Performance regression in nightly with target-cpu=native on znver4

[nsarlin-zama]

Code

Hi, I have a regression when I try to run benchmarks using the nightly compiler on a crate that does elliptic curve operations:
In this repo: https://github.com/zama-ai/tfhe-rs, on branch v1.0 , in the crate tfhe-zk-pok, running the bench with RUSTFLAGS="-C target-cpu=znver4" cargo +nightly-2025-02-18 bench -p tfhe-zk-pok --bench pke_v2 -- pke_zk_verify_v2::PKEV2.

The machine running the bench is an aws hpc7 with an AMD EPYC 9R14 CPU.
Depending on the bench I get around 30% slower timings.

A bisect shows that the regression was introduced with nightly 2025-02-18. Stable toolchains do not seem to be impacted. The slowdown is only visible with -C target-cpu=znver4.

Version it worked on

All stable toolchains, and nightly before 2025-02-18.

Version with regression

nightly 2025-02-18 and after. The latest one as of today (2025-04-04) also has the regression.

Thank you for your help.

[lqd]

2025-02-18 sounds recent enough to have a more precise bisection, to a given PR with https://github.com/rust-lang/cargo-bisect-rustc/ -- could you try to do it?

[nikic]

2025-02-18 is probably the LLVM 20 upgrade.

[lqd]

Ah indeed, #135763 landed on the 17th.

[nsarlin-zama]

I can confirm from cargo-bisect-rustc (great tool btw!) that the regressing commit is the one introducing llvm-20: ce36a96