Support Android ndk versions r23-beta3 and up

[ghost]

Since android ndk version r23-beta3, libgcc has been replaced with libunwind. This moves the linking of libgcc/libunwind into the unwind crate where we check if the system compiler can find libunwind and fall back to libgcc if needed.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[Mark-Simulacrum]

@petrochenkov -- do you have a read on this PR perhaps?

It seems like this could break downstream users if they're relying on libgcc linked in (due to std asking for it), even though they sort of shouldn't be doing so. Ultimately seems ok, though, especially if upstream is going to drop it...

[petrochenkov]

If this passes CI (which uses an old NDK?) then I'm happy with this.

We are linking compiler-builtins anyway so Rust code is covered, and if libgcc is required by libc then we'll know it from CI, I guess. That's certainly a possibility because a random old NDK that I have at hand (android-ndk-r17c-aarch64-21) does this

-lgcc -lc -ldl -lgcc

[Mark-Simulacrum]

@bors r+ rollup=iffy

[bors]

📌 Commit f077233ab84a97c71300cc37e94d1d1c7d584349 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit f077233ab84a97c71300cc37e94d1d1c7d584349 with merge 144163b601605bc1e85ec5580115659ca3faf399...

[bors]

💔 Test failed - checks-actions

[ghost]

There seem to be two main issues:

1. __sync_* missing

These are normally defined by compiler-builtins but they are only exposed on target_os = "linux" and not android:
https://github.com/rust-lang/compiler-builtins/blob/master/src/lib.rs#L52-L53

This should be changed to include android as it also uses them.

Implemented here: rust-lang/compiler-builtins#423

2. Missing unwinder

libgcc included an unwinder which we are now missing. ndk r23 beta 3 now includes libunwind as a replacement. So we could check in the unwind build script if the C compiler can link with libunwind and fall back to libgcc otherwise.

Any other ideas are appreciated.

[ghost]

The mentioned issues should now be resolved as compiler-builtins now exposes the missing functions. And we now try to find the newer libunwind using the C compiler and fall back to libgcc if it cannot be found.

[petrochenkov]

@ATiltedTree
To clarify, you are going to use this for custom builds of the standard library, right?
The NDK on CI is still old, and will always result in linking to libgcc in released Rust toolchains.

@bors r+

[bors]

📌 Commit 965997b has been approved by petrochenkov

[ghost]

To clarify, you are going to use this for custom builds of the standard library, right?

Yes, I am going to use this for custom std builds.

The NDK on CI is still old, and will always result in linking to libgcc in released Rust toolchains.

And that is fine as long as I can do custom builds with newer toolchains.

[bors]

⌛ Testing commit 965997b with merge 96acff98ad88f528eac1e23f2c1f6ce47f4b0f2b...

[phatblat]

Confirmed that @ssrlive's dummy libgcc.a workaround #85806 (comment) works with both NDK versions 23.1.7779620 and 25.1.8937393 with rust version 1.66.1 android targets.

Here's a one-liner script to drop these libgcc.a files next to every libunwind.a.

find "${ndk_dir}" -name "libunwind.a" \
    -execdir bash -c 'echo "INPUT(-lunwind)" > libgcc.a' \;

ndk_dir can be a specific NDK version dir (e.g. ~/Library/Android/sdk/ndk/25.1.8937393) or just ~ to get them all.

[cjdelisle]

In case you are using busybox find (e.g. alpine linux), this should work:

find "${ndk_dir}" -name 'libunwind.a' | \
  sed 's@libunwind.a$@libgcc.a@' | \
  while read x; do
    echo "INPUT(-lunwind)" > $x
  done