`noreturn` on diverging functions makes LLVM trash the link register on thumb targets

[jonas-schievink]

Downstream issue: rust-embedded/cortex-m-rt#139

It looks like LLVM is extremely eager to overwrite the Link Register without saving it as soon as noreturn is put on a function. Since rustc does that for any -> ! function, which includes many parts of the panic machinery, this effectively makes obtaining a backtrace on panic on embedded systems impossible.

This is somewhat analogous to omission of frame pointers on x86, so maybe -Cforce-frame-pointers can be repurposed to prevent this behavior on Thumb/ARM targets? I'm not sure there's even an LLVM feature to control this specifically, but I suppose rustc could always omit the noreturn.

Since this is only important in debug builds (that may be optimized, however), it might make sense to couple this behavior to whether debug assertions are enabled, but I'm not sure about that. What do you think? Would a patch doing something of this sort be accepted?

[hanna-kruppe]

Note that LLVM can also infer noreturn on its own, not in all cases where rustc can add it, but certainly in many simple cases such as panic-abort's panic handler.

Have you tried -Cforce-frame-pointer and found it didn't help? At a glance it seems that the ARM backend respects the usual way to request the frame pointer to be preserved (the frame-pointer function attribute), so I would expect -Cforce-frame-pointer to work already.

[jonas-schievink]

Thanks, that really does look like it'd fix this, but according to rust-embedded/cortex-m-rt#139 (comment) it doesn't help.

[hanna-kruppe]

Oh, right. A pre-built libcore, at least the mononorphic non-inline parts such as core::panicking::panic_fmt, won't usually be affected by the -C flag. Does rebuilding libcore (via Xargo or cargo -Z build-std=...) help?

[MabezDev]

Oh, right. A pre-built libcore, at least the mononorphic non-inline parts such as core::panicking::panic_fmt, won't usually be affected by the -C flag.

Ah, I always for get about that!

Running the minimal example with cargo +nightly run -Z build-std=core and "-C", "force-frame-pointers=yes" does indeed fix the corrupted backtrace! 🎉

Breakpoint 3, rust_begin_unwind (_info=0x2000feb8)
    at /home/mabez/.cargo/registry/src/github.com-1ecc6299db9ec823/panic-halt-0.2.0/src/lib.rs:32
32          loop {
(gdb) bt
#0  rust_begin_unwind (_info=0x2000feb8)
    at /home/mabez/.cargo/registry/src/github.com-1ecc6299db9ec823/panic-halt-0.2.0/src/lib.rs:32
#1  0x080008de in core::panicking::panic_fmt (fmt=..., 
    location=0x8003a40 <.Lanon.b0bb6e8da484f667ddcf6a6acf4928ab.9>)
    at /home/mabez/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/panicking.rs:85
#2  0x08002972 in core::slice::slice_index_len_fail (index=1024, len=1)
    at /home/mabez/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/slice/mod.rs:2674
#3  0x08000800 in <core::ops::range::Range<usize> as core::slice::SliceIndex<[T]>>::index (self=..., slice=...)
    at /home/mabez/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/slice/mod.rs:2838
#4  0x08000830 in <core::ops::range::RangeTo<usize> as core::slice::SliceIndex<[T]>>::index (self=..., slice=...)
    at /home/mabez/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/slice/mod.rs:2880
#5  0x08000516 in core::slice::<impl core::ops::index::Index<I> for [T]>::index (
    self=..., index=...)
    at /home/mabez/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/slice/mod.rs:2656
#6  0x0800043c in backtrace::__cortex_m_rt_main () at src/main.rs:19

What are the performance implications of forcing framepointers? Would forcing frame pointers for thumb* targets (Does this affect all baremetal targets?) be such a bad idea?

[jonas-schievink]

Is it possible to use -Cforce-frame-pointers=off to forcefully disable them again? If so, I think there's not much harm in enabling them by default for these targets. At most, we'll pay for a bit of stack space since LR is saved slightly more often, as well as slightly higher register pressure, I think?

[MabezDev]

Is it possible to use -Cforce-frame-pointers=off to forcefully disable them again?

Not sure I follow; do you mean for downstream crates? I think if the core crate is built with framepointers, the downstream crates shouldn't have to have it enabled. I'm not sure how we could test that theory though.

[jonas-schievink]

I want to know if it would be possible to opt out of frame pointers again if we decide to enable them by default

[hanna-kruppe]

I am reasonably sure that -Cforce-frame-pointers will always override the target's default, so it should work as an opt-out (except of course for pre-built machine code in sysroot crates).