Godbolt's Law

I’ve been spending a lot of time over the last few days making my code run faster. This is a fairly common pastime of mine, I’ve always enjoyed optimising code, and sometimes the speedups you can get from innocuous changes are frankly amazing. However, in my long experience of this kind of activity, I’ve noticed a pretty consistent law, which as far as I can tell nobody else has caught on to. So, flame-retardant suit fully adorned, I claim my own law — Godbolt’s Law:


Godbolt’s Law — if any single optimisation makes a routine run two or more times faster, then you’ve broken the code.


A staggeringly potent law, I’m sure you’ll agree, borne out over many long times where I’ve been carefully noting down execution times, then discovered that I’ve sped things up by a factor of three! ‘Hurrah’, think I, and proceed to dance around the room, to the amusement of my colleagues (many of whom have probably cottoned on to this Law already). Excitedly I begin writing the check-in notes for this fantastic speed up, and then, just before I commit the change, I remember to actually check the results — and yes, you’ve sussed it, the reason it’s going three times faster is because I’ve broken it and now it’s not actually doing any work at all. Arse!

In fact, oftentimes I’ve actually checked in code like this, so excited have I been about the speedup. Premature optimisation may be the root of all evil, but premature checkins are the root of the root of all evil.

Maybe there’s some kind of way an automatic metric can be used to check this; an automatic build system perhaps can do performance analysis and alert management if suddenly things are blazing along — either someone’s done something so amazing they should be rewarded…or else Godbolt’s Law is at work and the gremlins are now running amok in the source code!

Filed under: Coding
Posted at 22:33:56 BST on 4th April 2005.

About Matt Godbolt

Matt Godbolt is a C++ developer working in Chicago in the finance industry.