Avoid a hash allocation when keyword splatting empty hash when calling ruby2_keywords method

Treat this similar to keyword splatting nil, using goto ignore.
However, keep previous behavior if the method accepts a keyword
splat, to avoid double hash allocation.

This also can avoid an array allocation when calling a method
that doesn't have any splat parameters but supports literal
keyword parameters, because ignore_keyword_hash_p was not
ignoring the keyword hash in that case.

This change doesn't remove the empty ruby2_keywords hash from
the array, which caused an assertion failure if the method
being called accepted keywords in some cases.  Modify the
assertion to handle this case.  An alternative approach would
add a flag to the args struct so the args_argc calculation could
handle this case and report the correct argc, but such an approach
would likely be slower.

test/ruby/test_allocation.rb

@@ -273,7 +273,7 @@ class TestAllocation < Test::Unit::TestCase
         check_allocations(0, 1, "keyword(**hash1, **empty_hash#{block})")
         check_allocations(1, 0, "keyword(*empty_array, *empty_array, **empty_hash#{block})")
 
-        check_allocations(1, 0, "keyword(*r2k_empty_array#{block})")
+        check_allocations(0, 0, "keyword(*r2k_empty_array#{block})")
         check_allocations(1, 1, "keyword(*r2k_array#{block})")
 
         check_allocations(0, 1, "keyword(*empty_array, a: 2, **empty_hash#{block})")
@@ -379,7 +379,7 @@ class TestAllocation < Test::Unit::TestCase
         check_allocations(1, 1, "required_and_keyword(*array1, *empty_array, a: 2, **empty_hash#{block})")
         check_allocations(1, 1, "required_and_keyword(*array1, *empty_array, **hash1, **empty_hash#{block})")
 
-        check_allocations(1, 0, "required_and_keyword(*r2k_empty_array1#{block})")
+        check_allocations(0, 0, "required_and_keyword(*r2k_empty_array1#{block})")
         check_allocations(1, 1, "required_and_keyword(*r2k_array1#{block})")
 
         # Currently allocates 1 array unnecessarily due to splatarray true
@@ -724,7 +724,7 @@ class TestAllocation < Test::Unit::TestCase
         check_allocations(1, 1, "r2k(1, **empty_hash, a: 2#{block})")
 
         check_allocations(1, 0, "r2k(1, **nil#{block})")
-        check_allocations(1, 1, "r2k(1, **empty_hash#{block})")
+        check_allocations(1, 0, "r2k(1, **empty_hash#{block})")
         check_allocations(1, 1, "r2k(1, **hash1#{block})")
         check_allocations(1, 1, "r2k(1, *empty_array, **hash1#{block})")
         check_allocations(1, 1, "r2k(1, **hash1, **empty_hash#{block})")
@@ -732,17 +732,17 @@ class TestAllocation < Test::Unit::TestCase
 
         check_allocations(1, 0, "r2k(1, *empty_array#{block})")
         check_allocations(1, 1, "r2k(1, **hash1, **empty_hash#{block})")
-        check_allocations(1, 1, "r2k(1, *empty_array, *empty_array, **empty_hash#{block})")
+        check_allocations(1, 0, "r2k(1, *empty_array, *empty_array, **empty_hash#{block})")
 
         check_allocations(1, 1, "r2k(*array1, a: 2#{block})")
 
         check_allocations(1, 0, "r2k(*array1, **nill#{block})")
-        check_allocations(1, 1, "r2k(*array1, **empty_hash#{block})")
+        check_allocations(1, 0, "r2k(*array1, **empty_hash#{block})")
         check_allocations(1, 1, "r2k(*array1, **hash1#{block})")
         check_allocations(1, 1, "r2k(*array1, *empty_array, **hash1#{block})")
 
         check_allocations(1, 0, "r2k(*array1, *empty_array#{block})")
-        check_allocations(1, 1, "r2k(*array1, *empty_array, **empty_hash#{block})")
+        check_allocations(1, 0, "r2k(*array1, *empty_array, **empty_hash#{block})")
 
         check_allocations(1, 1, "r2k(*array1, *empty_array, a: 2, **empty_hash#{block})")
         check_allocations(1, 1, "r2k(*array1, *empty_array, **hash1, **empty_hash#{block})")

vm_args.c

@@ -535,6 +535,10 @@ ignore_keyword_hash_p(VALUE keyword_hash, const rb_iseq_t * const iseq, unsigned
         }
     }
 
+    if (RHASH_EMPTY_P(keyword_hash) && !ISEQ_BODY(iseq)->param.flags.has_kwrest) {
+        goto ignore;
+    }
+
     if (!(*kw_flag & VM_CALL_KW_SPLAT_MUT) &&
         (ISEQ_BODY(iseq)->param.flags.has_kwrest ||
          ISEQ_BODY(iseq)->param.flags.ruby2_keywords)) {
@@ -856,7 +860,18 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
             args_setup_kw_parameters_from_kwsplat(ec, iseq, keyword_hash, klocals, remove_hash_value);
         }
         else {
-            VM_ASSERT(args_argc(args) == 0);
+#if VM_CHECK_MODE > 0
+            if (args_argc(args) != 0) {
+                VM_ASSERT(ci_flag & VM_CALL_ARGS_SPLAT);
+                VM_ASSERT(!(ci_flag & (VM_CALL_KWARG | VM_CALL_KW_SPLAT | VM_CALL_KW_SPLAT_MUT)));
+                VM_ASSERT(!kw_flag);
+                VM_ASSERT(!ISEQ_BODY(iseq)->param.flags.has_rest);
+                VM_ASSERT(RARRAY_LENINT(args->rest) > 0);
+                VM_ASSERT(RB_TYPE_P(rest_last, T_HASH));
+                VM_ASSERT(FL_TEST_RAW(rest_last, RHASH_PASS_AS_KEYWORDS));
+                VM_ASSERT(args_argc(args) == 1);
+            }
+#endif
             args_setup_kw_parameters(ec, iseq, NULL, 0, NULL, klocals);
         }
     }