Support running LLVM backend in a separate thread

src/Compilation.zig

@@ -2442,7 +2442,15 @@ fn flush(
         try link.File.C.flushEmitH(zcu);
 
         if (zcu.llvm_object) |llvm_object| {
-            try emitLlvmObject(comp, arena, default_artifact_directory, null, llvm_object, prog_node);
+            try emitLlvmObject(
+                comp,
+                arena,
+                default_artifact_directory,
+                null,
+                llvm_object,
+                prog_node,
+                tid,
+            );
         }
     }
 }
@@ -2725,11 +2733,15 @@ pub fn emitLlvmObject(
     bin_emit_loc: ?EmitLoc,
     llvm_object: LlvmObject.Ptr,
     prog_node: std.Progress.Node,
+    tid: Zcu.PerThread.Id,
 ) !void {
     const sub_prog_node = prog_node.start("LLVM Emit Object", 0);
     defer sub_prog_node.end();
 
     try llvm_object.emit(.{
+        .zcu = comp.zcu.?,
+        .tid = tid,
+    }, .{
         .pre_ir_path = comp.verbose_llvm_ir,
         .pre_bc_path = comp.verbose_llvm_bc,
         .bin_path = try resolveEmitLoc(arena, default_artifact_directory, bin_emit_loc),

src/Sema.zig

@@ -10235,6 +10235,8 @@ fn finishFunc(
         // lower this fn type.
         const unresolved_stack_trace_ty = try sema.getBuiltinType("StackTrace");
         try unresolved_stack_trace_ty.resolveFields(pt);
+
+        if (zcu.stack_trace_type == .none) zcu.stack_trace_type = unresolved_stack_trace_ty.toIntern();
     }
 
     return Air.internedToRef(if (opt_func_index != .none) opt_func_index else func_ty);

src/Zcu.zig

@@ -221,6 +221,7 @@ panic_messages: [PanicId.len]InternPool.Nav.Index.Optional = .{.none} ** PanicId
 /// The panic function body.
 panic_func_index: InternPool.Index = .none,
 null_stack_trace: InternPool.Index = .none,
+stack_trace_type: InternPool.Index = .none,
 
 generation: u32 = 0,
 

src/link.zig

@@ -1348,14 +1348,22 @@ pub const File = struct {
         arena: Allocator,
         llvm_object: LlvmObject.Ptr,
         prog_node: std.Progress.Node,
+        tid: Zcu.PerThread.Id,
     ) !void {
-        return base.comp.emitLlvmObject(arena, .{
-            .root_dir = base.emit.root_dir,
-            .sub_path = std.fs.path.dirname(base.emit.sub_path) orelse "",
-        }, .{
-            .directory = null,
-            .basename = base.zcu_object_sub_path.?,
-        }, llvm_object, prog_node);
+        return base.comp.emitLlvmObject(
+            arena,
+            .{
+                .root_dir = base.emit.root_dir,
+                .sub_path = std.fs.path.dirname(base.emit.sub_path) orelse "",
+            },
+            .{
+                .directory = null,
+                .basename = base.zcu_object_sub_path.?,
+            },
+            llvm_object,
+            prog_node,
+            tid,
+        );
     }
 
     pub const C = @import("link/C.zig");

src/link/Coff.zig

@@ -2216,7 +2216,7 @@ pub fn flushModule(coff: *Coff, arena: Allocator, tid: Zcu.PerThread.Id, prog_no
     const diags = &comp.link_diags;
 
     if (coff.llvm_object) |llvm_object| {
-        try coff.base.emitLlvmObject(arena, llvm_object, prog_node);
+        try coff.base.emitLlvmObject(arena, llvm_object, prog_node, tid);
         return;
     }
 

src/link/Elf.zig

@@ -811,7 +811,7 @@ pub fn flushModule(self: *Elf, arena: Allocator, tid: Zcu.PerThread.Id, prog_nod
     const diags = &comp.link_diags;
 
     if (self.llvm_object) |llvm_object| {
-        try self.base.emitLlvmObject(arena, llvm_object, prog_node);
+        try self.base.emitLlvmObject(arena, llvm_object, prog_node, tid);
         const use_lld = build_options.have_llvm and comp.config.use_lld;
         if (use_lld) return;
     }

src/link/MachO.zig

@@ -354,7 +354,7 @@ pub fn flushModule(self: *MachO, arena: Allocator, tid: Zcu.PerThread.Id, prog_n
     const diags = &self.base.comp.link_diags;
 
     if (self.llvm_object) |llvm_object| {
-        try self.base.emitLlvmObject(arena, llvm_object, prog_node);
+        try self.base.emitLlvmObject(arena, llvm_object, prog_node, tid);
     }
 
     const sub_prog_node = prog_node.start("MachO Flush", 0);

src/link/Wasm.zig

@@ -2626,7 +2626,7 @@ pub fn flushModule(wasm: *Wasm, arena: Allocator, tid: Zcu.PerThread.Id, prog_no
     const comp = wasm.base.comp;
     const diags = &comp.link_diags;
     if (wasm.llvm_object) |llvm_object| {
-        try wasm.base.emitLlvmObject(arena, llvm_object, prog_node);
+        try wasm.base.emitLlvmObject(arena, llvm_object, prog_node, tid);
         const use_lld = build_options.have_llvm and comp.config.use_lld;
         if (use_lld) return;
     }

src/target.zig

@@ -730,7 +730,7 @@ pub inline fn backendSupportsFeature(backend: std.builtin.CompilerBackend, compt
             else => false,
         },
         .separate_thread => switch (backend) {
-            .stage2_llvm => false,
+            .stage2_llvm => true,
             else => true,
         },
     };