Wildcard pattern not optimized in match expression

[mitiko]

The following snippet generates branches (with -C opt-level=3)

pub fn func1(a: u16, b: u16, v: u16) -> u16 {
    match (a == v, b == v) {
        (true, false) => 0,
        (false, true) => u16::MAX,
        _ => 1 << 15, // half
    }
}

whereas spelling it out to the compiler does not:

pub fn func2(a: u16, b: u16, v: u16) -> u16 {
    match (a == v, b == v) {
        (true, false) => 0,
        (false, true) => u16::MAX,
        (true, true) => 1 << 15, // half
        (false, false) => 1 << 15, // half
    }
}

I believe this breaks the zero cost abstraction promise.
In theory these do mean different things but for constant 16-bit integers on a modern 64-bit system they should be optimized away.

Other quirks:

• only partially spelling it out to the compiler with (true, true) | (false, false) still generates branches
• using guard clauses generates branchless code but with more instructions

Godbolt link: https://rust.godbolt.org/z/cWoboKM3d

[clubby789]

@rustbot label +I-slow +A-llvm

[veera-sivarajan]

Looks like this is fixed now: https://rust.godbolt.org/z/G93zG8jdb