Implement returns.

src/backend.rs

@@ -361,11 +361,20 @@ pub fn pass_outgoing_args(ctx: &mut Context, arity: u32) {
     }
 }
 
-pub fn call_direct(ctx: &mut Context, index: u32) {
+pub fn call_direct(ctx: &mut Context, index: u32, return_arity: u32) {
+    assert!(return_arity == 0 || return_arity == 1);
+
     let label = &ctx.func_starts[index as usize].1;
     dynasm!(ctx.asm
         ; call =>*label
     );
+
+    if return_arity == 1 {
+        dynasm!(ctx.asm
+            ; push rax
+        );
+        ctx.sp_depth.reserve(1);
+    }
 }
 
 pub fn prologue(ctx: &mut Context, stack_slots: u32) {

src/function_body.rs

@@ -222,13 +222,11 @@ pub fn translate(
             }
             Operator::Call { function_index } => {
                 let callee_ty = translation_ctx.func_type(function_index);
-                assert!(callee_ty.returns.len() == 0, "is not supported");
 
-                // TODO: ensure that this function is locally defined
-                // We would like to support imported functions at some point
+                // TODO: this implementation assumes that this function is locally defined.
 
                 pass_outgoing_args(&mut ctx, callee_ty.params.len() as u32);
-                call_direct(&mut ctx, function_index);
+                call_direct(&mut ctx, function_index, callee_ty.returns.len() as u32);
             }
             _ => {
                 trap(&mut ctx);