llvm backend ignores `@ptrCast` on comptime-known values

[mitchellh]

Zig Version

0.11.0-dev.86+b83e4d965

Steps to Reproduce and Observed Behavior

typedef struct Origin {
    unsigned long x;
    unsigned long y;
    unsigned long z;
} Origin;

typedef struct Size {
    unsigned long width;
    unsigned long height;
    unsigned long depth;
} Size;

typedef struct Region {
    Origin origin;
    Size size;
} Region;

void do_the_thing(Region in) {
    // Modification is REQUIRED for the crash
    in.origin.x = 0;
}

const std = @import("std");

extern "c" fn do_the_thing(*anyopaque, *anyopaque, ...) void;

const Region = extern struct {
    origin: extern struct {
        x: c_ulong,
        y: c_ulong,
        z: c_ulong,
    },

    size: extern struct {
        width: c_ulong,
        height: c_ulong,
        depth: c_ulong,
    },
};

pub fn main() void {
    const new_fn = @ptrCast(*const fn (Region) callconv(.C) void, &do_the_thing);

    @call(.{}, new_fn, .{
        Region{
            .origin = .{ .x = 9, .y = 9, .z = 9 },
            .size = .{ .width = 9, .height = 9, .depth = 9 },
        },
    });
}

$ zig run -lc test.c test.zig
Segmentation fault at address 0xffffffffffffffe7
???:?:?: 0x104963c85 in _main (???)
zsh: abort      ./abi-test

Expected Behavior

It should call the C function and work.

Note it works FINE if the function signature is correct to begin with, i.e. the extern "c" is properly typed.

What the #$@! am I doing this for? This looks like a really stupid thing to do. Well, this is how objc_msgSend works and I'm interfacing with Objective-C with a type signature like this. I have to cast a function pointer to a different type so that arguments are passed correctly for the C ABI.

Note that other function signatures do work fine. This specific function signature is causing the segfault.

[gwenzek]

WDYM by "other function signatures do work fine" ? which functions are you changing signatures ? What happen if you declare extern "c" fn do_the_thing(Region) void; ?

[Vexu]

The LLVM backend is completely ignoring the @ptrCast because it is done on a comptime-known value. The issue can be worked around by making new_fn mutable.

[mitchellh]

WDYM by "other function signatures do work fine" ? which functions are you changing signatures ?

I mean that if you ptrCast other type signatures (where the func expects it), this works. I haven’t exhaustively tested type signatures but I do call at least a few dozen others and it works. This is just a specific case my application is calling that isn’t working.

What happen if you declare extern "c" fn do_the_thing(Region) void; ?

This fixes it, but this is not possible in my case due to the use case stated in the issue (obcj_msgSend). obcj_msgSend has a prototype of void f(void *, void *, …) and expects you to cast it to various other function pointer types depending on the target Objective-C selector.

[mitchellh]

The LLVM backend is completely ignoring the @ptrCast because it is done on a comptime-known value. The issue can be worked around by making new_fn mutable.

That does work in this specific case but doesn't work in my actual application which has a more complex usage... let me try to minimize that one.

[mitchellh]

@Vexu https://github.com/ziglang/zig/issues/13605 Here is another issue where var doesn't fix it. I opened it as another issue since it seems to be slightly different since var does fix this but not that issue. If you want to merge them, feel free.

[Vexu]

They are separate, in the second case we're not adding a byval attribute to the function call parameter, I'll give these issues more descriptive titles.

[mikdusan]

this seems to work (not crashing) on master 0.12.0-dev.1297+a9e66ed73. I did update: test.zig and tried on both linux and macos:

const std = @import("std");

extern "c" fn do_the_thing(*anyopaque, *anyopaque, ...) void;

const Region = extern struct {
    origin: extern struct {
        x: c_ulong,
        y: c_ulong,
        z: c_ulong,
    },

    size: extern struct {
        width: c_ulong,
        height: c_ulong,
        depth: c_ulong,
    },
};

pub fn main() void {
    const new_fn = @as(*const fn (Region) callconv(.C) void, @ptrCast(&do_the_thing));

    @call(.auto, new_fn, .{
        Region{
            .origin = .{ .x = 9, .y = 9, .z = 9 },
            .size = .{ .width = 9, .height = 9, .depth = 9 },
        },
    });
}