YJIT: Properly deal with cfunc splat when no args needed

bootstraptest/test_yjit.rb

@@ -3584,3 +3584,13 @@ def a.test = :test
 assert_equal 'true', %q{
   1.send(:==, 1, *[])
 }
+
+# Test send with splat to a cfunc
+assert_equal '2', %q{
+  def foo
+    Integer.sqrt(4, *[])
+  end
+  # call twice to deal with constant exiting
+  foo
+  foo
+}

yjit/src/codegen.rs

@@ -4864,23 +4864,29 @@ fn gen_send_cfunc(
 
     // push_splat_args does stack manipulation so we can no longer side exit
     if flags & VM_CALL_ARGS_SPLAT != 0 {
-        if flags & VM_CALL_OPT_SEND != 0 {
-            // FIXME: This combination is buggy.
-            // For example `1.send(:==, 1, *[])` fails to adjust the stack properly
-            gen_counter_incr!(asm, send_cfunc_splat_send);
-            return CantCompile;
-        }
         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 {
             gen_counter_incr!(asm, send_cfunc_toomany_args);
             return CantCompile;
         }
-        // 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;
+
+        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;
+        }
         push_splat_args(required_args, ctx, asm, ocb, side_exit)
     }