YJIT: Support arity=-2 cfuncs

This type of cfuncs shows up as consume a lot of cycles in profiles of
the lobsters benchmark, even though in the stats they don't happen that
frequently. Might be a bug in the profiling, but these calls are not
too bad to support, so might as well do it.

bootstraptest/test_yjit.rb

@@ -4678,3 +4678,19 @@ def test(klass, args)
 
   test(KwInit, [Hash.ruby2_keywords_hash({1 => 1})])
 }
+
+# arity=-2 cfuncs
+assert_equal '["", "1/2", [0, [:ok, 1]]]', %q{
+  def test_cases(file, chain)
+    new_chain = chain.allocate # to call initialize directly
+    new_chain.send(:initialize, [0], ok: 1)
+
+    [
+      file.join,
+      file.join("1", "2"),
+      new_chain.to_a,
+    ]
+  end
+
+  test_cases(File, Enumerator::Chain)
+}

yjit/src/codegen.rs

@@ -6174,9 +6174,9 @@ fn gen_send_cfunc(
     let variable_splat = flags & VM_CALL_ARGS_SPLAT != 0 && cfunc_argc == -1;
     let block_arg = flags & VM_CALL_ARGS_BLOCKARG != 0;
 
-    // If the function expects a Ruby array of arguments
-    if cfunc_argc < 0 && cfunc_argc != -1 {
-        gen_counter_incr(asm, Counter::send_cfunc_ruby_array_varg);
+    // If it's a splat and the method expects a Ruby array of arguments
+    if cfunc_argc == -2 && flags & VM_CALL_ARGS_SPLAT != 0 {
+        gen_counter_incr(asm, Counter::send_cfunc_splat_neg2);
         return None;
     }
 
@@ -6461,7 +6461,19 @@ fn gen_send_cfunc(
             asm.stack_opnd(argc),
         ]
     }
-    else {
+    // Variadic method taking a Ruby array
+    else if cfunc_argc == -2 {
+        // Slurp up all the arguments into an array
+        let stack_args = asm.lea(asm.ctx.sp_opnd(-argc * SIZEOF_VALUE_I32));
+        let args_array = asm.ccall(
+            rb_ec_ary_new_from_values as _,
+            vec![EC, passed_argc.into(), stack_args]
+        );
+
+        // Example signature:
+        // VALUE neg2_method(VALUE self, VALUE argv)
+        vec![asm.stack_opnd(argc), args_array]
+    } else {
         panic!("unexpected cfunc_args: {}", cfunc_argc)
     };
 

yjit/src/stats.rs

@@ -354,7 +354,7 @@ make_counters! {
     send_refined_method,
     send_private_not_fcall,
     send_cfunc_kw_splat_non_nil,
-    send_cfunc_ruby_array_varg,
+    send_cfunc_splat_neg2,
     send_cfunc_argc_mismatch,
     send_cfunc_block_arg,
     send_cfunc_toomany_args,