Proposal: Allow `@export` to take an arbitrary pointer as its first argument

[alexrp]

Proposal

Allow e.g. this:

comptime {
    @export(@as(*void, @ptrFromInt(0x2C)), .{ .name = "__tls_array" });
}

Motivated by this std code:

zig/lib/std/os/windows/tls.zig

Lines 11 to 20 in 80f2aeb

	comptime {
	if (builtin.cpu.arch == .x86 and !builtin.abi.isGnu() and builtin.zig_backend != .stage2_c) {
	// The __tls_array is the offset of the ThreadLocalStoragePointer field
	// in the TEB block whose base address held in the %fs segment.
	asm (
	\\ .global __tls_array
	\\ __tls_array = 0x2C
	);
	}
	}

Presumably it'll also be useful in certain embedded and OS scenarios.

Implementation

• Lowering is straightforward for self-hosted backends.
• Lowering in the LLVM backend will probably emit global assembly that's roughly equivalent to the above std code.
• Lowering in the C backend is an open question. For GCC-compatible compilers, we can obviously just emit global assembly like the LLVM backend, but there doesn't seem to be anything usable for MSVC.
  • Fortunately the motivating use case is not relevant here.