spirv: make the backend compile again

Unfortunately, the self-hosted SPIR-V backend is quite tightly coupled
with the self-hosted SPIR-V linker through its `Object` concept (which
is much like `llvm.Object`). Reworking this would be too much work for
this branch. So, for now, I have introduced a special case (similar to
the LLVM backend's special case) to the codegen logic when using this
backend. We will want to delete this special case at some point, but it
need not block this work.

src/Zcu/PerThread.zig

@@ -4461,6 +4461,22 @@ fn runCodegenInner(pt: Zcu.PerThread, func_index: InternPool.Index, air: *Air) e
     }
 
     const lf = comp.bin_file orelse return error.NoLinkFile;
+
+    // TODO: self-hosted codegen should always have a type of MIR; codegen should produce that MIR,
+    // and the linker should consume it. However, our SPIR-V backend is currently tightly coupled
+    // with our SPIR-V linker, so needs to work more like the LLVM backend. This should be fixed to
+    // unblock threaded codegen for SPIR-V.
+    if (lf.cast(.spirv)) |spirv_file| {
+        assert(pt.tid == .main); // SPIR-V has a lot of shared state
+        spirv_file.object.updateFunc(pt, func_index, air, &liveness) catch |err| {
+            switch (err) {
+                error.OutOfMemory => comp.link_diags.setAllocFailure(),
+            }
+            return error.CodegenFail;
+        };
+        return error.BackendDoesNotProduceMir;
+    }
+
     return codegen.generateFunction(lf, pt, zcu.navSrcLoc(nav), func_index, air, &liveness) catch |err| switch (err) {
         error.OutOfMemory,
         error.CodegenFail,

src/codegen/spirv.zig

@@ -250,12 +250,12 @@ pub const Object = struct {
         self: *Object,
         pt: Zcu.PerThread,
         func_index: InternPool.Index,
-        air: Air,
-        liveness: Air.Liveness,
+        air: *const Air,
+        liveness: *const Air.Liveness,
     ) !void {
         const nav = pt.zcu.funcInfo(func_index).owner_nav;
         // TODO: Separate types for generating decls and functions?
-        try self.genNav(pt, nav, air, liveness, true);
+        try self.genNav(pt, nav, air.*, liveness.*, true);
     }
 
     pub fn updateNav(

src/link.zig

@@ -758,8 +758,8 @@ pub const File = struct {
         assert(base.comp.zcu.?.llvm_object == null);
         switch (base.tag) {
             .lld => unreachable,
+            .spirv => unreachable, // see corresponding special case in `Zcu.PerThread.runCodegenInner`
             inline else => |tag| {
-                if (tag == .spirv) @panic("MLUGG TODO");
                 dev.check(tag.devFeature());
                 return @as(*tag.Type(), @fieldParentPtr("base", base)).updateFunc(pt, func_index, mir, maybe_undef_air);
             },

src/link/SpirV.zig

@@ -111,24 +111,6 @@ pub fn deinit(self: *SpirV) void {
     self.object.deinit();
 }
 
-pub fn updateFunc(
-    self: *SpirV,
-    pt: Zcu.PerThread,
-    func_index: InternPool.Index,
-    air: Air,
-    liveness: Air.Liveness,
-) link.File.UpdateNavError!void {
-    if (build_options.skip_non_native) {
-        @panic("Attempted to compile for architecture that was disabled by build configuration");
-    }
-
-    const ip = &pt.zcu.intern_pool;
-    const func = pt.zcu.funcInfo(func_index);
-    log.debug("lowering function {}", .{ip.getNav(func.owner_nav).name.fmt(ip)});
-
-    try self.object.updateFunc(pt, func_index, air, liveness);
-}
-
 pub fn updateNav(self: *SpirV, pt: Zcu.PerThread, nav: InternPool.Nav.Index) link.File.UpdateNavError!void {
     if (build_options.skip_non_native) {
         @panic("Attempted to compile for architecture that was disabled by build configuration");

src/target.zig

@@ -850,8 +850,8 @@ pub inline fn backendSupportsFeature(backend: std.builtin.CompilerBackend, compt
         },
         .separate_thread => switch (backend) {
             .stage2_llvm => false,
-            // MLUGG TODO
             .stage2_c, .stage2_wasm => true,
+            // TODO: most self-hosted backends should be able to support this without too much work.
             else => false,
         },
     };