Minimal proof of concept of universal binaries support for CMakeToolchain

[czoido]

Changelog: Feature: Add basic support in CMakeToolchain for universal binaries.
Docs: conan-io/docs#3642

[jcar87]

This looks great!

While we have got requests to support something like this, one detail that is often neglected is that this may not work at all . That is, if the build scripts have logic that is conditional on the target architecture (whether this is CMake, autotools, etc), it may not be possible to generate a universal binary in the same pass, or worse, it may generate one but one of the two is missing some crucial options. By this I mean any logic that is contingent on, say, whether CMAKE_CROSSCOMPILING is enabled, or having logic around CMAKE_SYSTEM_PROCESSOR - etc.

So while it "may be possible" for a binary to exist with arch=x86_64-armv8 - it may be that it is impossible to generate it in a single pass, and may require generating the files "independently" for each architecture and them "fusing" them together. Which makes me wonder if we should consider a way of creating a multi-arch package from two previous existing single-arch packages. Food for thought :D

[memsharded]

So while it "may be possible" for a binary to exist with arch=x86_64-armv8 - it may be that it is impossible to generate it in a single pass, and may require generating the files "independently" for each architecture and them "fusing" them together.

I think this idea has potential, I'd probably try to prioritize this, as it would be a way more general solution, valid for any build system, not adding extra complexity on the recipe+build-system side. It sounds a bit complex with the current model but with some ideas and building blocks, like what it is done in compatibility, it might be doable.

[czoido]

This looks great!

While we have got requests to support something like this, one detail that is often neglected is that this may not work at all . That is, if the build scripts have logic that is conditional on the target architecture (whether this is CMake, autotools, etc), it may not be possible to generate a universal binary in the same pass, or worse, it may generate one but one of the two is missing some crucial options. By this I mean any logic that is contingent on, say, whether CMAKE_CROSSCOMPILING is enabled, or having logic around CMAKE_SYSTEM_PROCESSOR - etc.

So while it "may be possible" for a binary to exist with arch=x86_64-armv8 - it may be that it is impossible to generate it in a single pass, and may require generating the files "independently" for each architecture and them "fusing" them together. Which makes me wonder if we should consider a way of creating a multi-arch package from two previous existing single-arch packages. Food for thought :D

Yes, absolutely true, this will not work in some cases. For those cases maybe we should use something similar to this: conan-io/conan-extensions#116
Anyway, I think this could be useful in some cases.

[memsharded]

Looking good, thanks!