Backtraces broken on s390x

[cuviper]

run-pass/backtrace.rs and run-pass/backtrace-debuginfo.rs are both broken on s390x since Rust 1.28. When std is built without debuginfo, the tests hang with their backtracing processes stuck in a loop. With std debuginfo, it fails with <unknown> output like:

thread 'main' panicked at 'bad output: thread 'main' panicked at 'explicit panic', [...]/rust/src/test/run-pass/backtrace.rs:25:9
stack backtrace:
   0: rust_metadata_std_afee429493f25d7710e1d9899ebe9736
   1: rust_metadata_std_afee429493f25d7710e1d9899ebe9736
   2: rust_metadata_std_afee429493f25d7710e1d9899ebe9736
   3: rust_metadata_std_afee429493f25d7710e1d9899ebe9736
   4: std::panicking::rust_panic_with_hook
   5: <unknown>
   6: <unknown>
   7: <unknown>
   8: <unknown>
   9: rust_metadata_std_afee429493f25d7710e1d9899ebe9736
  10: rust_metadata_std_afee429493f25d7710e1d9899ebe9736
  11: __rust_maybe_catch_panic
  12: std::rt::lang_start_internal
  13: <unknown>
  14: __libc_start_main
  15: <unknown>
', [...]/rust/src/test/run-pass/backtrace.rs:60:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.

I believe the problem is this part of #50955:
https://github.com/rust-lang/rust/pull/50955/files#diff-df8879cba69175008878e97cee6d7d91R113
where target.contains("64") doesn't identify that s390x-unknown-linux-gnu is a 64-bit platform.

See also rust-lang/backtrace-rs#122. I plan to update that here when it's published, and make a similar fix in rust/src/libstd/build.rs.

[cuviper]

#53377 fixes the backtrace when std is built with debuginfo, but I'm still getting a hang without.

In GDB, it looks like the unwinder gets to the __rust_try that catches main panics, then loops unwinding that same PC forever. This part of the regression is older -- happens on 1.27.2 as well, but 1.26.2 is fine. Binaries from the older version don't appear to have a distinct __rust_try symbol at all, but maybe that's just inlined differently.

I'm bisecting for that hang now.

[cuviper]

Bisecting pointed me at #50188 (cc @alexcrichton), which added target-feature attributes to all functions. For s390x the base features actually include a removal, "-vector". But it doesn't look like the features are applied to the synthesized __rust_try (intrinsics::try), and as a result it doesn't get inlined anymore since it has more features (w/ vector) than the callee.

• We probably should apply target-features to __rust_try as well.
• I don't know why the non-inlined __rust_try can't be unwound -- maybe we need to force uwtable?

By default GDB backtrace stops at crate main, but set backtrace past-main on tries to go further:

(gdb) bt
#0  0x000002aa00034064 in std::sys::unix::backtrace::tracing::imp::trace_fn ()
#1  0x000003fffdd0a0ac in _Unwind_Backtrace () from /lib64/libgcc_s.so.1
#2  0x000002aa00033fbc in std::sys::unix::backtrace::tracing::imp::unwind_backtrace ()
#3  0x000002aa00039872 in std::sys_common::backtrace::print ()
#4  0x000002aa00029252 in std::panicking::default_hook::{{closure}} ()
#5  0x000002aa00028ede in std::panicking::default_hook ()
#6  0x000002aa0002979c in std::panicking::rust_panic_with_hook ()
#7  0x000002aa000180c6 in std::panicking::begin_panic (msg=..., file_line_col=0x2aa00063d30) at /home/nfs/jistone/rust/src/libstd/panicking.rs:391
#8  0x000002aa0001d4e4 in backtrace::foo () at ./src/test/run-pass/backtrace.rs:25
#9  0x000002aa0001ec88 in backtrace::main () at ./src/test/run-pass/backtrace.rs:111
#10 0x000002aa00017f3c in std::rt::lang_start::{{closure}} () at /home/nfs/jistone/rust/src/libstd/rt.rs:74
#11 0x000002aa0002933a in std::panicking::try::do_call ()
#12 0x000002aa00042ddc in __rust_try ()
Backtrace stopped: previous frame identical to this frame (corrupt stack?)

Maybe Rust's trace_fn could have a similar defensive check, at least.

[alexcrichton]

Nice find @cuviper! Definitely makes sense to me, and this should be fixed up (at least the target-feature business) in #53437