Avoid array allocation for f(*empty_ary, **hash) when def f(x)

This avoids an array allocation when calling a method that does
not accept a positional splat or keywords with both a positional
splat and keywords.  Previously, Ruby would dup the positional
splat to append the keyword splat to it. Then it would flatten
the dupped positional splat array to the VM stack.

This flattens the given positional splat to the VM stack, then
adds the keyword splat hash after the last positional splat
element on the VM stack, avoiding the need to modify
the positional splat array.

test/ruby/test_allocation.rb

@@ -123,8 +123,7 @@ class TestAllocation < Test::Unit::TestCase
         check_allocations(0, 0, "required(*r2k_empty_array1#{block})")
         check_allocations(0, 1, "required(*r2k_array#{block})")
 
-        # Currently allocates 1 array unnecessarily
-        check_allocations(1, 1, "required(*empty_array, **hash1, **empty_hash#{block})")
+        check_allocations(0, 1, "required(*empty_array, **hash1, **empty_hash#{block})")
       RUBY
     end
 
@@ -148,8 +147,7 @@ class TestAllocation < Test::Unit::TestCase
         check_allocations(0, 0, "optional(*r2k_empty_array1#{block})")
         check_allocations(0, 1, "optional(*r2k_array#{block})")
 
-        # Currently allocates 1 array unnecessarily
-        check_allocations(1, 1, "optional(*empty_array, **hash1, **empty_hash#{block})")
+        check_allocations(0, 1, "optional(*empty_array, **hash1, **empty_hash#{block})")
       RUBY
     end
 

vm_args.c

@@ -672,9 +672,32 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
         }
         else if (!ISEQ_BODY(iseq)->param.flags.has_kwrest && !ISEQ_BODY(iseq)->param.flags.has_kw) {
             converted_keyword_hash = check_kwrestarg(converted_keyword_hash, &kw_flag);
-            arg_rest_dup(args);
-            rb_ary_push(args->rest, converted_keyword_hash);
-            keyword_hash = Qnil;
+            if (ISEQ_BODY(iseq)->param.flags.has_rest) {
+                arg_rest_dup(args);
+                rb_ary_push(args->rest, converted_keyword_hash);
+                keyword_hash = Qnil;
+            }
+            else {
+                // Avoid duping rest when not necessary
+                // Copy rest elements and converted keyword hash directly to VM stack
+                const VALUE *argv = RARRAY_CONST_PTR(args->rest);
+                int j, i=args->argc, rest_len = RARRAY_LENINT(args->rest);
+                if (rest_len) {
+                    CHECK_VM_STACK_OVERFLOW(ec->cfp, rest_len+1);
+                    given_argc += rest_len;
+                    args->argc += rest_len;
+                    for (j=0; rest_len > 0; rest_len--, i++, j++) {
+                        locals[i] = argv[j];
+                    }
+                }
+                locals[i] = converted_keyword_hash;
+                given_argc--;
+                args->argc++;
+                args->rest = Qfalse;
+                ci_flag &= ~(VM_CALL_ARGS_SPLAT|VM_CALL_KW_SPLAT);
+                keyword_hash = Qnil;
+                goto arg_splat_and_kw_splat_flattened;
+            }
         }
         else {
             keyword_hash = converted_keyword_hash;
@@ -768,6 +791,7 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
         FL_SET_RAW(flag_keyword_hash, RHASH_PASS_AS_KEYWORDS);
     }
 
+  arg_splat_and_kw_splat_flattened:
     if (kw_flag && ISEQ_BODY(iseq)->param.flags.accepts_no_kwarg) {
         rb_raise(rb_eArgError, "no keywords accepted");
     }