassing. On my project, after making changes to one of the prerequisites libraries for the application I’m working on, a regular incremental takes 10 minutes; a subbuild incremental takes just 77 seconds:

Standard incremental:
609s
Subbuild incremental:
77s

Not bad! Read on for more about how subbuilds work and how you can get SparkBuild, a free gmake- and NMAKE-compatible build tool, so you can try subbuilds yourself.
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Makefile performance: built-in rules

Like any system that has evolved over many years, GNU Make is rife with appendages of questionable utility. One area this is especially noticeable is the collection of built-in rules in gmake. These rules make it possible to do things like compile a C source file to an executable without even having a makefile, or compile and link several source files with a makefile that simply names the executable and each of the objects that go into it.

But this convience comes at a price. Although some of the built-in rules are still relevant in modern environments, many are obsolete or uncommonly used at best. When’s the last time you compiled Pascal code, or used SCCS or RCS as your version control system? And yet every time you run a build, gmake must check every source file against each of these rules, on the off chance that one of them might apply. A simple tweak to your GNU Make command-line is all it takes to get a performance improvement of up to 30% out of your makefiles. Don’t believe me? Read on.
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Friday Fun: Generating Fibonacci Numbers with GNU Make

Nobody would ever claim that GNU Make is a general purpose programming language, but with a little work, we can coerce it into generating Fibonacci numbers for us. Why bother? Because we can.
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Rules with Multiple Outputs in GNU Make

I recently wrote an article for CM Crossroads exploring various strategies for handling rules that generate multiple output files in GNU make. If you’ve ever struggled with this problem, you should check out the article. I don’t want to spoil the exciting conclusion, but it turns out that the only way to really correctly capture this relationship in GNU make syntax is with pattern rules. That’s great if your input and output files share a common stem (eg, “parser” in parser.i, parser.c and parser.h), but if your files don’t adhere to that convention, you’re stuck with one of the alternatives, each of which have some strange caveats and limitations.

Here’s a question for you: if ElectricAccelerator had an extension that allowed you to explicitly mark a non-pattern rule as having multiple outputs, would you use it? For example:

#pragma multi
something otherthing: input
	@echo Generating something and otherthing from input...

What do you think? Comments encouraged.