Proposal: Link-time initialization of globals

[N00byEdge]

I believe there should be a concept of link-time initialization of global variables. There is for example a boot protocol called stivale2 that reads a structure straight from one of your ELF sections, and traverses a linked list of tags (pointers encoded as integers) in your memory before jumping to the entry point:

In C:

static uint8_t stack[4096];

__attribute__((section(".stivale2hdr"), used))
static struct stivale2_header stivale_hdr = {
    .entry_point = &_entry,
    .stack = (uintptr_t)stack + sizeof(stack),
    .flags = (1 << 1) | (1 << 2),
    .tags = (uintptr_t)&framebuffer_hdr_tag
};

static struct stivale2_header_tag_framebuffer framebuffer_hdr_tag = {
    .tag = {
        .identifier = STIVALE2_HEADER_TAG_FRAMEBUFFER_ID,
        .next = (uint64_t)&terminal_hdr_tag
    },
    .framebuffer_width  = 0,
    .framebuffer_height = 0,
    .framebuffer_bpp    = 0
};

// ...

If you attempt to do the same thing in Zig, you're hit with something akin to "unable to evaluate constant expression" if you just have a single pointer-to-int conversion within any of your globals.

I propose that instead the link time pointers are tagged as exactly that, "link time known", so that you actually trigger the error only if you access them within a comptime context.

The workaround for now is to declare these structs in C and linking the C file to your executable, or implementing the creation of these structs in the linker script, none of which have access to zig comptime values, or to get access to zig constants through inline assembly:

export fn stivale_hdr() linksection(".stivale2hdr") callconv(.Naked) void {
    @setRuntimeSafety(false);
    asm volatile(
        \\ .quad stivale2Entry
        \\ .quad stack + %[stack_size]
        \\ .quad 6 // flags
        \\ .quad first_tag
        :
        : [stack_size] "i" (@sizeOf(stack))
    )
    unreachable;
}

[matu3ba]

As I understand it to make this reliable, this would require 1. adjusting the memory model in #6396 and comptime memory model #9646 to have a value for "initialize this value at link time" and 2. the linker speak to error out, if it cant fill at emitting the binary/lib (or represent that state, but thats even more complexity)

Alternatively, no guarantee for initialization may be given. However, this could introduce silent failures, ie when there are default values.