[CMake] command in add_custom_command launched even if output not stale

[CREMARENCO Cosmin]

Hello,

We use CMake and among other things CMake is used for generating code at compilation time.

We have a custom command like so:
add_custom_command(OUTPUT O0, O1, O2...On
                   launch-generation-command
                   DEPENDS I0, I1, I2, ....In
   VERBATIM)
So there are multiple inputs and multiple outputs (why not launch the command for each input and have only one output? - it would take too much time combined, a jvm is launched every time and there are a lot of generated files).
The command that is launched is intelligent enough to not touch output if its corresponding input has not changed.

Now I'm coming to the problem: when I touch an input Ix, I expect that output Ox is regenerated. That works great, make will launch the custom command and Ox will be generated and compiled afterwards. But, subsequent runs of make will still launch the custom command which is not ok. By digging in the generated Makefiles I see that cmake will hang the custom command under one of the outputs (arbitrarily?). If that output is not exactly the input that I touch, given that the generator will only output code for what has really changed, the target will always be invalid and make will always launch the custom command. Is there a solution for this?
It seems obvious to me that if the oldest output is more recent than the newest input then the custom command shouldn't be called.

Visual Studio is even worse. Besides launching the custom command every time, it will recompile all outputs because, my guess is that after the custom command has run it won't even bother to check that the .obj is more recent than the corresponding .cpp.

Sorry for the somewhat terse mail and thanks in advance!

Cosmin


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