rust: cross build broken on armv*hf-musl

[Gottox]

In the last few days we made huge steps forward to get a cross (and native non-x86) environment for rust.

@Cogitri, @jnbr, and I fixed all glibc platforms and aarch64 for both musl and glibc. The only platforms that remained broken are armv6hf-musl and armv7hf-musl.

The error occurs during cross building while linking rustc. It seems that libunwind is built with a wrong configuration. As noted in https://github.com/rust-lang/rust/blob/1.29.1/src/libunwind/libunwind.rs#L178, _Unwind_GetIP is implemented as a macro which is handled below the comment. As you can see in the buildlog, the linker complains about the missing function which indicates, that at https://github.com/rust-lang/rust/blob/1.29.1/src/libunwind/libunwind.rs#L96 the wrong branch is taken.

https://gist.github.com/Gottox/ebb16c18150b5d6daec7c67daaacd4b4

[Cogitri]

Well, the easiest way to find out is removing the supposedly wrong branch, I guess :D
https://gist.githubusercontent.com/Cogitri/3877c5ef3a08a8ad68a26a7b81d2df21/raw/b5fed95dbb0ccb2d970a12aefb906fb8f1f1461e/diff

Currently building with this - not that I actually want to use it, but it'd be good to know if it's actually caused by this.

[Gottox]

wouldn't that break the stage1 compiler?

[Cogitri]

Oh, oops, yes. Currently testing some other stuff, let's see...

[Cogitri]

Hm, I've reversed the logic of that function and it still returns the same error, I start doubting that the error actually comes from that...
https://gist.github.com/48ed2b3a37f3184aba866a3d2e1f7f98

[Duncaen]

I looked into it again but couldn't find a better explanation.
there is backtrace-sys/libbacktrace, but I think the files that actually use _Unwind_GetIP{,Info} are not actually used.

I don't have a machine to do test builds, so what I found was by looking at the sources.
Maybe a comparison between builds armv7 with glibc and musl would be interesting.

[jnbr]

Comparing the libstd-xxx.so files on glibc and musl shows that in both cases the correct branch is taken in libunwind/libunwind.rs. The problem is, that musl additionally links against _unwind_GetIP which comes from unwind() from src/vendor/backtrace-sys/src/libbacktrace/backtrace.c.

[Duncaen]

The difference is the the default include path order is different in musl and glibc.
Musl builds use libunwinds unwind.h and glibc builds use the unwind.h file provided by gcc.

They both provide the same api/prototypes, except that libgcc_s.so does not provide some functions (_Unwind_GetIP being one of them) for arm and defines a macro instead.
This is not directly a problem for other architectures where libgcc_s.so provides all the defined functions but still a mess and I don't really know how to work around it.

There are a few possible solutions I can think of:

• Get rid of libunwind entirely if this would be possible.
• Rename the header in libunwind.
• Use #include_next <unwind.h> in libunwinds header, but this would still mean glibc and musl builds use different header files.

[jnbr]

libunwind wasn't used before rust-1.28. Re-adding some old patches should make it work without again. Waiting for the build to complete ...

[Duncaen]

Nice I got the same feedback from musls freenode channel and a patch smaeul/rust@e3b9058

[jnbr]

we're currently building rust without crt-static, also bootstrap/sanity.rs errors out if there is no libunwind.a