Return value changed upon returning to callee (x86_64-unknown-none)

[elB4RTO]

When compiling for the target x86_64-unknown-none, using an opt-level higher than 0 resulted in a return value being changed upon returning to the callee.

The value is checked just before returning and it is a None, but right after returning the value becomes a Some. This doesn't happen when compiling with opt-level = 0, but any value higher than that triggers this behavior.

I've been unable to setup a minimum reproducible example, as the issue seems to appear in rare situations only, possibly depending on the context. Please refer to this PR for more information: elB4RTO/Rebel#8

I can provide the compiled binaries and/or the assembly dump of the faulty sections if needed

Compiler info

Output of rustc --version --verbose:

rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: x86_64-unknown-linux-gnu
release: 1.87.0
LLVM version: 20.1.1

Target triple: x86_64-unknown-none

[ultimaweapon]

This seems like you are hitting by an UB caused by incorrect unsafe code. I experienced this kind of issue before and it is caused by aliasing violation (e.g. you accidentally create a mutable reference while there are other immutable references).

[bjorn3]

The usage of &raw const for pointer which you will write through is questionable at best. Maybe you can try getting your code to run in miri?