Miscompilation of libcore with -Zmir-opt-level=3

[bjorn3]

Compile the standard library with -Zmir-opt-level=3 and then running libcore's test suite with this version of the standard library results in several tests failing. I am seeing this on cg_clif's CI (https://github.com/bjorn3/rustc_codegen_cranelift/actions/runs/4203400456/jobs/7292743420) but I expect it to be reproducible using LLVM too.

Meta

Last known good version: rustc 1.69.0-nightly (065852def 2023-02-13)

First known bad version:

rustc 1.69.0-nightly (9a7cc6c32 2023-02-16)
binary: rustc
commit-hash: 9a7cc6c32f1a690f86827e4724bcda85e506ef35
commit-date: 2023-02-16
host: aarch64-unknown-linux-gnu
release: 1.69.0-nightly
LLVM version: 15.0.7

Regression range: 065852d...9a7cc6c

#107449 stands out as a changed MIR opt.

[saethlin]

Try adding -Zmir-enable-passes=-DataflowConstProp to your RUSTFLAGS instead of lowering your MIR opt level. That makes the miscompiles go away for me and suggests that #107411 is the cause.

Turning off CopyProp has no effect, so it seems very unlikely that pass is involved. Turning off MIR inlining also makes the miscompiles go away, but that's probably just because that makes a lot of other MIR opts just not fire.

[saethlin]

Also cc #105706 + #105736 which are stuck in "I'm not sure how to make this work" limbo, but might/should have prevented this.

[bjorn3]

I can confirm the miscompilation is now fixed.