Zig: Compiler crash using a field in a packed struct

[josephg]

const std = @import("std");

const InlineString = packed struct {
    bytes: [10]u8,
};

pub fn main() !void {
    var s = InlineString {
        .bytes = [_]u8{0} ** 10,
    };

    std.mem.copy(u8, &s.bytes, "hi");
}

This code crashes the compiler:

$ zig build-exe compiler_crash.zig
[1]    52084 segmentation fault  zig build-exe compiler_crash.zig

The compiler only crashes if you use a packed struct.

(Version: [email protected] from homebrew on macos)

[pixelherodev]

This is a duplicate, I think.

[pixelherodev]

tl;dr: there's some bugs with the current implementation of packed structures. I don't remember the exact trigger, but it's something along the lines of "non-power-of-two field size."

[SpexGuy]

there's some bugs with the current implementation of packed structures

That's true, but they aren't compiler crashes. We should at least make this error.
That said it's worth noting @josephg that even with the compiler crash fixed, the InlineString structure ends up being 16 bytes due to these bugs. They will be addressed by #3133, but there are still pieces that need to be figured out so it could take a while. In the meantime I recommend using struct or extern struct and putting align(1) on fields where necessary.

It looks like what's happening is the type of &s.bytes is *align(:0:16) [10]u8, which can't coerce to the type []u8 required for the parameter to copy because of its weird bit-alignment (which is a result of the aforementioned bugs). Instead of causing an error though, this coercion case ends in a compiler crash. I was able to find this minimal repro:

pub fn main() void {
    var s: *align(:0:16) [10]u8 = undefined;
    _ = @as([]u8, s);
}

[alexnask]

Here is another case of this:

const std = @import("std");

const S = packed struct {
    arr: [5]u8,
};

pub fn main() !void {
    var a: S = undefined;
    std.debug.warn("{}\n", .{a.arr[0..2]});
}

gdb backtrace at crash:

(gdb) run build-exe test.zig
Starting program: /mnt/c/dev/zig/build-linux/zig build-exe test.zig
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Code Generation [402/531] std.mem.bytesAsSlice...
Program received signal SIGSEGV, Segmentation fault.
0x000000000e1a9ca8 in llvm::PointerType::get(llvm::Type*, unsigned int) ()
(gdb) bt
#0  0x000000000e1a9ca8 in llvm::PointerType::get(llvm::Type*, unsigned int) ()
#1  0x000000000e0c6f9d in LLVMBuildInBoundsGEP ()
#2  0x000000000917be6f in ir_render_slice (g=0x10b026c0, executable=0x154005b0, instruction=0x154a6a60) at /mnt/c/dev/zig/src/codegen.cpp:5582
#3  0x000000000918041c in ir_render_instruction (g=0x10b026c0, executable=0x154005b0, instruction=0x154a6a60) at /mnt/c/dev/zig/src/codegen.cpp:6624
#4  0x0000000009180b21 in ir_render (g=0x10b026c0, fn_entry=0x15400530) at /mnt/c/dev/zig/src/codegen.cpp:6744
#5  0x00000000091871ea in do_code_gen (g=0x10b026c0) at /mnt/c/dev/zig/src/codegen.cpp:8144
#6  0x00000000091928f6 in codegen_build_and_link (g=0x10b026c0) at /mnt/c/dev/zig/src/codegen.cpp:10917
#7  0x0000000009161e1a in main0 (argc=3, argv=0x7ffffffedbf8) at /mnt/c/dev/zig/src/main.cpp:1671
#8  0x000000000916299e in main (argc=3, argv=0x7ffffffedbf8) at /mnt/c/dev/zig/src/main.cpp:1858
(gdb) q

[cartr]

Another case of non-power-of-two-length arrays in packed structs causing a compiler crash:

const S = packed struct {
    data: [5]u8,
};

fn returnZeros() [5]u8 {
    const result = [_]u8 { 0 } ** 5;
    return result;
}

export fn x() void {
    const s = S {
        .data = returnZeros()
    };
}

Attempting to compile this produces the following error:

broken LLVM module found: Call parameter type does not match function signature!
  %1 = bitcast [8 x i8]* %0 to i64*, !dbg !57
 [5 x i8]*  call fastcc void @returnZeros(i64* sret %1), !dbg !58

This is a bug in the Zig compiler.
Unable to dump stack trace: debug info stripped