'Apparent no-op in Rust assembly output?

I was studying the x86_64 assembly for the following function:

/// Returns the greatest power of two less than or equal to `self`, or 0 otherwise.
pub const fn prev_power_of_two(n: u64) -> u64 {
    // n = 0 gives highest_bit_set_idx = 0.
    let highest_bit_set_idx = 63 - (n|1).leading_zeros();
    // Binary AND of highest bit with n is a no-op, except zero gets wiped.
    (1 << highest_bit_set_idx) & n
}

When compiling with -C opt=level=3 we get the following assembly on nightly and 1.50.0:

example::prev_power_of_two:
        mov     rax, rdi
        or      rax, 1
        bsr     rcx, rax
        xor     ecx, 63
        xor     cl, 63
        mov     eax, 1
        shl     rax, cl
        and     rax, rdi
        ret

Everything seems good, except I can't explain these two instructions:

    xor     ecx, 63
    xor     cl, 63

As far as I can see, these are a no-op. Why are they generated?

assemblyrustbit-manipulationx86-64


Solution 1:[1]

This problem has been fixed. Rust nightly no longer generates these xor instructions.

See Godbolt

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1 Robin Lindner

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