miscompilation possibly related to union layout

[Techatrix]

Zig Version

0.11.0-dev.3978+711b4e93e

Steps to Reproduce and Observed Behavior

const std = @import("std");

const Inner = union {
    integer: u32,
    string: [8]u8, // modifying the size of this field will change the behavior
                   // you could try out [4]u8 or [12]u8
};

const Outer = union {
    unreachable_inner: Inner,
    params: [16]u8,
};

pub fn foo(params: [16]u8) Outer {
    var id: ?u32 = null;
    std.mem.doNotOptimizeAway(&id);
    if (id) |id_obj| {
        return .{ .unreachable_inner = .{ .integer = id_obj } };
    }
    return .{ .params = params };
}

pub fn main() void {
    var val1 = [16]u8{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 };
    std.debug.print("{any}\n", .{val1});
    const val2 = foo(val1);
    std.debug.print("{any}\n", .{val2.params});
}

running zig run file.zig -O ReleaseSafe will output:

{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 }
{ 0, 1, 2, 3, 4, 5, 6, 7, 0, 9, 10, 11, 12, 13, 14, 15 }

There is a zero at index 8.
This only happens in ReleaseSafe. I did not observe this behavior in 0.10.1.

https://godbolt.org/z/qc3MsW7d6

Expected Behavior

{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 }
{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 }

[jacobly0]

We output llvm for unions in the same way as clang does, as can be seen by the fact that this bug reproduces with clang.

[andrewrk]

Moving this to the 0.12.0 milestone since it's an issue to be solved in LLVM 17.

[henrikolsen549]

Not fixed by a63a1c5 (LLVM 17).

This miscompilation frequently happens when parsing a JSON value with two or more nested objects, so maybe it makes sense to implement the workaround suggested in the LLVM issue.

[henrikolsen549]

Fixed by #17963.

[jacobly0]

Fixed by #17963.

That's a workaround, the issue stays open to track the upstream bug and a revert of the workaround.

[henrikolsen549]

@jacobly0, with LLVM 18 the workaround can probably now be reverted.