Merge topic 'docs-UNITY_BUILD' into release-3.16

fcacd8ce Help: Improve readability and fix inaccuracies in unity build docs
Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !4019

Help/prop_sf/SKIP_UNITY_BUILD_INCLUSION.rst

 SKIP_UNITY_BUILD_INCLUSION
 --------------------------
 
-Is this source file skipped by :prop_tgt:`UNITY_BUILD` feature.
+Setting this property to true ensures the source file will be skipped by
+unity builds when its associated target has its :prop_tgt:`UNITY_BUILD`
+property set to true.  The source file will instead be compiled on its own
+in the same way as it would with unity builds disabled.
 
-This property helps with "ODR (One definition rule)" problems
-that one would run into when using an :prop_tgt:`UNITY_BUILD`.
+This property helps with "ODR (One definition rule)" problems where combining
+a particular source file with others might lead to build errors or other
+unintended side effects.

Help/prop_tgt/UNITY_BUILD.rst

 UNITY_BUILD
 -----------
 
-Should the target source files be processed into batches for
-faster compilation. This feature is known as "Unity build",
-or "Jumbo build".
-
-The ``C`` and ``CXX`` source files are grouped separately.
-
-This property is initialized by the value of the
-:variable:`CMAKE_UNITY_BUILD` variable if it is set when
-a target is created.
-
-.. note::
-
-  It's not recommended to directly set :prop_tgt:`UNITY_BUILD`
-  to ``ON``, but to instead set :variable:`CMAKE_UNITY_BUILD` from
-  the command line.  However, it IS recommended to set
-  :prop_tgt:`UNITY_BUILD` to ``OFF`` if you need to ensure that a
-  target doesn't get a unity build.
-
-The batch size can be specified by setting
-:prop_tgt:`UNITY_BUILD_BATCH_SIZE`.
-
-The batching of source files is done by adding new sources files
-which will ``#include`` the source files, and exclude them from
-building by setting :prop_sf:`HEADER_FILE_ONLY` to ``ON``.
+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.  The :prop_tgt:`UNITY_BUILD_BATCH_SIZE` property controls the upper
+limit on how many sources can be combined per unity source file.
+
+Unity builds are not currently supported for all languages.  CMake version
+|release| 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 :variable:`CMAKE_UNITY_BUILD`
+variable when a target is created.
 
 .. note::
 
-  Marking the original sources with :prop_sf:`HEADER_FILE_ONLY`
-  is considered an implementation detail that may change in the
-  future because it does not work well in combination with
-  the :variable:`CMAKE_EXPORT_COMPILE_COMMANDS` variable.
+  Projects should not directly set the ``UNITY_BUILD`` property or its
+  associated :variable:`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 :variable:`CMAKE_UNITY_BUILD`
+  variable on the :manual:`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.
 
 ODR (One definition rule) errors
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-Since multiple source files are included into one source file,
-it can lead to ODR errors. This section contains properties
-which help fixing these errors.
-
-The source files marked by :prop_sf:`SKIP_UNITY_BUILD_INCLUSION`
-will be skipped from unity build.
-
-The source files that have :prop_sf:`COMPILE_OPTIONS`,
-:prop_sf:`COMPILE_DEFINITIONS`, :prop_sf:`COMPILE_FLAGS`, or
-:prop_sf:`INCLUDE_DIRECTORIES` will also be skipped.
-
-With the :prop_tgt:`UNITY_BUILD_CODE_BEFORE_INCLUDE` and
-:prop_tgt:`UNITY_BUILD_CODE_AFTER_INCLUDE` one can specify code
-to be injected in the unity source file before and after every
-``#include`` statement.
-
-.. note::
-
-  The order of source files defined in the ``CMakeLists.txt`` will
-  be preserved into the generated unity source files. This can
-  be used to manually enforce a specific grouping based on the
-  :prop_tgt:`UNITY_BUILD_BATCH_SIZE` target property.
+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:
+
+* Any source file that has a non-empty :prop_sf:`COMPILE_OPTIONS`,
+  :prop_sf:`COMPILE_DEFINITIONS`, :prop_sf:`COMPILE_FLAGS`, or
+  :prop_sf:`INCLUDE_DIRECTORIES` source property will not be combined
+  into a unity source.
+
+* Projects can prevent an individual source file from being combined into
+  a unity source by setting its :prop_sf:`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.
+
+* The :prop_tgt:`UNITY_BUILD_CODE_BEFORE_INCLUDE` and
+  :prop_tgt:`UNITY_BUILD_CODE_AFTER_INCLUDE` target properties can be used
+  to inject code into the unity source files before and after every
+  ``#include`` statement.
+
+* The order of source files added to the target via commands like
+  :command:`add_library`, :command:`add_executable` or
+  :command:`target_sources` will be preserved in the generated unity source
+  files.  This can be used to manually enforce a specific grouping based on
+  the :prop_tgt:`UNITY_BUILD_BATCH_SIZE` target property.

