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thumbv6m-none-eabi inline asm clobber_abi("C") fails with register 'LR' allocated for constraint '{r14}' does not match required type #163173
Description
The following minimized example fails to compile on the latest nightly, but works on stable and the previous nightly:
#![no_std]
#[no_mangle]
pub fn test() {
unsafe { core::arch::asm!("nop", clobber_abi("C")) }
}
#[panic_handler]
fn panic(_: &core::panic::PanicInfo) -> ! {
loop {}
}Current nightly:
❯ rustc +nightly-2026-09-22 --version
rustc 1.100.0-nightly (1303417c4 2026-09-21)
❯ rustc +nightly-2026-09-22 --target thumbv6m-none-eabi --crate-type staticlib -o repro.a repro.rs
error: register 'LR' allocated for constraint '{r14}' does not match required type
--> repro.rs:6:32
|
6 | unsafe { core::arch::asm!("nop", clobber_abi("C")) }
| ^^^
error: aborting due to 1 previous error
Prior nightly:
❯ rustc +nightly-2026-09-21 --version
rustc 1.100.0-nightly (bba531001 2026-09-20)
❯ rustc +nightly-2026-09-21 --target thumbv6m-none-eabi --crate-type staticlib -o repro.a repro.rs
Stable:
❯ rustc +stable --version
rustc 1.98.1 (48a229cea 2026-09-01)
❯ rustc +stable --target thumbv6m-none-eabi --crate-type staticlib -o repro.a repro.rs
This can also be reproduced with the examples of embassy-rp and zeptos, and likely anything that calls into the RP2040 boot ROM for flash operations.
Activity
zachs18 commentedon Sep 23, 2026
searched nightlies: from nightly-2026-09-21 to nightly-2026-09-23
regressed nightly: nightly-2026-09-22
searched commit range: bba5310...1303417
regressed commit: f45772e
bisected with cargo-bisect-rustc v0.6.11
Host triple: x86_64-unknown-linux-gnu
Reproduce with:
Currently testing with rust-lang/cargo-bisect-rustc#438 to check which PR in the rollup #163095 may have caused the regression. #163062 seems likely.
Edit: Yes,
cargo-bisect-rustcsaid dbcd41d (edit 2: and Reverting 3de0624 seems to fix the regression), so seems like #163062 caused the regression. CC @malezjaa @folkertdevmalezjaa commentedon Sep 23, 2026
Well for now, I think we can revert the ARM r14 change and keep the AArch64 x30 change. That should fix the regression. I can look for a proper fix for {r14} later today.
folkertdev commentedon Sep 23, 2026
Hmm yeah,
{r14}works for arm but not for thumb, I think that needs further LLVM changes.Can you revert the arm part and add a test for thumb (that would have caught this regression)?
Rollup merge of rust-lang#163214 - malezjaa:revert-r14-register-name,…
Rollup merge of rust-lang#163214 - malezjaa:revert-r14-register-name,…
Unrolled build for #163214 in rollup 163244
[ARM] fix `r14` aliasing `lr` on `thumb-1` (#225746)
Automerge: [ARM] fix `r14` aliasing `lr` on `thumb-1` (#225746)
Automerge: [ARM] fix `r14` aliasing `lr` on `thumb-1` (#225746)
[ARM] fix `r14` aliasing `lr` on `thumb-1` (#225746)
[ARM] fix `r14` aliasing `lr` on `thumb-1` (#225746)