PyStr::mul optimization for i=0 or str=''

Author: killme2008

Lib/test/string_tests.py

@@ -1188,6 +1188,8 @@ def test_extended_getslice(self):
                                     slice(start, stop, step))
 
     def test_mul(self):
+        self.assertTrue("('' * 3) is ''");
+        self.assertTrue("('a' * 0) is ''");
         self.checkequal('', 'abc', '__mul__', -1)
         self.checkequal('', 'abc', '__mul__', 0)
         self.checkequal('abc', 'abc', '__mul__', 1)

vm/src/builtins/str.rs

@@ -509,8 +509,13 @@ impl PyStr {
     #[pymethod(name = "__rmul__")]
     #[pymethod(magic)]
     fn mul(zelf: PyRef<Self>, value: isize, vm: &VirtualMachine) -> PyResult<PyRef<Self>> {
-        if value == 1 && zelf.class().is(&vm.ctx.types.str_type) {
-            // Special case: when some `str` is multiplied by `1`,
+        if value == 0 && zelf.class().is(&vm.ctx.types.str_type) {
+            // Special case: when some `str` is multiplied by `0`,
+            // returns the empty `str`.
+            return Ok(vm.ctx.empty_str.clone());
+        }
+        if (value == 1 || zelf.is_empty()) && zelf.class().is(&vm.ctx.types.str_type) {
+            // Special case: when some `str` is multiplied by `1` or is the empty `str`,
             // nothing really happens, we need to return an object itself
             // with the same `id()` to be compatible with CPython.
             // This only works for `str` itself, not its subclasses.

vm/src/vm/context.rs

@@ -25,6 +25,7 @@ pub struct Context {
     pub none: PyRef<PyNone>,
     pub empty_tuple: PyTupleRef,
     pub empty_frozenset: PyRef<PyFrozenSet>,
+    pub empty_str: PyRef<PyStr>,
     pub ellipsis: PyRef<PyEllipsis>,
     pub not_implemented: PyRef<PyNotImplemented>,
 
@@ -83,13 +84,16 @@ impl Context {
 
         let true_str = unsafe { string_pool.intern("True", types.str_type.clone()) }.into_pyref();
         let false_str = unsafe { string_pool.intern("False", types.str_type.clone()) }.into_pyref();
+        let empty_str = unsafe { string_pool.intern("", types.str_type.clone()) }.into_pyref();
 
         let context = Context {
             true_value,
             false_value,
             none,
             empty_tuple,
             empty_frozenset,
+            empty_str,
+
             ellipsis,
             not_implemented,