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>

1 files changed, 3 insertions, 0 deletions

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(