Merge pull request #3703 from killme2008/feat/issue-3702

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

Lib/test/string_tests.py

@@ -1188,6 +1188,8 @@ class MixinStrUnicodeUserStringTest:
                                     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/exceptions.rs

@@ -507,7 +507,7 @@ impl PyBaseException {
     pub(super) fn str(&self, vm: &VirtualMachine) -> PyStrRef {
         let str_args = vm.exception_args_as_string(self.args(), true);
         match str_args.into_iter().exactly_one() {
-            Err(i) if i.len() == 0 => PyStr::from("").into_ref(vm),
+            Err(i) if i.len() == 0 => vm.ctx.empty_str.clone(),
             Ok(s) => s,
             Err(i) => PyStr::from(format!("({})", i.format(", "))).into_ref(vm),
         }

vm/src/frame.rs

@@ -1125,7 +1125,7 @@ impl ExecutingFrame<'_> {
 
     #[cfg_attr(feature = "flame-it", flame("Frame"))]
     fn import(&mut self, vm: &VirtualMachine, module: Option<PyStrRef>) -> FrameResult {
-        let module = module.unwrap_or_else(|| PyStr::from("").into_ref(vm));
+        let module = module.unwrap_or_else(|| vm.ctx.empty_str.clone());
         let from_list = <Option<PyTupleTyped<PyStrRef>>>::try_from_object(vm, self.pop_value())?;
         let level = usize::try_from_object(vm, self.pop_value())?;
 

vm/src/stdlib/builtins.rs

@@ -305,7 +305,7 @@ mod builtins {
     ) -> PyResult<PyStrRef> {
         let format_spec = format_spec
             .into_option()
-            .unwrap_or_else(|| PyStr::from("").into_ref(vm));
+            .unwrap_or_else(|| vm.ctx.empty_str.clone());
 
         call_object_format(vm, value, None, format_spec.as_str())
     }

vm/src/stdlib/io.rs

@@ -2588,7 +2588,7 @@ mod _io {
                     }
                 }
                 if chunks.is_empty() {
-                    PyStr::from("").into_ref(vm)
+                    vm.ctx.empty_str.clone()
                 } else if chunks.len() == 1 {
                     chunks.pop().unwrap()
                 } else {
@@ -2850,7 +2850,7 @@ mod _io {
             } else if let Some(cur_line) = cur_line {
                 cur_line.slice_pystr(vm)
             } else {
-                PyStr::from("").into_ref(vm)
+                vm.ctx.empty_str.clone()
             };
             Ok(line)
         }
@@ -3009,7 +3009,7 @@ mod _io {
             append: Option<PyStrRef>,
             vm: &VirtualMachine,
         ) -> PyStrRef {
-            let empty_str = || PyStr::from("").into_ref(vm);
+            let empty_str = || vm.ctx.empty_str.clone();
             let chars_pos = std::mem::take(&mut self.decoded_chars_used).bytes;
             let decoded_chars = match std::mem::take(&mut self.decoded_chars) {
                 None => return append.unwrap_or_else(empty_str),

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>,
 
@@ -83,6 +84,7 @@ 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,
@@ -90,6 +92,8 @@ impl Context {
             none,
             empty_tuple,
             empty_frozenset,
+            empty_str,
+
             ellipsis,
             not_implemented,