Performance regression between 1.78 and 1.79

We were upgrading from rust 1.78 to 1.79 and later 1.81 and we discovered a performance regression in our applications in certain cases up to 10%.
In our benchmarks we were using iai-callgrind to measure instructions and we have seen a significant increase there.

Unfortunately, I can't provide exact code as they are proprietary but in the nature they have long callchains with lots of small functions.

The same issue was reported here.

I have bisected the 1.78 and 1.79 version and found that the following change causes performance regression: https://github.com/rust-lang/rust/commit/3412f0174934778dd803081dc7c4b5864309350f#diff-deee82aaf9baf43ab05d939355f6249fdacf8959bc0e06c9574283453f838ee9R702

Release cargo profile

codegen-units = 1
opt-level = 3
lto = "thin"

Workaround

Adding debug = 1 to the profile in release mode essentially disables this change and the performance degradation is not there anymore as expected.

Start by inspecting commit 3412f0174934778dd803081dc7c4b5864309350f and its referenced diff around line R702. Reproduce the regression with a suitable benchmark using iai-callgrind and the reported release profile; done means confirming and isolating the instruction increase between Rust 1.78 and later versions.