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:

// 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!