Fix unwinding through Reset

[jonas-schievink]

Unwinders may detect the end of the program by seeing 0xFFFFFFFF in lr, which is why code to ensure that it has that value was added in #293. However, the bl main overwrites that value with the current program counter. This PR saves the old lr value on the stack, and adds debuginfo entries to allow an external unwinder to restore the value.

This fixes knurling-rs/probe-run#277

[rust-highfive]

r? @therealprof

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[jonathanpallant]

This might be worth an explanation, as it's not something you often come across in ARM asm.

Maybe link to https://sourceware.org/binutils/docs/as/CFI-directives.html, or include their note:

.cfi_def_cfa defines a rule for computing CFA as: take address from register and add offset to it.

[jonas-schievink]

Added a comment and link

[jonathanpallant]

Can confirm it resolves the upstream issue though:

PS C:\Users\Test\Documents\knurling\app-template> cargo run --bin hello
    Blocking waiting for file lock on build directory
   Compiling cortex-m-rt v0.7.0 (https://github.com/knurling-rs/cortex-m-rt.git?branch=fix-unwind-through-reset#9c88eab9)
   Compiling stm32f4xx-hal v0.10.1
   Compiling my_project v0.1.0 (C:\Users\Test\Documents\knurling\app-template)
    Finished dev [optimized + debuginfo] target(s) in 15.88s
     Running `probe-run --chip STM32F429ZITx target\thumbv7em-none-eabi\debug\hello`
(HOST) INFO  flashing program (7.93 KiB)
(HOST) INFO  success!
────────────────────────────────────────────────────────────────────────────────
Hello, world!
└─ hello::__cortex_m_rt_main @ src\bin\hello.rs:8
────────────────────────────────────────────────────────────────────────────────
(HOST) INFO  device halted without error

[jonathanpallant]

bors r+

[bors]

🔒 Permission denied

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[jonathanpallant]

Ugh. Wearing the wrong hat.

[thejpster]

bors r+

[thejpster]

^ have switched hats

[bors]

Build succeeded:

• build-other (macOS-latest)
• build-other (windows-latest)
• ci-linux (1.39.0)
• ci-linux (stable)
• Rustfmt