gh-101525: Use only safe identical code folding with BOLT

Misc/NEWS.d/next/Build/2026-09-11-19-37-05.gh-issue-101525.DGIpL3.rst

@@ -0,0 +1,2 @@
+When using :option:`--enable-bolt`, enable safe identical code folding on
+BOLT versions new enough to support it (LLVM 20+).

configure.ac

@@ -2230,6 +2230,27 @@ 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 ${LLVM_BOLT} supports safe identical code folding],
+    [py_cv_bolt_icf_safe],
+    [
+      WITH_SAVE_ENV([
+        CFLAGS="$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])])
+      ])
+    ]
+  )
+  if test "$py_cv_bolt_icf_safe" = no; then
+    py_bolt_icf_flag=""
+  fi
 fi
 
 dnl Enable BOLT of libpython if built and used by the python3 binary.
@@ -2289,7 +2310,7 @@ then
      -reorder-blocks=ext-tsp
      -reorder-functions=cdsort
      -split-functions
-     -icf=0
+     ${py_bolt_icf_flag}
      -inline-all
      -split-eh
      -reorder-functions-use-hot-size