From c771cbe8f9b13d674b74d5f81ab36a5e20febb7d Mon Sep 17 00:00:00 2001 From: "Miss Islington (bot)" <31488909+miss-islington@users.noreply.github.com> Date: Tue, 24 May 2022 01:52:06 -0700 Subject: gh-93065: Fix HAMT to iterate correctly over 7-level deep trees (GH-93066) (GH-93145) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Also while there, clarify a few things about why we reduce the hash to 32 bits. Co-authored-by: Eli Libman Co-authored-by: Yury Selivanov Co-authored-by: Łukasz Langa (cherry picked from commit c1f5c903a7e4ed27190488f4e33b00d3c3d952e5) --- Include/internal/pycore_hamt.h | 14 ++++++++- Lib/test/test_context.py | 35 ++++++++++++++++++++++ Misc/ACKS | 1 + .../2022-05-21-23-21-37.gh-issue-93065.5I18WC.rst | 5 ++++ Python/hamt.c | 14 +++++++-- 5 files changed, 65 insertions(+), 4 deletions(-) create mode 100644 Misc/NEWS.d/next/Core and Builtins/2022-05-21-23-21-37.gh-issue-93065.5I18WC.rst diff --git a/Include/internal/pycore_hamt.h b/Include/internal/pycore_hamt.h index 85e35c5..4d64288 100644 --- a/Include/internal/pycore_hamt.h +++ b/Include/internal/pycore_hamt.h @@ -5,7 +5,19 @@ # error "this header requires Py_BUILD_CORE define" #endif -#define _Py_HAMT_MAX_TREE_DEPTH 7 + +/* +HAMT tree is shaped by hashes of keys. Every group of 5 bits of a hash denotes +the exact position of the key in one level of the tree. Since we're using +32 bit hashes, we can have at most 7 such levels. Although if there are +two distinct keys with equal hashes, they will have to occupy the same +cell in the 7th level of the tree -- so we'd put them in a "collision" node. +Which brings the total possible tree depth to 8. Read more about the actual +layout of the HAMT tree in `hamt.c`. + +This constant is used to define a datastucture for storing iteration state. +*/ +#define _Py_HAMT_MAX_TREE_DEPTH 8 extern PyTypeObject _PyHamt_Type; diff --git a/Lib/test/test_context.py b/Lib/test/test_context.py index 3132cea..b1aece4 100644 --- a/Lib/test/test_context.py +++ b/Lib/test/test_context.py @@ -535,6 +535,41 @@ class HamtTest(unittest.TestCase): self.assertEqual(len(h4), 2) self.assertEqual(len(h5), 3) + def test_hamt_collision_3(self): + # Test that iteration works with the deepest tree possible. + # https://github.com/python/cpython/issues/93065 + + C = HashKey(0b10000000_00000000_00000000_00000000, 'C') + D = HashKey(0b10000000_00000000_00000000_00000000, 'D') + + E = HashKey(0b00000000_00000000_00000000_00000000, 'E') + + h = hamt() + h = h.set(C, 'C') + h = h.set(D, 'D') + h = h.set(E, 'E') + + # BitmapNode(size=2 count=1 bitmap=0b1): + # NULL: + # BitmapNode(size=2 count=1 bitmap=0b1): + # NULL: + # BitmapNode(size=2 count=1 bitmap=0b1): + # NULL: + # BitmapNode(size=2 count=1 bitmap=0b1): + # NULL: + # BitmapNode(size=2 count=1 bitmap=0b1): + # NULL: + # BitmapNode(size=2 count=1 bitmap=0b1): + # NULL: + # BitmapNode(size=4 count=2 bitmap=0b101): + # : 'E' + # NULL: + # CollisionNode(size=4 id=0x107a24520): + # : 'C' + # : 'D' + + self.assertEqual({k.name for k in h.keys()}, {'C', 'D', 'E'}) + def test_hamt_stress(self): COLLECTION_SIZE = 7000 TEST_ITERS_EVERY = 647 diff --git a/Misc/ACKS b/Misc/ACKS index a55706d..4ea1fa1 100644 --- a/Misc/ACKS +++ b/Misc/ACKS @@ -1062,6 +1062,7 @@ Robert Li Xuanji Li Zekun Li Zheao Li +Eli Libman Dan Lidral-Porter Robert van Liere Ross Light diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-05-21-23-21-37.gh-issue-93065.5I18WC.rst b/Misc/NEWS.d/next/Core and Builtins/2022-05-21-23-21-37.gh-issue-93065.5I18WC.rst new file mode 100644 index 0000000..ea80165 --- /dev/null +++ b/Misc/NEWS.d/next/Core and Builtins/2022-05-21-23-21-37.gh-issue-93065.5I18WC.rst @@ -0,0 +1,5 @@ +Fix contextvars HAMT implementation to handle iteration over deep trees. + +The bug was discovered and fixed by Eli Libman. See +`MagicStack/immutables#84 `_ +for more details. diff --git a/Python/hamt.c b/Python/hamt.c index c3cb4e6..908c253 100644 --- a/Python/hamt.c +++ b/Python/hamt.c @@ -409,14 +409,22 @@ hamt_hash(PyObject *o) return -1; } - /* While it's suboptimal to reduce Python's 64 bit hash to + /* While it's somewhat suboptimal to reduce Python's 64 bit hash to 32 bits via XOR, it seems that the resulting hash function is good enough (this is also how Long type is hashed in Java.) Storing 10, 100, 1000 Python strings results in a relatively shallow and uniform tree structure. - Please don't change this hashing algorithm, as there are many - tests that test some exact tree shape to cover all code paths. + Also it's worth noting that it would be possible to adapt the tree + structure to 64 bit hashes, but that would increase memory pressure + and provide little to no performance benefits for collections with + fewer than billions of key/value pairs. + + Important: do not change this hash reducing function. There are many + tests that need an exact tree shape to cover all code paths and + we do that by specifying concrete values for test data's `__hash__`. + If this function is changed most of the regression tests would + become useless. */ int32_t xored = (int32_t)(hash & 0xffffffffl) ^ (int32_t)(hash >> 32); return xored == -1 ? -2 : xored; -- cgit v0.12