Help/prop_tgt/UNITY_BUILD_BATCH_SIZE.rst

 UNITY_BUILD_BATCH_SIZE
 ----------------------
 
-Specifies how many source code files will be included into a
-:prop_tgt:`UNITY_BUILD` source file.
+Specifies the maximum number of source files that can be combined into any one
+unity source file when unity builds are enabled by the :prop_tgt:`UNITY_BUILD`
+target property.  The original source files will be distributed across as many
+unity source files as necessary to honor this limit.
 
-If the property is not set, CMake will use the value provided
-by :variable:`CMAKE_UNITY_BUILD_BATCH_SIZE`.
+The initial value for this property is taken from the
+:variable:`CMAKE_UNITY_BUILD_BATCH_SIZE` variable when the target is created.
+If that variable has not been set, the initial value will be 8.
 
-By setting it to value `0` the generated unity source file will
-contain all the source files that would otherwise be split
-into multiple batches. It is not recommended to do so, since it
-would affect performance.
+The batch size needs to be selected carefully.  If set too high, the size of
+the combined source files could result in the compiler using excessive memory
+or hitting other similar limits.  In extreme cases, this can even result in
+build failure.  On the other hand, if the batch size is too low, there will be
+little gain in build performance.
+
+Although strongly discouraged, the batch size may be set to a value of 0 to
+combine all the sources for the target into a single unity file, regardless of
+how many sources are involved.  This runs the risk of creating an excessively
+large unity source file and negatively impacting the build performance, so
+a value of 0 is not generally recommended.

Help/prop_tgt/UNITY_BUILD_CODE_AFTER_INCLUDE.rst

@@ -2,7 +2,18 @@ UNITY_BUILD_CODE_AFTER_INCLUDE
 ------------------------------
 
 Code snippet which is included verbatim by the :prop_tgt:`UNITY_BUILD`
-feature just after the `#include` statement of the targeted source
-files.
+feature just after every ``#include`` statement in the generated unity
+source files.  For example:
 
-This could be something like `#undef NOMINMAX`.
+.. code-block:: cmake
+
+  set(after [[
+  #if defined(NOMINMAX)
+  #undef NOMINMAX
+  #endif
+  ]])
+  set_target_properties(myTarget PROPERTIES
+    UNITY_BUILD_CODE_AFTER_INCLUDE "${after}"
+  )
+
+See also :prop_tgt:`UNITY_BUILD_CODE_BEFORE_INCLUDE`.

Help/prop_tgt/UNITY_BUILD_CODE_BEFORE_INCLUDE.rst

@@ -2,7 +2,18 @@ UNITY_BUILD_CODE_BEFORE_INCLUDE
 -------------------------------
 
 Code snippet which is included verbatim by the :prop_tgt:`UNITY_BUILD`
-feature just before the `#include` statement of the targeted source
-files.
+feature just before every ``#include`` statement in the generated unity
+source files.  For example:
 
-This could be something like `#define NOMINMAX`.
+.. code-block:: cmake
+
+  set(before [[
+  #if !defined(NOMINMAX)
+  #define NOMINMAX
+  #endif
+  ]])
+  set_target_properties(myTarget PROPERTIES
+    UNITY_BUILD_CODE_BEFORE_INCLUDE "${before}"
+  )
+
+See also :prop_tgt:`UNITY_BUILD_CODE_AFTER_INCLUDE`.

Help/variable/CMAKE_UNITY_BUILD.rst

 CMAKE_UNITY_BUILD
 -----------------
 
-Initializes the :prop_tgt:`UNITY_BUILD` target property on targets
-as they are created.  Set to ``ON`` to batch compilation of multiple
-sources within each target.  This feature is known as "Unity build",
-or "Jumbo build".  By default this variable is not set and so does
-not enable unity builds on targets.
+This variable is used to initialize the :prop_tgt:`UNITY_BUILD`
+property of targets when they are created.  Setting it to true
+enables batch compilation of multiple sources within each target.
+This feature is known as a *Unity* or *Jumbo* build.
+
+Projects should not set this variable, it is intended as a developer
+control to be set on the :manual:`cmake(1)` command line or other
+equivalent methods.  The developer must have the ability to enable or
+disable unity builds according to the capabilities of their own machine
+and compiler.
+
+By default, this variable is not set, which will result in unity builds
+being disabled.
 
 .. note::
   This option currently does not work well in combination with

Help/variable/CMAKE_UNITY_BUILD_BATCH_SIZE.rst

 CMAKE_UNITY_BUILD_BATCH_SIZE
 ----------------------------
 
-Default value for :prop_tgt:`UNITY_BUILD_BATCH_SIZE` of targets.
-
-By default ``CMAKE_UNITY_BUILD_BATCH_SIZE`` is set to ``8``.
+This variable is used to initialize the :prop_tgt:`UNITY_BUILD_BATCH_SIZE`
+property of targets when they are created.  It specifies the default upper
+limit on the number of source files that may be combined in any one unity
+source file when unity builds are enabled for a target.