x86 backend miscompilation w/ packed structs and bitwise operation

[MasonRemaley]

Zig Version

0.14.0-dev.616+ab4eeb770

Steps to Reproduce and Observed Behavior

I know the x86 backend isn't passing all tests yet anyway, so I'm not sure if documenting these sorts of bugs is useful or not--if not let me know! I'm just very excited about how fast my project compiles under this backend haha.

Here's a reproduction of the problem:

const std = @import("std");
pub fn main() void {
    // Create a packed struct
    const Packed = packed struct {
        should_be_true: bool,
    };

    // Create a variable and then set it to zero. If we set it immediately to zero the bug
    // doesn't occur, presumably because the bitwise math can then be trivially optimized out.
    var zero: u32 = 5;
    zero -= 5;
    std.debug.assert(zero == 0); // <-- All good

    // This bitwise math results in true as expected
    const should_be_true = (zero & 1) != 1;
    std.debug.assert(should_be_true); // <-- All good

    // Store the same bitwise math in this packed struct. Now the value is true instead of
    // false.
    const result: Packed = .{ .should_be_true = (zero & 1) != 1 };
    std.debug.assert(result.should_be_true); // <-- This fails
}

Expected Behavior

I expected the result of the bitwise operation to be the same in both cases.

[mlugg]

Yeah, we're at a point where it's worth reporting bugs in x86_64 self-hosted; thanks!

[xtexx]

This seems to be the same bug as #20113. It looks like that #23256 also fixed this.