Options (of various degrees)

There are a number of ways to make a piece of code execute conditionally in OpenQL, global- and pass options being the most obvious ones. But there are more, with different use cases, so let’s go over them.

Pass options

Wherever possible, pass options are what should be used to configure things related to passes.

Global options

Global options are primarily a remnant from before we had passes. They should now only be used for backward compatibility, and for things that cannot be turned into pass options due to needing to be available outside the context of a pass. If you really need them, they are defined in src/ql/com/options.cc.

Platform JSON configuration file options

Anything that has access to the IR can read arbitrary JSON from the platform configuration data structure; the raw JSON is always maintained, and unrecognized keys are silently ignored (somewhat by design). So, whenever you have an optional bit of code that’s logically related to the platform and that has access to the platform, this is what you should use. Usually, though, it makes more sense to use a pass option.

Preprocessor-based options

The C preprocessor can be used to set options at compile-time. There are various ways to do this (beyond just #define) in the context of CMake, which are preferred to the usual C++ way, because they don’t require a user to modify code in order to change the option.


All preprocessor macros should use the QL_ prefix, to avoid name clashes with libraries we’re depending on, or (if it ever happens) the codebase of software depending on OpenQL within C++.

CMake target_compile_definitions

These are useful for pieces of code that shouldn’t even be attempted to be compiled when the option is off. However, the associated preprocessor directive must not be used in public header files! After all, if OpenQL would be installed and a user would link against it, they wouldn’t use the CMakeLists file at all, and thus you’d end up with arbitrary discrepancies in the header files! When nevertheless used, they should be added at the bottom of the “OpenQL library target” section of the main CMakeLists.txt file.

Whether the macro is defined or now would normally be controlled via a CMake option/cache variable, created with the option() function. These directives should be placed at the top of the main CMakeLists.txt file, in the “Configuration options” section.

CMake configure_file options

When the above is not possible because the definition is also needed in public header files, you can instead use ql/config.h, again in conjunction with an option() directive. To do so:

  • add a #cmakedefine directive to src/ql/config.h.template;

  • make sure that the name of the preprocessor directive exists as a yes/no CMake variable when config.h is created (also in the “OpenQL library target” section of the main CMakeLists.txt file); and

  • ensure that ql/config.h is included in all files that use the definition.

The latter is of course very important, otherwise you’ll get the same kind of discrepancies that you would for target_compile_definitions. Basically, this just shifts that problem. Choose the one that’s more convenient.

#define in some header file somewhere

You can do this if you want, but it should only be done for things that rarely change. After all, you can’t change the option without changing sources.