gh-145098: Run Apple Silicon macOS CI on macos-26 (Tahoe)

.github/workflows/build.yml

@@ -200,10 +200,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        # macos-14 is M1, macos-15-intel is Intel.
+        # macos-26 is Apple Silicon, macos-15-intel is Intel.
         # macos-15-intel only runs tests against the GIL-enabled CPython.
         os:
-        - macos-14
+        - macos-26
         - macos-15-intel
         free-threading:
         - false
@@ -387,7 +387,7 @@ jobs:
       matrix:
         include:
           - arch: aarch64
-            runs-on: macos-14
+            runs-on: macos-26
           - arch: x86_64
             runs-on: ubuntu-24.04
 
@@ -404,7 +404,7 @@ jobs:
     needs: build-context
     if: needs.build-context.outputs.run-ios == 'true'
     timeout-minutes: 60
-    runs-on: macos-14
+    runs-on: macos-26
     steps:
       - uses: actions/checkout@v6
         with:
@@ -417,10 +417,10 @@ jobs:
       # https://github.com/actions/runner-images/issues/12751.
       - name: Select Xcode version
         run: |
-          sudo xcode-select --switch /Applications/Xcode_15.4.app
+          sudo xcode-select --switch /Applications/Xcode.app
 
       - name: Build and test
-        run: python3 Apple ci iOS --fast-ci --simulator 'iPhone SE (3rd generation),OS=17.5'
+        run: python3 Apple ci iOS --fast-ci --simulator 'iPhone 17,OS=latest'
 
   build-wasi:
     name: 'WASI'

.github/workflows/jit.yml

@@ -86,7 +86,7 @@ jobs:
             runner: macos-15-intel
           - target: aarch64-apple-darwin/clang
             architecture: aarch64
-            runner: macos-14
+            runner: macos-26
           - target: x86_64-unknown-linux-gnu/gcc
             architecture: x86_64
             runner: ubuntu-24.04

.github/workflows/tail-call.yml

@@ -69,7 +69,7 @@ jobs:
           - target: x86_64-apple-darwin/clang
             runner: macos-15-intel
           - target: aarch64-apple-darwin/clang
-            runner: macos-14
+            runner: macos-26
     steps:
       - uses: actions/checkout@v6
         with:

Include/internal/pycore_dict.h

@@ -171,7 +171,9 @@ typedef enum {
     DICT_KEYS_SPLIT = 2
 } DictKeysKind;
 
-/* See dictobject.c for actual layout of DictKeysObject */
+/* See dictobject.c for the full DictKeysObject memory layout.
+ * A PyDictKeysObject* points to the fixed-size header below.
+ */
 struct _dictkeysobject {
     Py_ssize_t dk_refcnt;
 
@@ -211,7 +213,8 @@ struct _dictkeysobject {
        - 4 bytes if dk_size <= 0xffffffff (int32_t*)
        - 8 bytes otherwise (int64_t*)
 
-       Dynamically sized, SIZEOF_VOID_P is minimum. */
+       Dynamically sized. This is the start of the variable-sized tail that
+       immediately follows the fixed-size header. */
     char dk_indices[];  /* char is required to avoid strict aliasing. */
 
     /* "PyDictKeyEntry or PyDictUnicodeEntry dk_entries[USABLE_FRACTION(DK_SIZE(dk))];" array follows:

Objects/dictobject.c

@@ -17,6 +17,7 @@ As of Python 3.6, this is compact and ordered. Basic idea is described here:
 layout:
 
 +---------------------+
+| struct _dictkeysobj |
 | dk_refcnt           |
 | dk_log2_size        |
 | dk_log2_index_bytes |
@@ -25,13 +26,17 @@ As of Python 3.6, this is compact and ordered. Basic idea is described here:
 | dk_usable           |
 | dk_nentries         |
 +---------------------+
-| dk_indices[]        |
+| dk_indices[]        |  <-- starts at &dk->dk_indices[0]
 |                     |
 +---------------------+
-| dk_entries[]        |
+| dk_entries[]        |  <-- starts at DK_ENTRIES(dk)
 |                     |
 +---------------------+
 
+A PyDictKeysObject* points to the start of the fixed-size header.
+The variable-sized part begins at dk_indices and is followed immediately by
+the entry array.
+
 dk_indices is actual hashtable.  It holds index in entries, or DKIX_EMPTY(-1)
 or DKIX_DUMMY(-2).
 Size of indices is dk_size.  Type of each index in indices varies with dk_size: