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cc @serge-sans-paille @erlend-aasland: This version should no longer miscompile Python: it avoids the type punning issue thanks to memcpy(). I wrote a comment in Py_CLEAR() to explain the rationale for memcpy().

This PR no longer reintroduces the miscompilation bug: issue #99701. I tested manually.

I tested this PR with this local patch (otherwise the output is just too verbose!):

diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 8209132ebc..cf243c8ef9 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -1891,9 +1891,6 @@ Py_FinalizeEx(void)
     /* Destroy all modules */
     finalize_modules(tstate);
 
-    /* Print debug stats if any */
-    _PyEval_Fini();
-
     /* Flush sys.stdout and sys.stderr (again, in case more was printed) */
     if (flush_std_files() < 0) {
         status = -1;

Test command:

rm ~/pyprefix -rf; git clean -fdx
./configure --enable-pystats --prefix ~/pyprefix CC=gcc CFLAGS="-O3 -fstrict-aliasing"
make && make install
./python -m test test_capi test_itertools

The test succeeded with CC=gcc CFLAGS="-O3 -fstrict-aliasing" and CC=clang LD=clang CFLAGS="-O3 -fstrict-aliasing".

I updated my PR to use __typeof__() if available to still emit compiler warnings in Py_SETREF(dst, src) if src has not the same type than dst. On Windows, the memcpy() implementation is used since __typeof__() is not available: memcpy() avoids the type punning issue, but it does not emit a compiler warning if src has the same type.

My previous attempt PR #99739 always used __typeof__(), but this function is not available on Windows with MSVC.

I ran my test for 4 cases and all succeeded:

• GCC with __typeof__()
• GCC with memcpy()
• clang with __typeof__()
• clang with memcpy()

To also emit a warning in the memcpy() implementation, @serge-sans-paille suggested to me using sizeof(dst=src). It works because sizeof() does not evalute its argument at runtime, it only infers the argument type. MSVC emits a warning with sizeof(dst=src).

I'm not sure about this one. Maybe this enhancement can be made later, once the initial issue #98724 is fixed with my current PR implementation. So it's easy to revert if something goes wrong.

🤖 New build scheduled with the buildbot fleet by @vstinner for commit 86d8878 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

buildbot/ARM64 Windows PR

test_ast.test_recursion_direct() crash with a stack overflow. It sounds unrelated to this change. The ARM64 Windows 3.x buildbot is sick for 2 days: it fails with "exception" (don't build+test Python).

test_parse_in_error (test.test_ast.ASTHelpers_Test.test_parse_in_error) ... ok
Windows fatal exception: stack overflow
Current thread 0x0000142c (most recent call first):
  File "C:\Workspace\buildarea\pull_request.linaro-win-arm64\build\Lib\test\test_ast.py", line 1252 in test_recursion_direct

Apart of the sick "ARM64 Windows 3.x" buildbot, all buildbots were happy with this change.

I fixed a fix typos. I amended the commit message.

buildbot/ARM64 Windows PR: test_ast.test_recursion_direct() crash with a stack overflow

FYI I proposed #100104 to fix this bug (unrelated to this change).