New in version 3.16.
When this property is set to true, the target source files will be combined into batches for faster compilation. This is done by creating a (set of) unity sources which #include the original sources, then compiling these unity sources instead of the originals. This is known as a Unity or Jumbo build.
CMake provides different algorithms for selecting which sources are grouped together into a bucket. Algorithm selection is decided by the UNITY_BUILD_MODE target property, which has the following acceptable values:
BATCH When in this mode CMake determines which files are grouped together. The UNITY_BUILD_BATCH_SIZE property controls the upper limit on how many sources can be combined per unity source file.GROUP When in this mode each target explicitly specifies how to group source files. Each source file that has the same UNITY_GROUP value will be grouped together. Any sources that don’t have this property will be compiled individually. The UNITY_BUILD_BATCH_SIZE property is ignored when using this mode.If no explicit UNITY_BUILD_MODE has been specified, CMake will default to BATCH.
Unity builds are not currently supported for all languages. CMake version 3.19.0-rc3 supports combining C and CXX source files. For targets that mix source files from more than one language, CMake will separate the languages such that each generated unity source file only contains sources for a single language.
This property is initialized by the value of the CMAKE_UNITY_BUILD variable when a target is created.
Note
Projects should not directly set the UNITY_BUILD property or its associated CMAKE_UNITY_BUILD variable to true. Depending on the capabilities of the build machine and compiler used, it might or might not be appropriate to enable unity builds. Therefore, this feature should be under developer control, which would normally be through the developer choosing whether or not to set the CMAKE_UNITY_BUILD variable on the cmake(1) command line or some other equivalent method. However, it IS recommended to set the UNITY_BUILD target property to false if it is known that enabling unity builds for the target can lead to problems.
When multiple source files are included into one source file, as is done for unity builds, it can potentially lead to ODR errors. CMake provides a number of measures to help address such problems:
COMPILE_OPTIONS, COMPILE_DEFINITIONS, COMPILE_FLAGS, or INCLUDE_DIRECTORIES source property will not be combined into a unity source.SKIP_UNITY_BUILD_INCLUSION source property to true. This can be a more effective way to prevent problems with specific files than disabling unity builds for an entire target.UNITY_BUILD_CODE_BEFORE_INCLUDE and UNITY_BUILD_CODE_AFTER_INCLUDE target properties can be used to inject code into the unity source files before and after every #include statement.add_library(), add_executable() or target_sources() will be preserved in the generated unity source files. This can be used to manually enforce a specific grouping based on the UNITY_BUILD_BATCH_SIZE target property.
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https://cmake.org/cmake/help/v3.19/prop_tgt/UNITY_BUILD.html