Commit dae9a808 authored by Craig Scott's avatar Craig Scott

Help: Reorganise target_precompile_headers() docs for readability

Incorporates clarification of behavior when trying to specify
REUSE_FROM on a target that already has its own precompile
headers set.

Fixes: #19970
parent 28bc41ca
......@@ -3,33 +3,21 @@ target_precompile_headers
Add a list of header files to precompile.
Precompiling header files can speed up compilation by creating a partially
processed version of some header files, and then using that version during
compilations rather than repeatedly parsing the original headers.
Main Form
^^^^^^^^^
.. code-block:: cmake
target_precompile_headers(<target>
<INTERFACE|PUBLIC|PRIVATE> [header1...]
[<INTERFACE|PUBLIC|PRIVATE> [header2...] ...])
target_precompile_headers(<target> REUSE_FROM <other_target>)
Adds header files to :prop_tgt:`PRECOMPILE_HEADERS` or
:prop_tgt:`INTERFACE_PRECOMPILE_HEADERS` target properties.
The second signature will reuse an already precompiled header file artefact
from another target. This is done by setting the
:prop_tgt:`PRECOMPILE_HEADERS_REUSE_FROM` to ``<other_target>`` value.
The ``<other_target>`` will become a dependency of ``<target>``.
.. note::
The second signature will require the same set of compiler options,
compiler flags, compiler definitions for both ``<target>``, and
``<other_target>``. Compilers (e.g. GCC) will issue a warning if the
precompiled header file cannot be used (``-Winvalid-pch``).
Precompiling header files can speed up compilation by creating a partially
processed version of some header files, and then using that version during
compilations rather than repeatedly parsing the original headers.
The command adds header files to the :prop_tgt:`PRECOMPILE_HEADERS` and/or
:prop_tgt:`INTERFACE_PRECOMPILE_HEADERS` target properties of ``<target>``.
The named ``<target>`` must have been created by a command such as
:command:`add_executable` or :command:`add_library` and must not be an
:ref:`ALIAS target <Alias Targets>`.
......@@ -38,25 +26,36 @@ The ``INTERFACE``, ``PUBLIC`` and ``PRIVATE`` keywords are required to
specify the scope of the following arguments. ``PRIVATE`` and ``PUBLIC``
items will populate the :prop_tgt:`PRECOMPILE_HEADERS` property of
``<target>``. ``PUBLIC`` and ``INTERFACE`` items will populate the
:prop_tgt:`INTERFACE_PRECOMPILE_HEADERS` property of ``<target>``.
(:ref:`IMPORTED targets <Imported Targets>` only support ``INTERFACE`` items.)
Repeated calls for the same ``<target>`` append items in the order called.
:prop_tgt:`INTERFACE_PRECOMPILE_HEADERS` property of ``<target>``
(:ref:`IMPORTED targets <Imported Targets>` only support ``INTERFACE`` items).
Repeated calls for the same ``<target>`` will append items in the order called.
The list of header files is used to generate a header file named
``cmake_pch.h|xx`` which is used to generate the precompiled header file
(``.pch``, ``.gch``, ``.pchi``) artifact. The ``cmake_pch.h|xx`` header
file will be force included (``-include`` for GCC, ``/FI`` for MSVC) to
all source files, so sources do not need to have ``#include "pch.h"``.
Header file names specified with angle brackets (e.g. ``<unordered_map>``) or
explicit double quotes (escaped for the :manual:`cmake-language(7)`,
e.g. ``[["other_header.h"]]``) will be treated as is, and include directories
must be available for the compiler to find them. Other header file names
(e.g. ``project_header.h``) are interpreted as being relative to the current
source directory (e.g. :variable:`CMAKE_CURRENT_SOURCE_DIR`) and will be
included by absolute path.
Arguments to ``target_precompile_headers`` may use "generator expressions"
Arguments to ``target_precompile_headers()`` may use "generator expressions"
with the syntax ``$<...>``.
See the :manual:`cmake-generator-expressions(7)` manual for available
expressions. See the :manual:`cmake-compile-features(7)` manual for
information on compile features and a list of supported compilers.
The ``$<COMPILE_LANGUAGE:...>`` generator expression is particularly
useful for specifying a language-specific header to precompile for
only one language (e.g. ``CXX`` and not ``C``).
Usage
^^^^^
only one language (e.g. ``CXX`` and not ``C``). For example:
.. code-block:: cmake
target_precompile_headers(<target>
target_precompile_headers(myTarget
PUBLIC
project_header.h
"$<$<COMPILE_LANGUAGE:CXX>:cxx_only.h>"
......@@ -65,20 +64,6 @@ Usage
<unordered_map>
)
The list of header files is used to generate a header file named
``cmake_pch.h|xx`` which is used to generate the precompiled header file
(``.pch``, ``.gch``, ``.pchi``) artifact. The ``cmake_pch.h|xx`` header
file will be force included (``-include`` for GCC, ``/FI`` for MSVC) to
all source files, so sources do not need to have ``#include "pch.h"``.
Header file names specified with angle brackets (e.g. ``<unordered_map>``) or
explicit double quotes (escaped for the :manual:`cmake-language(7)`,
e.g. ``[["other_header.h"]]``) will be treated as is, and include directories
must be available for the compiler to find them. Other header file names
(e.g. ``project_header.h``) are interpreted as being relative to the current
source directory (e.g. :variable:`CMAKE_CURRENT_SOURCE_DIR`) and will be
included by absolute path.
When specifying angle brackets inside a :manual:`generator expression
<cmake-generator-expressions(7)>`, be sure to encode the closing ``>``
as ``$<ANGLE-R>``. For example:
......@@ -88,13 +73,38 @@ as ``$<ANGLE-R>``. For example:
target_precompile_headers(mylib PRIVATE
"$<$<COMPILE_LANGUAGE:C>:<stddef.h$<ANGLE-R>>"
"$<$<COMPILE_LANGUAGE:CXX>:<cstddef$<ANGLE-R>>"
)
)
Reusing Precompile Headers
^^^^^^^^^^^^^^^^^^^^^^^^^^
The command also supports a second signature which can be used to specify that
one target re-uses a precompiled header file artefact from another target
instead of generating its own:
.. code-block:: cmake
target_precompile_headers(<target> REUSE_FROM <other_target>)
This form sets the :prop_tgt:`PRECOMPILE_HEADERS_REUSE_FROM` property to
``<other_target>`` and adds a dependency such that ``<target>`` will depend
on ``<other_target>``. CMake will halt with an error if the
:prop_tgt:`PRECOMPILE_HEADERS` property of ``<target>`` is already set when
the ``REUSE_FROM`` form is used.
.. note::
The ``REUSE_FROM`` form requires the same set of compiler options,
compiler flags and compiler definitions for both ``<target>`` and
``<other_target>``. Some compilers (e.g. GCC) may issue a warning if the
precompiled header file cannot be used (``-Winvalid-pch``).
See Also
^^^^^^^^
For disabling precompile headers for specific targets there is the
property :prop_tgt:`DISABLE_PRECOMPILE_HEADERS`.
To disable precompile headers for specific targets, see the
:prop_tgt:`DISABLE_PRECOMPILE_HEADERS` target property.
For skipping certain source files there is the source file property
:prop_sf:`SKIP_PRECOMPILE_HEADERS`.
To prevent precompile headers from being used when compiling a specific
source file, see the :prop_sf:`SKIP_PRECOMPILE_HEADERS` source file property.
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