[cmake-developers] RFC: CMake precompiled header support and custom compiler based implementation

[Daniel Pfeifer]

On Wed, Mar 4, 2015 at 8:53 PM, Brad King <brad.king at kitware.com> wrote:

> On 03/03/2015 07:43 AM, Adam Strzelecki wrote:
> > On 03/01/2015 08:02 PM, Ben Boeckel wrote:
> >> On 02/28/2015 11:59 AM, Adam Strzelecki wrote:
> >>      target_precompiled_header(<target> <path/to/precompiled_header.h>)
> >>
> >>      target_precompiled_header(<target> <path/to/precompiled_header.h>
> SHARED
> >>
> <other_target_to_share_precompiled_header_with>)
> >> Could we rename this? Currently target_* functions are all related to
> >> usage requirements and these are not usage requirements.
> >
> > Honestly I don't understand your request. What would be then the name
> > that better represents what the function does.
>
> Currently the target_* commands are:
>
>  target_compile_definitions
>  target_compile_features
>  target_compile_options
>  target_include_directories
>  target_link_libraries
>  target_sources
>
> They all have the general signature
>
>  target_command(<target> [options] [<SCOPE> <items>...]...)
>
> where <SCOPE> is INTERFACE, PUBLIC, or PRIVATE and affects whether the
> settings apply to sources in the target itself and/or its dependents.
> The proposed target_precompiled_header signature does not fit into
> this layout.  Either a different command name is needed or one needs
> to define the command signature and semantics to fit the above layout.
> Do PCHs fit into usage requirements in any way?
>

That would be nice, right?
I pushed some work to https://github.com/purpleKarrot/CMake/commits/pch
Expect it to be incomplet and incorrekt. It certainly needs a couple of
iterations, but the direction should be clear:


add_library(foo foo.c)
target_include_directories(foo PUBLIC include)
target_precompile_headers(foo PUBLIC foo.h)

add_library(bar INTERFACE)
target_include_directories(bar INTERFACE include)
target_precompile_headers(bar INTERFACE bar.h)

add_executable(foobar foobar.c)
target_link_libraries(foobar foo bar)


There is a new command called "target_precompile_headers". This name is
chosen carefully.
* It says "precompile headers", not "precompiled header". This command is
used to set a list of headers "to precompile". It is not used to specify a
single prebuilt file.
* It has a "target_" prefix, and the expectations about usage requirements
should be satisfied.


Things to keep in mind:
* We probably want to precompile headers based on configuration.
* For generators that support it (Makefile + Ninja), we want to precompile
headers based on the compile language.
* We probably want to enable/disable the use of PCH globally, on a
per-target-level, and on a per-source-level.

> In meantime https://github.com/nanoant/CMakePCHCompiler is just an
implementation that "emulates" desired behavior using many tricks. But I
don't want this to be a part of CMake, but instead I want this to be
handled natively adding some extra variables:

Good. That means we are on the same page. I was a little confused when I
first saw your approach. :-)

cheers, Daniel
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