[mypyc] Simplify IR generated for "for" loops over strings (#19434)

Add unsafe list get item primitive. The new primitive just calls the
primary get item primitive, but we could later provide an optimized
primitive if this turns out to be a performance bottleneck.

Author: JukkaL

mypyc/irbuild/for_helpers.py

@@ -76,6 +76,7 @@
 from mypyc.primitives.misc_ops import stop_async_iteration_op
 from mypyc.primitives.registry import CFunctionDescription
 from mypyc.primitives.set_ops import set_add_op
+from mypyc.primitives.str_ops import str_get_item_unsafe_op
 from mypyc.primitives.tuple_ops import tuple_get_item_unsafe_op
 
 GenFunc = Callable[[], None]
@@ -772,6 +773,8 @@ def unsafe_index(builder: IRBuilder, target: Value, index: Value, line: int) ->
         return builder.primitive_op(list_get_item_unsafe_op, [target, index], line)
     elif is_tuple_rprimitive(target.type):
         return builder.call_c(tuple_get_item_unsafe_op, [target, index], line)
+    elif is_str_rprimitive(target.type):
+        return builder.call_c(str_get_item_unsafe_op, [target, index], line)
     else:
         return builder.gen_method_call(target, "__getitem__", [index], None, line)
 

mypyc/lib-rt/CPy.h

@@ -727,6 +727,7 @@ static inline char CPyDict_CheckSize(PyObject *dict, Py_ssize_t size) {
 char CPyStr_Equal(PyObject *str1, PyObject *str2);
 PyObject *CPyStr_Build(Py_ssize_t len, ...);
 PyObject *CPyStr_GetItem(PyObject *str, CPyTagged index);
+PyObject *CPyStr_GetItemUnsafe(PyObject *str, Py_ssize_t index);
 CPyTagged CPyStr_Find(PyObject *str, PyObject *substr, CPyTagged start, int direction);
 CPyTagged CPyStr_FindWithEnd(PyObject *str, PyObject *substr, CPyTagged start, CPyTagged end, int direction);
 PyObject *CPyStr_Split(PyObject *str, PyObject *sep, CPyTagged max_split);

mypyc/lib-rt/str_ops.c

@@ -117,6 +117,11 @@ PyObject *CPyStr_GetItem(PyObject *str, CPyTagged index) {
     }
 }
 
+PyObject *CPyStr_GetItemUnsafe(PyObject *str, Py_ssize_t index) {
+    // This is unsafe since we don't check for overflow when doing <<.
+    return CPyStr_GetItem(str, index << 1);
+}
+
 // A simplification of _PyUnicode_JoinArray() from CPython 3.9.6
 PyObject *CPyStr_Build(Py_ssize_t len, ...) {
     Py_ssize_t i;

mypyc/primitives/str_ops.py

@@ -95,6 +95,15 @@
     error_kind=ERR_MAGIC,
 )
 
+# This is unsafe since it assumes that the index is within reasonable bounds.
+# In the future this might do no bounds checking at all.
+str_get_item_unsafe_op = custom_op(
+    arg_types=[str_rprimitive, c_pyssize_t_rprimitive],
+    return_type=str_rprimitive,
+    c_function_name="CPyStr_GetItemUnsafe",
+    error_kind=ERR_MAGIC,
+)
+
 # str[begin:end]
 str_slice_op = custom_op(
     arg_types=[str_rprimitive, int_rprimitive, int_rprimitive],

mypyc/test-data/irbuild-tuple.test

@@ -272,7 +272,7 @@ L4:
     a = r6
     return 1
 
-[case testTupleBuiltFromStr_64bit]
+[case testTupleBuiltFromStr]
 def f2(val: str) -> str:
     return val + "f2"
 
@@ -292,10 +292,9 @@ def test():
     r2 :: bit
     r3 :: tuple
     r4, r5 :: native_int
-    r6, r7, r8, r9 :: bit
-    r10, r11, r12 :: int
-    r13, x, r14 :: str
-    r15 :: native_int
+    r6, r7 :: bit
+    r8, x, r9 :: str
+    r10 :: native_int
     a :: tuple
 L0:
     r0 = 'abc'
@@ -308,30 +307,17 @@ L1:
     r5 = CPyStr_Size_size_t(source)
     r6 = r5 >= 0 :: signed
     r7 = r4 < r5 :: signed
-    if r7 goto L2 else goto L8 :: bool
+    if r7 goto L2 else goto L4 :: bool
 L2:
-    r8 = r4 <= 4611686018427387903 :: signed
-    if r8 goto L3 else goto L4 :: bool
+    r8 = CPyStr_GetItemUnsafe(source, r4)
+    x = r8
+    r9 = f2(x)
+    CPySequenceTuple_SetItemUnsafe(r3, r4, r9)
 L3:
-    r9 = r4 >= -4611686018427387904 :: signed
-    if r9 goto L5 else goto L4 :: bool
-L4:
-    r10 = CPyTagged_FromInt64(r4)
-    r11 = r10
-    goto L6
-L5:
-    r12 = r4 << 1
-    r11 = r12
-L6:
-    r13 = CPyStr_GetItem(source, r11)
-    x = r13
-    r14 = f2(x)
-    CPySequenceTuple_SetItemUnsafe(r3, r4, r14)
-L7:
-    r15 = r4 + 1
-    r4 = r15
+    r10 = r4 + 1
+    r4 = r10
     goto L1
-L8:
+L4:
     a = r3
     return 1