Redundant overflow check in a simple 3-line function

[glessard]

Description

From an implementation of the prefix(count:) function for an UnsafeBufferPointer-like struct:

    public func testOne(k: Int) -> Int {
        precondition(k >= 0)
        let dc = min(k, _count)
        return _count-dc
    }

The _count-dc line produces an overflow check (in -O), even though the possibility of overflow has been excluded by the previous operations.

Reproduction

struct Z {
    let _count: Int

    public func testOne(k: Int) -> Int {
        precondition(k >= 0)
        let dc = min(k, _count)
        return _count-dc
    }

    public func testTwo(k: Int) -> Int {
        precondition(k >= 0)
        let dc = min(k, _count)
        return _count&-dc
    }
}

Expected behavior

testTwo() produces one branch (in the precondition). I would expect the same of testOne(), since the precondition and the min() function combine to make an overflow impossible. However, testOne() produces two branches, one in the precondition and the other in the - operator.

Environment

Swift version 6.1-dev (LLVM 95f3fb07f8f5294, Swift 8ae66ec)
Target: x86_64-unknown-linux-gnu

Additional information

rdar://138518973