py/vm: Avoid heap-allocating slices when subscripting built-ins.

This commit adds a fast-path optimisation for when a BUILD_SLICE is
immediately followed by a LOAD/STORE_SUBSCR for a native type, to avoid
needing to allocate the slice on the heap.

In some cases (e.g. `a[1:3] = x`) this can result in no allocations at all.

We can't do this for instance types because the get/set/delattr
implementation may keep a reference to the slice.

Adds more tests to the basic slice tests to ensure that a stack-allocated
slice never makes it to Python, and also a heapalloc test that verifies
(when using bytecode) that assigning to a slice is no-alloc.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
Signed-off-by: Damien George <damien@micropython.org>

3 files changed, 56 insertions, 2 deletions

diff --git a/tests/basics/builtin_slice.py b/tests/basics/builtin_slice.py
index df84d5c57..5197a7cad 100644
--- a/tests/basics/builtin_slice.py
+++ b/tests/basics/builtin_slice.py
@@ -1,11 +1,44 @@
 # test builtin slice
 
+# ensures that slices passed to user types are heap-allocated and can be
+# safely stored as well as not overriden by subsequent slices.
+
 # print slice
 class A:
     def __getitem__(self, idx):
-        print(idx)
+        print("get", idx)
+        print("abc"[1:])
+        print("get", idx)
+        return idx
+
+    def __setitem__(self, idx, value):
+        print("set", idx)
+        print("abc"[1:])
+        print("set", idx)
+        self.saved_idx = idx
+        return idx
+
+    def __delitem__(self, idx):
+        print("del", idx)
+        print("abc"[1:])
+        print("del", idx)
         return idx
-s = A()[1:2:3]
+
+
+a = A()
+s = a[1:2:3]
+a[4:5:6] = s
+del a[7:8:9]
+
+print(a.saved_idx)
+
+# nested slicing
+print(A()[1 : A()[A()[2:3:4] : 5]])
+
+# tuple slicing
+a[1:2, 4:5, 7:8]
+a[1, 4:5, 7:8, 2]
+a[1:2, a[3:4], 5:6]
 
 # check type
 print(type(s) is slice)
diff --git a/tests/micropython/heapalloc_slice.py b/tests/micropython/heapalloc_slice.py
new file mode 100644
index 000000000..62d96595c
--- /dev/null
+++ b/tests/micropython/heapalloc_slice.py
@@ -0,0 +1,18 @@
+# slice operations that don't require allocation
+try:
+    from micropython import heap_lock, heap_unlock
+except (ImportError, AttributeError):
+    heap_lock = heap_unlock = lambda: 0
+
+b = bytearray(range(10))
+
+m = memoryview(b)
+
+heap_lock()
+
+b[3:5] = b"aa"
+m[5:7] = b"bb"
+
+heap_unlock()
+
+print(b)
diff --git a/tests/run-tests.py b/tests/run-tests.py
index 0eaee5278..fe338d7ff 100755
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -855,6 +855,9 @@ def run_tests(pyb, tests, args, result_dir, num_threads=1):
             "micropython/emg_exc.py"
         )  # because native doesn't have proper traceback info
         skip_tests.add(
+            "micropython/heapalloc_slice.py"
+        )  # because native doesn't do the stack-allocated slice optimisation
+        skip_tests.add(
             "micropython/heapalloc_traceback.py"
         )  # because native doesn't have proper traceback info
         skip_tests.add(