Spooky action at a distance when calling a `callconv(.c)` function that returns an `extern struct`

[freakmangd]

Zig Version

0.16.0-dev.235+377a8b2a3

Steps to Reproduce and Observed Behavior

pub fn main() void {
    var cond = false;
    _ = &cond;

    const num: f32 = if (cond) 100 else 200;

    var a = f();
    _ = &a;

    std.debug.print("{}\n", .{num});
}

pub fn f() callconv(.c) A {
    return .{};
}

pub const A = extern struct {
    x: f32 = 0,
    y: f32 = 0,
    z: f32 = 0,
};

On Fedora linux x86_64, running this snippet with zig run src/main.zig outputs 0
Running with zig run src/main.zig -fllvm outputs 200

Notes:

Inserting a print before f is called gives a correct output of 200 twice.

Replacing

const num: f32 = if (cond) 100 else 200;

with

var num: f32 = 200;
if (cond) num = 100;

also gives the correct output of 200.

Expected Behavior

Expected an output of 200 with the original snippet

[ibotha]

I encountered something that may be related when compiling wgpu-native-zig. llvm works fine but the zig compiler causes a runtime crash.

There is a short discussion here about it bronter/wgpu_native_zig#37

This is for zig 0.15.1 but this is probably related

[jacobly0]

No longer repros on master, possibly fixed by cbbb67c.