[FreeBSD] Adding FreeBSD support

[michael-yuji]

This commit adds required conditional compilation blocks to enable building and running
on FreeBSD (tested on x86_64 FreeBSD 14.1-RELEASE-p6).

Also implements FreeBSD synchronization shims using _umtx_op(2)

[lin72h]

It's my early Christmas gift. Thanks for working on this.

[michael-yuji]

It's my early Christmas gift. Thanks for working on this.

unfortunately this PR alone here is not enough. I do have patches to other swift components to get it fully working and going to PR them to coming weeks. But even with those, you'll need to have a functional swiftc (by bootstrapping a swift 5.8+) on FreeBSD to build them. personally I've done a multi-stages bootstrap until I now have a toolchain ready.

you also need a patch to the base libc++ to workaround this issue

[lin72h]

it's very impressive work especially the bootstrap part!

[finagolfin]

Nice, there is some work to get Swift 6 bootstrapping again, #77815, that you'll want to follow.

Pinging @mhjacobson and @3405691582, who added BSD support before and may be able to help review.

[3405691582]

As a heads up, the code uses LLVM target triples and the build system has a few places where it assumes that the platform calls the architecture with the same name as LLVM does. This isn't a problem for FreeBSD on what LLVM calls x86_64, since they apparently both seem to use the same spelling, but for what LLVM calls aarch64, both OpenBSD and FreeBSD call it arm64. This causes headaches.

In the past, I've tried to keep some meaningful separation between platform spelling and LLVM spelling, but I'm reconsidering that in #77879. You probably won't need to do anything right now for this pr if you're just targeting x86_64, though, and what you have seems good to me, but keep this in mind if you're going to also be working on arm64.

[khng300]

As a heads up, the code uses LLVM target triples and the build system has a few places where it assumes that the platform calls the architecture with the same name as LLVM does. This isn't a problem for FreeBSD on what LLVM calls x86_64, since they apparently both seem to use the same spelling, but for what LLVM calls aarch64, both OpenBSD and FreeBSD call it arm64. This causes headaches.

In the past, I've tried to keep some meaningful separation between platform spelling and LLVM spelling, but I'm reconsidering that in #77879. You probably won't need to do anything right now for this pr if you're just targeting x86_64, though, and what you have seems good to me, but keep this in mind if you're going to also be working on arm64.

Which is not correct for the FreeBSD part. On FreeBSD itself, arm64 is the TARGET, meanwhile aarch64 is the TARGET_ARCH and MACHINE_ARCH. Below is what I got from a build of the FreeBSD tree on my side:

CMD cc -target aarch64-unknown-freebsd15.0 --sysroot=

For reference how we build the -target tuple in the tree:

Makefile.inc1:		MACHINE_ARCH=${TARGET_ARCH}
Makefile:	TARGET=${_TARGET} TARGET_ARCH=${_TARGET_ARCH} ${_MAKEARGS}
Makefile:_TARGET_ARCH=	${TARGET:S/arm64/aarch64/:S/riscv/riscv64/:S/arm/armv7/}
./Makefile.inc1:MACHINE_TRIPLE?=${MACHINE_ARCH:S/amd64/x86_64/}-${MACHINE_TRIPLE_ABI}-freebsd${OS_REVISION}

[michael-yuji]

@swift-ci please smoke test

[michael-yuji]

@swift-ci please test

[egorzhdan]

This looks amazing! I have just one comment about a C++ interop test.

[CodingCarpincho]

@shahmishal Could you or someone else kick off the CI here so we can get Michael’s work merged?

[3405691582]

@swift-ci please test.

[jrtc27]

Probably shouldn't be reformatted if you're not otherwise touching the file?

[michael-yuji]

@swift-ci please test