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Build: Support make's and ninja's restat optimization

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After running a build command, make by default stat()s the command's
output, and if it wasn't touched, then it cancels all build steps
that were scheduled only because this command was expected to change
the output.

Ninja has the same feature, but it's opt-in behind the per-command
"restat = 1" setting. However, CMake enables it by default for all
custom commands.

Use Meta/write-only-on-difference.sh to write the output to a temporary
file, and then copy the temporary file only to the final location if the
contents of the output have changed since last time.
write-only-on-difference.sh automatically creates the output's parent
directory, so stop doing that in CMake.

Reduces the number of build steps that run after touching a file
in LibCore from 522 to 312.

Since we now no longer trigger the CMake special case "If COMMAND
specifies an executable target name (created by the add_executable()
command), it will automatically be replaced by the location of the
executable created at build time", we now need to use qualified paths to
the generators.

Somewhat related to #2877.
This commit is contained in:
Nico Weber 2020-08-02 09:54:06 -04:00 • committed by Andreas Kling
parent 1d08cab9ab
commit d8b6314018
2 changed files with 8 additions and 11 deletions
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4
CMakeLists.txt
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@ -51,6 +51,8 @@ add_subdirectory(DevTools/FormCompiler)
add_subdirectory(Libraries/LibWeb/CodeGenerators)
add_subdirectory(AK/Tests)
set(write_if_different ${CMAKE_SOURCE_DIR}/Meta/write-only-on-difference.sh)
function(serenity_install_headers target_name)
file(GLOB_RECURSE headers RELATIVE ${CMAKE_CURRENT_SOURCE_DIR} "*.h")
foreach(header ${headers})
@ -96,7 +98,7 @@ function(compile_ipc source output)
set(source ${CMAKE_CURRENT_SOURCE_DIR}/${source})
add_custom_command(
OUTPUT ${output}
COMMAND IPCCompiler ${source} > ${output}
COMMAND ${write_if_different} ${output} ${CMAKE_BINARY_DIR}/DevTools/IPCCompiler/IPCCompiler ${source}
VERBATIM
DEPENDS IPCCompiler
MAIN_DEPENDENCY ${source}