try_compile does not honor own variables

Code
1

Hi,

I have a toolchain file that sets a custom CMAKE_SYSTEM_NAME. I also have the Platform/-Files that I need. The problem is that try_compile() cannot see those.

I tried:

cmake_minimum_required(VERSION 3.22)

set ( CMAKE_TOOLCHAIN_FILE ${CMAKE_CURRENT_LIST_DIR}/CMakeModules/toolchain.cmake )
set ( CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/CMakeModules )
set ( CMAKE_TRY_COMPILE_PLATFORM_VARIABLES CMAKE_MODULE_PATH )

project ( MyProject LANGUAGES C )

include ( CheckIPOSupported )
check_ipo_supported ( RESULT _IPO_SUPPORTED OUTPUT IPO_SUPPORTED_CHECK_OUTPUT LANGUAGES C )

But still I get:

System is unknown to cmake, create:
Platform/XXX to use this system, please post your config file on discourse.cmake.org so it can be added to cmake

The system itself is not of public interest. More interesting is that I also use --debug-trycompile and the IPO test does NOT have CMAKE_MODULE_PATH in its CMakeCache.txt file (CMakeFiles/_CMakeLTOTest-C/bin/CMakeCache.txt).
This means that CMAKE_TRY_COMPILE_PLATFORM_VARIABLES essentially does not work.
I also tried another variable name and this also does not appear in the try-compile cache file.

Any insights?

2

Note: These messages come from the try_compile(), not from project() in the main project!

3

This means that CMAKE_TRY_COMPILE_PLATFORM_VARIABLES essentially does not work.

It is always dangerous to make such definitive statements without carefully considering the question…

CheckIPOSupported relies on the project mode of try_compile command and the documentation of this command clearly states, regarding CMAKE_TRY_COMPILE_PLATFORM_VARIABLES:

This variable is meant for use only in toolchain files and is only honored by the try_compile() command for the source files form, not when given a whole project.

So, it is perfectly normal that your setting is ignored!

4

Sorry, didn’t wanted to blame anyone. Thanks for solving this mystery :slight_smile:

So what shall we do about the project mode as used in CheckIPOSupported?
Is it feasible to process the CMAKE_TRY_COMPILE_PLATFORM_VARIABLES in the CheckIPOSupported cmake file and ammend the CMAKE_ARGS list?