[PATCH] D119804: [docs] HowToCrossCompileLLVM.rst: update cmake options

Sat Feb 19 05:11:19 PST 2022

rengolin added a comment.

In D119804#3333462 <https://reviews.llvm.org/D119804#3333462>, @rengolin wrote:

> I think you forgot to upload the new diff for the GCC comment. It's looking the same as before for me.

Hm, there wasn't any. But I did get a comment on the email that I wanted to reply to and it isn't here. Won't change the acceptance of the patch, it's more as FYI.

> If you're compiling with GCC, you can use architecture options for your target, and the compiler driver will detect everything that it needs

This is a common thread between Clang and GCC.

In GCC, the target is defined at compile time, so if you use `target-gcc`, all of the options are already chosen. Paths, headers, libraries, even sysroot is already set.

Clang also has `target-clang` (as a symlink to `clang`), but that's a shortcut to `clang -target target`, which may have some headers but doesn't have libraries, sysroot and may not get the right path (if the platform compiler is GCC and compiled in unexpected ways).

That is why we need the other flags and why this document was written in the first place.

We have debated for years if we want to have a default LLVM sysroot+libs+headers, and some side projects have started in that direction, but we shouldn't use a different toolchain from the system one if we want to link with objects compiled with the system compiler (statically or dynamically).

In FreeBSD, Apple and OpenMandriva, Android, the system compiler is clang, so all libraries, paths and the sysroot should be easy to find and known, and clang does what you expect, too.

But on other Linux distros, Windows and systems where the system toolchain isn't clang, you'll have that problem again.

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