borrowing_sub chain is not recognized if sub result is unused

[DaniPopes]

Code

I tried this code (godbolt):

#[no_mangle]
pub fn lt_thru_sub(a: &[u64; 4], b: &[u64; 4]) -> bool {
    let mut borrow = false;
    for i in 0..a.len() {
        (_, borrow) = borrowing_sub(a[i], b[i], borrow);
    }
    borrow
}

#[inline]
fn borrowing_sub(lhs: u64, rhs: u64, borrow: bool) -> (u64, bool) {
    let (result, borrow_1) = lhs.overflowing_sub(rhs);
    let (result, borrow_2) = result.overflowing_sub(borrow as u64);
    (result, borrow_1 | borrow_2)
}

I expected to see this happen: compiles down to a few sub and sbb instructions (bigint subtraction)

Instead, this happened: compiles to a bunch of cmp, set*, or, and

If the first result of borrowing_sub is observed, like accumulating it or storing it to an out pointer, then it will codegen the sbb chain. As an example, you can uncomment the black_box line in the godbolt link above

Version it worked on

It most recently worked on: 1.66

rustc 1.66.0 (69f9c33d7 2022-12-12)
binary: rustc
commit-hash: 69f9c33d71c871fc16ac445211281c6e7a340943
commit-date: 2022-12-12
host: x86_64-unknown-linux-gnu
release: 1.66.0
LLVM version: 15.0.2

Version with regression

rustc --version --verbose:

rustc 1.67.0 (fc594f156 2023-01-24)
binary: rustc
commit-hash: fc594f15669680fa70d255faec3ca3fb507c3405
commit-date: 2023-01-24
host: x86_64-unknown-linux-gnu
release: 1.67.0
LLVM version: 15.0.6

rustc 1.90.0-nightly (da58c0513 2025-07-03)
binary: rustc
commit-hash: da58c051315268a197ce280f6ba07bbd03c66535
commit-date: 2025-07-03
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7

---

Regressed in #103299 and/or #103479, cc @nikic

searched toolchains nightly-2022-10-29 through nightly-2022-12-09


********************************************************************************
Regression in nightly-2022-10-31
********************************************************************************

fetching https://static.rust-lang.org/dist/2022-10-30/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2022-10-30: 40 B / 40 B [=========================================================================] 100.00 % 1.38 MB/s converted 2022-10-30 to 5e9772042948002f9c6f60c4c81603170035fffa
fetching https://static.rust-lang.org/dist/2022-10-31/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2022-10-31: 40 B / 40 B [=========================================================================] 100.00 % 1.35 MB/s converted 2022-10-31 to 77e57db384aca99444c3b5f6a9c86bc58a804d89
looking for regression commit between 2022-10-30 and 2022-10-31
fetching (via remote github) commits from max(5e9772042948002f9c6f60c4c81603170035fffa, 2022-10-28) to 77e57db384aca99444c3b5f6a9c86bc58a804d89
ending github query because we found starting sha: 5e9772042948002f9c6f60c4c81603170035fffa
get_commits_between returning commits, len: 9
  commit[0] 2022-10-29: Auto merge of #103450 - cjgillot:elision-nodedup, r=Mark-Simulacrum
  commit[1] 2022-10-29: Auto merge of #103731 - Mark-Simulacrum:new-version, r=Mark-Simulacrum
  commit[2] 2022-10-30: Auto merge of #103745 - matthiaskrgr:rollup-hipjva8, r=matthiaskrgr
  commit[3] 2022-10-30: Auto merge of #103721 - RalfJung:miri, r=RalfJung
  commit[4] 2022-10-30: Auto merge of #103755 - Dylan-DPC:rollup-dl2hups, r=Dylan-DPC
  commit[5] 2022-10-30: Auto merge of #103010 - petrochenkov:effvisdoc, r=GuillaumeGomez
  commit[6] 2022-10-30: Auto merge of #103295 - ishitatsuyuki:ninja, r=cuviper
  commit[7] 2022-10-30: Auto merge of #103299 - nikic:usub-overflow, r=wesleywiser
  commit[8] 2022-10-30: Auto merge of #103479 - nikic:update-llvm-9, r=cuviper
ERROR: no CI builds available between 5e9772042948002f9c6f60c4c81603170035fffa and 77e57db384aca99444c3b5f6a9c86bc58a804d89 within last 167 days

@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged +A-LLVM