YJIT: Fix cfunc splat

Follow-up for cb8a040b7906c09d9d3ac3d3fe853f633005024f.

bootstraptest/test_yjit.rb

@@ -3585,7 +3585,7 @@ assert_equal 'true', %q{
   1.send(:==, 1, *[])
 }
 
-# Test send with splat to a cfunc
+# Test empty splat with cfunc
 assert_equal '2', %q{
   def foo
     Integer.sqrt(4, *[])
@@ -3594,3 +3594,14 @@ assert_equal '2', %q{
   foo
   foo
 }
+
+# Test non-empty splat with cfunc
+assert_equal 'Hello World', %q{
+  def bar
+    args = ["Hello "]
+    greeting = "World"
+    greeting.insert(0, *args)
+    greeting
+  end
+  bar
+}

yjit/src/codegen.rs

@@ -4864,6 +4864,7 @@ fn gen_send_cfunc(
 
     // push_splat_args does stack manipulation so we can no longer side exit
     if flags & VM_CALL_ARGS_SPLAT != 0 {
+        assert!(cfunc_argc >= 0);
         let required_args : u32 = (cfunc_argc as u32).saturating_sub(argc as u32 - 1);
         // + 1 because we pass self
         if required_args + 1 >= C_ARG_OPNDS.len() as u32 {
@@ -4871,22 +4872,13 @@ fn gen_send_cfunc(
             return CantCompile;
         }
 
-        if required_args == 0 {
-            // The splat is not going to supply us any arguments
-            // so we set the argc to the number of arguments
-            // the cfunc expects.
-            // We still need to do all the splat logic because someone
-            // could pass in a non-zero sized array and we need to
-            // exit to error.
-            argc = cfunc_argc;
-            passed_argc = argc;
-        } else {
-            // We are going to assume that the splat fills
-            // all the remaining arguments. In the generated code
-            // we test if this is true and if not side exit.
-            argc = required_args as i32;
-            passed_argc = argc;
-        }
+        // We are going to assume that the splat fills
+        // all the remaining arguments. So the number of args
+        // should just equal the number of args the cfunc takes.
+        // In the generated code we test if this is true
+        // and if not side exit.
+        argc = cfunc_argc;
+        passed_argc = argc;
         push_splat_args(required_args, ctx, asm, ocb, side_exit)
     }