Failure to remove alloca

[Kmeakin]

str::starts_with(char) and str::ends_with(char) converts the 4-byte character codepoint into a 1-4 byte buffer on the stack, then compares against the start/end of the str's bytes contents. LLVM is able to remove the stack allocation when the char is a single byte, but if it is 2 or more, it is not able:

https://godbolt.org/z/zjhanPx7j
https://alive2.llvm.org/ce/z/MiAEjE

#[no_mangle]
pub fn src1(s: &str) -> bool {
    s.starts_with('a')
}

#[no_mangle]
pub fn tgt1(s: &str) -> bool {
    s.starts_with("a")
}

#[no_mangle]
pub fn src2(s: &str) -> bool {
    s.ends_with('a')
}

#[no_mangle]
pub fn tgt2(s: &str) -> bool {
    s.ends_with("a")
}

#[no_mangle]
pub fn src3(s: &str) -> bool {
    s.starts_with('£')
}

#[no_mangle]
pub fn tgt3(s: &str) -> bool {
    s.ends_with("£")
}

#[no_mangle]
pub fn src4(s: &str) -> bool {
    s.ends_with('£')
}

#[no_mangle]
pub fn tgt4(s: &str) -> bool {
    s.ends_with("£")
}

[efriedma-quic]

Dup of #52701?