Increased code size due to unnecessary memcpy libcall in `MaybeUninit::assume_init`

[piegamesde]

After updating the compiler toolchain, I noticed that my binary got about 100 bytes larger. In my extremely size constrained environment, this is an increase of 5%. After a lot of minimizing, I tracked the main increase down to MaybeUninit::assume_init. It adds a new call to libc's memcpy. As far as I can tell, the code is moving the value on the stack a few bytes around, which is pointless (alignment should not be the issue here).

This might be related to #61011

Code

Godbolt link

Version it worked on

The last version I got it to run on was nightly-2022-02-01, but it might be working until as late as 2022-02-28 (I had some unrelated compile errors that made bisecting hard).

Version with regression

The older nightly with regression is 2022-03-12

rustc --version --verbose:

rustc 1.61.0-nightly (335ffbfa5 2022-03-11)
binary: rustc
commit-hash: 335ffbfa547df94ac236f5c56130cecf99c8d82b
commit-date: 2022-03-11
host: x86_64-unknown-linux-gnu
release: 1.61.0-nightly
LLVM version: 14.0.0

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@piegamesde thanks for the details. Do you happen to have the time to run a bisection on the target platform you're using (riscv32i-unknown-none-elf) and pin the exact commit the regression started? That would help a lot.

@rustbot label -I-prioritize +P-medium

[piegamesde]

I already did a preliminary bisect, hence the date range above. I could not get any closer because my code did not compile during the days in between. This was before minimization though, I will try again with the smaller code and hope that it will keep compiling. However, I due to the tooling I use the best granularity I will be able to provide will be nightly builds and not individual commits.

(btw your Zulip link is broken)

[apiraino]

ok thank you so much. being able to single out the nightly is probably enough

(and I have fixed the link, thanks for the notice)

[piegamesde]

I tried bisecting with the following one-liner: nix-shell -p 'with (import <nixpkgs> { overlays = [(import (builtins.fetchTarball https://github.com/mozilla/nixpkgs-mozilla/archive/master.tar.gz))]; }); ((rustChannelOf { date = "2022-02-01"; channel = "nightly"; }).rust.override { targets = [ "riscv32i-unknown-none-elf" ]; })' --pure --run 'rustc --version && rm -f bug.s && rustc bug.rs --target=riscv32i-unknown-none-elf -C opt-level=s -C panic=abort -C codegen-units=1 -C lto=fat -C codegen-units=1 --cfg feature=\"panic_immediate_abort\" --crate-type lib --emit asm && cat bug.s. bug.rs is the repro source copied over from the Godbolt link.

The bad news is: I went back until the beginning of 2020 and all versions I tested emitted an additional memcopy for assume_init. So probably this is a case that never worked, and the regression I experienced must have happened on a different level.