panic_unwind: Use global_asm! for IMGREL relocations

[Darksonn]

View all comments

Rust consteval cannot construct 32-bit values that contain the offset between two symbols, but this is required for panic_unwind on some architectures because unwinding on Windows SEH uses image-base-relative pointers for the panic information (which makes them fit in 32-bit rather than 64-bit).

Currently, this is worked around by initializing these globals on panic with atomic stores. This works, but is undefined behavior as the panic runtime reads the values with non-atomic loads, which leads to a data race.

Instead, utilize global_asm! to create the globals with @IMGREL relocations so that the linker constructs the relative pointers for us at link or load time.

For context: #t-compiler > relative pointers in windows SEH panic_unwind @ 💬

AI assistance was involved with writing the assembly code.

[rustbot]

r? @LawnGnome

rustbot has assigned @LawnGnome.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: libs
• libs expanded to 12 candidates
• Random selection from 7 candidates

[Fulgen301]

(Nitpick: I'd clarify that they're offsets from the image base and not __ImageBase, even if that's the exact same address.)

View changes since the review

[LawnGnome]

Re-rolling, as my asm is absolutely not good enough for this.

@rustbot reroll