Compiler incorrectly assumes int will never be one

[AnttiParaoanu]

The following code tries to parse a decimal integer followed by a newline. For some reason the compiler assumes that the parsed number will never be one. opt-level has to be 2, 3, s or z

fn main() {
    let buffer = [49u8, 10];
    let mut a : u64 = 0;
    'read: loop {
        for c in &buffer {
            match c {
                48..=57 => {
                    a*= 10;
                    a+= *c as u64 - 48;
                }
                10 => {
                    break 'read;
                }
                _ => {
                    unsafe { std::hint::unreachable_unchecked() };
                }
            }
        }
    }
    if a == 1 {
        println!("What did you expect?");
    }
}

I expected to see this happen: When running the program it prints "What did you expect?"

Instead, this happened: The program doesn't print anything, just exits

Meta

rustc --version --verbose:

rustc 1.43.0-nightly (823ff8cf1 2020-03-07)
binary: rustc
commit-hash: 823ff8cf1397a5772b1f6954b60576202bf91836
commit-date: 2020-03-07
host: x86_64-unknown-linux-gnu
release: 1.43.0-nightly
LLVM version: 9.0

On godbolt the issue seems to go back to version 1.38

[ehuss]

Just out of curiosity, I bisected to #62592. Probably not helpful, but I see a note about changes to unreachability in jump tables in the release notes.

[Centril]

cc @nikic @cuviper @alexcrichton

[nikic]

Looks like the condition gets removed by instcombine: https://gist.github.com/nikic/5f13662ae7559c16fdcc2e3cd2b17e5d (Might be caused by a prior change though.)

[cuviper]

I found that the same IR optimizes correctly with LLVM master. I'm currently bisecting...

[nikic]

Based on the structure of the IR, my first guess would be the InstSimplify part of llvm/llvm-project@a7d992c.

[cuviper]

You're right, it's bad up to a7d992c0f2d2^ (1164d43855fd) and correct with a7d992c0f2d2.

[cuviper]

Also, it's fine with LLVM 8, consistent with @ehuss's bisection, so this is just a regression in LLVM 9.

edit: I bisected the regression specifically to llvm/llvm-project@0290a77

[tstellar]

Looks like the condition gets removed by instcombine: https://gist.github.com/nikic/5f13662ae7559c16fdcc2e3cd2b17e5d (Might be caused by a prior change though.)

Could you post dumps from both before and after instcombine ?