Tuesday, March 13, 2018

Hot reloading hardcoded parameters

Here is a trick that I cannot take any credit for, but that I finally implemented. I remember reading about it online several years ago, but I cannot find the reference again (it might have been on mollyrocket), so I'll write up the idea:

Everyone uses hardcoded parameters sometimes because it's fast and easy:

float someValue = 5.0f;

Once you have a parameter in the code, it's likely that you sooner or later want to tune that into some kind of sweet spot. With a hardcoded parameter the process often involves recompiling and restarting (unless you implemented code hot reloading, in which case it still involves recompiling) many times to try out different values. A popular approach is to add some form of config file to get rid of the recompile step. Config files can be hot reloaded to also get rid of the restart step, but config files require some extra work for each parameter. You need to name the parameter, and you need to add it to the config file.

The idea of parameter hot reloading is to use the code itself as config file. The file is already there, the context and naming is already there, the initial value is already there, and once you're done tweaking, the result is already in your code!

This can be done by wrapping the tweakable, hardcoded parameter in a macro:

float someValue = TWEAK(5.0f);

The macro expands to a function call that looks something like this, using the __FILE__ and __LINE__ built-ins:

float TweakValue(float v, const char* fileName, int lineNumber);

This function stores each tweakable value in a hash table using the file and line properties, so that it can be accessed quickly. The really sweet part is that since we know the file and line number we can periodically (say once a frame, or using some file modification event) check if the source file has changed, and when it changes, just parse out the new value. Note that this is rather trivial, since at this point we know exactly on what line to look for it and how to parse it, because it will be wrapped by a parenthesis right after the word TWEAK.

One limitation is that it only works for one tweakable parameter per line. It's probably possible to make it work for more than one, but that requires a lot more parsing. Note that this can be done for more than just floats. I've also added support for booleans, vectors and colors. The boolean especially can be useful to toggle between two implementations at run time:

if (TWEAK(true))
    doThis()
else 
    doThat()

Needless to say, in production builds, the TWEAK macro is just bypassed, adding zero overhead. Pretty neat isn't it?

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