Disable unsafe identical code folding in BOLT

cpython-unix/build-cpython.sh

@@ -271,6 +271,12 @@ if [ -n "${PYTHON_MEETS_MINIMUM_VERSION_3_12}" ]; then
     # https://github.com/python/cpython/issues/128514
     patch -p1 -i ${ROOT}/patch-configure-bolt-apply-flags-128514.patch
 
+    # Disable unsafe identical code folding. Objects/typeobject.c
+    # update_one_slot requires that wrap_binaryfunc != wrap_binaryfunc_l,
+    # despite the functions being identical.
+    # https://github.com/python/cpython/pull/134642
+    patch -p1 -i ${ROOT}/patch-configure-bolt-icf-safe.patch
+
     # Tweak --skip-funcs to work with our toolchain.
     patch -p1 -i ${ROOT}/patch-configure-bolt-skip-funcs.patch
 fi

cpython-unix/patch-configure-bolt-icf-safe.patch

@@ -0,0 +1,84 @@
+From 91fc5ae4a5a66a03931f8cd383abd2aa062bb0e9 Mon Sep 17 00:00:00 2001
+From: Geoffrey Thomas <[email protected]>
+Date: Sat, 24 May 2025 19:04:09 -0400
+Subject: [PATCH 1/1] Use only safe identical code folding with BOLT
+
+"Identical code folding" (ICF) is the feature of an optimizer to find that two
+functions have the same code and that they can therefore be deduplicated
+in the binary. While this is usually safe, it can cause observable
+behavior differences if the program relies on the fact that the two
+functions have different addresses.
+
+CPython relies on this in (at least) Objects/typeobject.c, which defines
+two functions wrap_binaryfunc() and wrap_binaryfunc_l() with the same
+implementation, and stores their addresses in the slotdefs array. If
+these two functions have the same address, update_one_slot() in that
+file will fill in slots it shouldn't, causing, for instances,
+classes defined in Python that inherit from some built-in types to
+misbehave.
+
+As of LLVM 20 (llvm/llvm-project#116275), BOLT has a "safe ICF" mode,
+where it looks to see if there are any uses of a function symbol outside
+function calls (e.g., relocations in data sections) and skips ICF on
+such functions. The intent is that this avoids observable behavior
+differences but still saves storage as much as possible.
+
+This version is about two months old at the time of writing. To support
+older LLVM versions, we have to turn off ICF entirely.
+
+This problem was previously noticed for Windows/MSVC in #53093 (and
+again in #24098), where the default behavior of PGO is to enable ICF
+(which they expand to "identical COMDAT folding") and we had to turn it
+off.
+---
+ configure    | 50 +++++++++++++++++++++++++++++++++++++++++++++++++-
+ configure.ac | 25 ++++++++++++++++++++++++-
+ 2 files changed, 73 insertions(+), 2 deletions(-)
+
+diff --git a/configure.ac b/configure.ac
+index 8d939f07505..25737e3f9d6 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -2129,6 +2129,29 @@ if test "$Py_BOLT" = 'true' ; then
+   else
+     AC_MSG_ERROR([merge-fdata is required for a --enable-bolt build but could not be found.])
+   fi
++
++  py_bolt_icf_flag="-icf=safe"
++  AC_CACHE_CHECK(
++    [whether ${LLVM_BOLT} supports safe identical code folding],
++    [py_cv_bolt_icf_safe],
++    [
++      saved_cflags="$CFLAGS"
++      saved_ldflags="$LDFLAGS"
++      CFLAGS="$CFLAGS_NODIST"
++      LDFLAGS="$LDFLAGS_NODIST"
++      AC_LINK_IFELSE(
++        [AC_LANG_PROGRAM([[]], [[]])],
++        [py_cv_bolt_icf_safe=no
++         ${LLVM_BOLT} -icf=safe -o conftest.bolt conftest$EXEEXT >&AS_MESSAGE_LOG_FD 2>&1 dnl
++         && py_cv_bolt_icf_safe=yes],
++        [AC_MSG_FAILURE([could not compile empty test program])])
++      CFLAGS="$saved_cflags"
++      LDFLAGS="$saved_ldflags"
++    ]
++  )
++  if test "$py_cv_bolt_icf_safe" = no; then
++    py_bolt_icf_flag=""
++  fi
+ fi
+
+ dnl Enable BOLT of libpython if built.
+@@ -2184,7 +2207,7 @@ then
+      -reorder-blocks=ext-tsp
+      -reorder-functions=cdsort
+      -split-functions
+      -split-strategy=cdsplit
+-     -icf=1
++     ${py_bolt_icf_flag}
+      -inline-all
+      -split-eh
+      -reorder-functions-use-hot-size
+-- 
+2.39.5 (Apple Git-154)
+