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Description
I would assume that the following two snippets of code would compile to identical assembly
pub fn convert_hex(d: char) -> Option<u32> {
if ('0'..='9').contains(&d) {
Some(u32::from(d) - u32::from('0'))
} else if ('A'..='Z').contains(&d) {
Some(10 + u32::from(d) - u32::from('A'))
} else {
None
}
}
pub fn convert_hex2(c: char) -> Option<u32> {
match c {
'0'..='9' => Some(u32::from(c) - u32::from('0')),
'A'..='Z' => Some(10 + u32::from(c) - u32::from('A')),
_ => None,
}
}However, as of rustc 1.65.0 convert_hex2 with the match blocks generates suboptimal code that is approximately 30% slower in a microbenchmark. Godbolt link showing asm comparison between 1.64.0 vs 1.69.0.
I don't think it's an LLVM regression since the emitted IR differs between 1.64 and 1.65: https://rust.godbolt.org/z/7b3EbGfj3
Version it worked on
It most recently worked on: 1.64.0
Version with regression
1.65.0 - nightly (as of 1.69.0)
@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged +A-codegen +C-bug +T-compiler +I-slow
I'll try to bisect the commit behind the change.