GH-130415: Narrow str to "" based on boolean tests (GH-130476)

Author: fluhus

Lib/test/test_capi/test_opt.py

@@ -1531,6 +1531,39 @@ def f(n):
         # But all of the appends we care about are still there:
         self.assertEqual(uops.count("_CALL_LIST_APPEND"), len("ABCDEFG"))
 
+def test_narrow_type_to_constant_str_empty(self):
+        def f(n):
+            trace = []
+            for i in range(n):
+                # Hopefully the optimizer can't guess what the value is.
+                # empty is always "", but we can only prove that it's a string:
+                false = i == TIER2_THRESHOLD
+                empty = "X"[:false]
+                trace.append("A")
+                if not empty:  # Kept.
+                    trace.append("B")
+                    if not empty:  # Removed!
+                        trace.append("C")
+                    trace.append("D")
+                    if empty:  # Removed!
+                        trace.append("X")
+                    trace.append("E")
+                trace.append("F")
+                if empty:  # Removed!
+                    trace.append("X")
+                trace.append("G")
+            return trace
+
+        trace, ex = self._run_with_optimizer(f, TIER2_THRESHOLD)
+        self.assertEqual(trace, list("ABCDEFG") * TIER2_THRESHOLD)
+        self.assertIsNotNone(ex)
+        uops = get_opnames(ex)
+        # Only one guard remains:
+        self.assertEqual(uops.count("_GUARD_IS_FALSE_POP"), 1)
+        self.assertEqual(uops.count("_GUARD_IS_TRUE_POP"), 0)
+        # But all of the appends we care about are still there:
+        self.assertEqual(uops.count("_CALL_LIST_APPEND"), len("ABCDEFG"))
+
 def global_identity(x):
     return x
 

Misc/ACKS

@@ -1065,6 +1065,7 @@ Keenan Lau
 Piers Lauder
 Ben Laurie
 Yoni Lavi
+Amit Lavon
 Simon Law
 Julia Lawall
 Chris Lawrence

Misc/NEWS.d/next/Core_and_Builtins/2025-02-22-22-49-00.gh-issue-130415.WyxBYS.rst

@@ -0,0 +1 @@
+Improve JIT's ability to optimize strings in boolean contexts.

Python/optimizer_bytecodes.c

@@ -426,7 +426,7 @@ dummy_func(void) {
 
     op(_TO_BOOL_STR, (value -- res)) {
         if (!optimize_to_bool(this_instr, ctx, value, &res)) {
-            res = sym_new_type(ctx, &PyBool_Type);
+            res = sym_new_truthiness(ctx, value, true);
             sym_set_type(value, &PyUnicode_Type);
         }
     }

Python/optimizer_cases.c.h

@@ -302,6 +302,9 @@ _Py_uop_sym_set_const(JitOptContext *ctx, JitOptSymbol *sym, PyObject *const_val
             else if (type == &PyLong_Type) {
                 _Py_uop_sym_set_const(ctx, value, Py_GetConstant(Py_CONSTANT_ZERO));
             }
+            else if (type == &PyUnicode_Type) {
+                _Py_uop_sym_set_const(ctx, value, Py_GetConstant(Py_CONSTANT_EMPTY_STR));
+            }
             // TODO: More types (GH-130415)!
             make_const(sym, const_val);
             return;