stage2: add a wasm backend

[ifreund]

Thus far, we only generate the type, function, export, and code
sections. These are sufficient to generate and export simple functions.

TODO:

• Create basic infrastructure for incremental compilation of the wasm container format.
• Implement trivial codegen
• Add wasm test cases
• Fix output files always having .o extension even with -ofmt=wasm

[andrewrk]

Fix output files always having .o extension even with -ofmt=wasm

This function should be modified to accept an (optional?) ObjectFormat parameter, and it should also be improved to take into account architecture. Wait, is that how it works though? Wasm uses the same extension for both objects and executables? I thought wasm objects were still .o

zig/lib/std/target.zig

Lines 1047 to 1049 in c12a262

	pub fn oFileExt(self: Target) [:0]const u8 {
	return self.abi.oFileExt();
	}

Called here:

zig/lib/std/zig.zig

Line 79 in c12a262

.Obj => return std.fmt.allocPrint(allocator, "{}{}", .{ root_name, target.oFileExt() }),

This C logic should be removed in favor of making binNameAlloc aware of .c as an object format:

zig/src-self-hosted/main.zig

Lines 474 to 477 in c12a262

	.yes_default_path => if (object_format != null and object_format.? == .c)
	try std.fmt.allocPrint(arena, "{}.c", .{root_name})
	else
	try std.zig.binNameAlloc(arena, root_name, target_info.target, output_mode, link_mode),

[andrewrk]

I'll look into this

[andrewrk]

I think this is the only change that needs to happen regarding this:

diff --git a/src-self-hosted/link.zig b/src-self-hosted/link.zig
index a835cc6b7..e89210763 100644
--- a/src-self-hosted/link.zig
+++ b/src-self-hosted/link.zig
@@ -186,7 +186,8 @@ pub const File = struct {
         };
     }
 
-    /// Must be called only after a successful call to `updateDecl`.
+    /// Can be called any time before or after the call to `updateDecl`. Same
+    /// with `allocateDeclIndexes`.
     pub fn updateDeclExports(
         base: *File,
         module: *Module,

Happy to discuss other possible ways of doing things

[ifreund]

Discovered a design flaw with regards to incremental compilation. On "freeing" a function decl we immediately mark both the function itself and the corresponding type in the types section as "dead." This breaks down however if the type is overwritten and reused before the function is erased from the binary as this may cause a type mismatch.

One way to solve this would be to keep the type corresponding to each function "alive" until the function is overwritten.

[ifreund]

I think this is ready to merge. The last remaining checkbox is orthogonal to these additions and could easily be done in another PR

[pannous]

Not sure if it's a bug: at least in wasi the main/start function always returns int

[pannous]

maybe (planned) / (alpha) would be appropriate here