Add support for WASI reactor in pure Zig-exe.

[mathetake]

This PR adds support for WASI reactor in pure Zig WASI targets. Notably this enables users to produce WASI reactor binary with build-exe command with -mexec-model=reactor just like zig cc, zig c++, and clang.

With -mexec-model=reactor flag, Zig 1) exports _initialize symbol which calls the user defined pub fn main, 2) does not define _start, and 3) does not execute proc_exit after main.

Please refer to https://github.com/WebAssembly/WASI/blob/main/design/application-abi.md for the ABI in detail.

Finally resolves #6757 together with #9052.

cc @kubkon

[kubkon]

Thanks @mathetake! I've got a few comments I'd like to see addressed before going forward with this PR.

[mathetake]

FYI: If I put this into a single statement,

    try buffer.writer().print(
        \\pub const wasi_exec_model = std.builtin.WasiExecModel.{};
        \\
    , .{if (comp.bin_file.options.wasi_exec_model) |model|
        std.zig.fmtId(@tagName(model))
    else
        std.zig.fmtId(@tagName(std.builtin.WasiExecModel.command))});

I got some weird error around LLVM:

make install
[ 77%] Built target embedded_softfloat
[ 78%] Built target opt_c_util
[ 83%] Built target zigcpp
[ 97%] Built target zigstage1
[ 98%] Built target zig0
[ 99%] Building self-hosted component /home/mathetake/zig/build/zig1.o
broken LLVM module found: Instruction does not dominate all uses!
  %369 = getelementptr inbounds %"Compilation.struct:3644:8", %"Compilation.struct:3644:8"* %39, i32 0, i32 0, !dbg !75434
  call fastcc void @std.zig.fmt.fmtId(%"std.fmt.Formatter(std.zig.fmt.formatId)"* sret(%"std.fmt.Formatter(std.zig.fmt.formatId)") %369, %"[]u8"* @13528), !dbg !75435
Instruction does not dominate all uses!
  %369 = getelementptr inbounds %"Compilation.struct:3644:8", %"Compilation.struct:3644:8"* %39, i32 0, i32 0, !dbg !75434
  %372 = bitcast %"std.fmt.Formatter(std.zig.fmt.formatId)"* %369 to i8*, !dbg !75435
Instruction does not dominate all uses!
  %369 = getelementptr inbounds %"Compilation.struct:3644:8", %"Compilation.struct:3644:8"* %39, i32 0, i32 0, !dbg !75434
  %373 = bitcast %"std.fmt.Formatter(std.zig.fmt.formatId)"* %369 to i8*, !dbg !75435

This is a bug in the Zig compiler.
Aborted (core dumped)
make[2]: *** [CMakeFiles/zig.dir/build.make:328: zig1.o] Error 134
make[1]: *** [CMakeFiles/Makefile2:148: CMakeFiles/zig.dir/all] Error 2
make: *** [Makefile:130: all] Error 2

[kubkon]

I believe this is because it's not enough to just add an enum to std.builtin. You need to make more tweaks to stage1 in C++. For starters, we need to have the exact same enum shared between both stage1 and stage2 which means you need add a corresponding enum to src/stage1/stage1.h. Then, you'll need to tweak logic for actually marshalling the data between stage1 and stage2 - have a look how this is for CodeModel or some other builtin structs.

If you get stuck, I'll be happy to help you out but will probably take a look at this in more detail tomorrow. Alternatively, we can also get @andrewrk involved if he's got a minute.

[mathetake]

Ah I see. Thanks for clarifying!

[mathetake]

so yeah I found that this also applies to other existing builtin types:

const std = @import("std");

pub fn main() void {
    std.debug.print("test {}", .{if (@rem(std.time.timestamp(), 2) == 0)
        std.zig.fmtId(@tagName(std.builtin.CodeModel.tiny))
    else
        std.zig.fmtId(@tagName(std.builtin.CodeModel.default))});
}

HEAD and 0.8.0 fails to compile this code with the same error like this:

zig build-exe main.zig
broken LLVM module found: Instruction does not dominate all uses!d.debug.ModuleDebug...
  %2 = getelementptr inbounds %"struct:4:33", %"struct:4:33"* %0, i32 0, i32 0, !dbg !16731
  call fastcc void @std.zig.fmt.fmtId(%"std.fmt.Formatter(std.zig.fmt.formatId)"* sret(%"std.fmt.Formatter(std.zig.fmt.formatId)") %2, %"[]u8.18"* @390), !dbg !16733
Instruction does not dominate all uses!
  %2 = getelementptr inbounds %"struct:4:33", %"struct:4:33"* %0, i32 0, i32 0, !dbg !16731
  %5 = bitcast %"std.fmt.Formatter(std.zig.fmt.formatId)"* %2 to i8*, !dbg !16733
Instruction does not dominate all uses!
  %2 = getelementptr inbounds %"struct:4:33", %"struct:4:33"* %0, i32 0, i32 0, !dbg !16731
  %6 = bitcast %"std.fmt.Formatter(std.zig.fmt.formatId)"* %2 to i8*, !dbg !16733

This is a bug in the Zig compiler.thread 3208559 panic:
Unable to dump stack trace: debug info stripped
zsh: abort (core dumped)  zig build-exe main.zig

and the same, if we split the line to two statements. the error disappears.

[mathetake]

fyi: I tweaked the C++ and around stage1.zig and Compilation.zig following CodeModel, but the error has never gone.

[mathetake]

Luckily the code with split statements just works and successfully produced Wasi Reactors!

[mathetake]

Thanks @kubkon for a quick look:) I hope 6769659 addressed your comments! PTAL!

[kubkon]

Looks great to me, thanks @mathetake! I just need to double check with @andrewrk that we don't need to add matching logic to the C++ codebase for setting WasiExecModel builtin.

[mathetake]

@andrewrk @kubkon I would appreciate it if you have any chance to take a look and upstream this! Thanks a lot:)

[andrewrk]

Ah I'm sorry for taking so long to get to this! Looking now.

[andrewrk]

Thanks for the contribution!

I have some requested improvements.

[andrewrk]

Based on my research it looks like every WASI program is either a command or a reactor. Is it meaningful for a program to be neither? Why is crt1_o even an option?

[mathetake]

yeah we should remove crt1_o in favor of crt1_command_o, where the latter lets wasm-ld call ctors rather than explicitly calling in crt.c. Also I believe this reduces the complexity here about null vs default stuff.

so I removed the support for crt1_o. cc @kubkon wdyt?

[mathetake]

Thank you so much @andrewrk, I hope I addressed your suggestion in 85b5503, and now it looks much better:)

PTAL when you have time, thanks!

[andrewrk]

Thanks for making the requested changes - just a couple small things left and then this looks ready to merge to me :)

[mathetake]

thanks for the quick review! addressed 943f656 here :)

[andrewrk]

Alright, this is ready to be merged, did you want to wait until @kubkon wakes up to discuss the question you asked? If not I'm happy to merge now.

[mathetake]

I can discuss offline with @kubkon later, so I'm happy to merge now. I believe it's easy to bring crt1.o support even if we want later:)