gh-96821: Add config option --with-strict-overflow

Doc/using/configure.rst

@@ -278,6 +278,11 @@ also be used to improve performance.
 
    Enable C-level code profiling with ``gprof`` (disabled by default).
 
+.. cmdoption:: --with-strict-overflow
+
+   Add ``-fstrict-overflow`` to the C compiler flags (by default we add
+   ``-fno-strict-overflow`` instead).
+
 
 .. _debug-build:
 

Misc/NEWS.d/next/Build/2022-09-14-10-38-15.gh-issue-96821.Zk2a9c.rst

@@ -0,0 +1,2 @@
+Explicitly mark C extension modules that need defined signed integer overflow,
+and add a configure option :option:`--with-strict-overflow`.

configure.ac

@@ -2082,6 +2082,35 @@ case $CC in
         fi
 esac
 
+dnl Historically, some of our code assumed that signed integer overflow
+dnl is defined behaviour via twos-complement.
+dnl Set STRICT_OVERFLOW_CFLAGS and NO_STRICT_OVERFLOW_CFLAGS depending on compiler support.
+dnl Pass the latter to modules that depend on such behaviour.
+_SAVE_VAR([CFLAGS])
+CFLAGS="-fstrict-overflow -fno-strict-overflow"
+AC_CACHE_CHECK([if $CC supports -fstrict-overflow and -fno-strict-overflow],
+               [ac_cv_cc_supports_fstrict_overflow],
+  AC_COMPILE_IFELSE(
+    [
+      AC_LANG_PROGRAM([[]], [[]])
+    ],[
+      STRICT_OVERFLOW_CFLAGS="-fstrict-overflow"
+      NO_STRICT_OVERFLOW_CFLAGS="-fno-strict-overflow"
+    ],[
+      STRICT_OVERFLOW_CFLAGS=""
+      NO_STRICT_OVERFLOW_CFLAGS=""
+    ])
+)
+_RESTORE_VAR([CFLAGS])
+
+AC_MSG_CHECKING([for --with-strict-overflow])
+AC_ARG_WITH([strict-overflow],
+  AS_HELP_STRING(
+    [--with-strict-overflow],
+    [choose between -fstrict-overflow or -fno-strict-overflow (default is no)]),[],
+    [with_strict_overflow=no])
+AC_MSG_RESULT([$with_strict_overflow])
+
 # Check if CC supports -Og optimization level
 _SAVE_VAR([CFLAGS])
 CFLAGS="-Og"
@@ -2112,15 +2141,8 @@ if test "${OPT-unset}" = "unset"
 then
     case $GCC in
     yes)
-        # For gcc 4.x we need to use -fwrapv so lets check if its supported
-        if "$CC" -v --help 2>/dev/null |grep -- -fwrapv > /dev/null; then
-           WRAP="-fwrapv"
-        fi
-
         if test -n "${cc_is_clang}"
         then
-            # Clang also needs -fwrapv
-            WRAP="-fwrapv"
             # bpo-30104: disable strict aliasing to compile correctly dtoa.c,
             # see Makefile.pre.in for more information
             CFLAGS_ALIASING="-fno-strict-aliasing"
@@ -2131,7 +2153,7 @@ then
 	    if test "$Py_DEBUG" = 'true' ; then
 		OPT="-g $PYDEBUG_CFLAGS -Wall"
 	    else
-		OPT="-g $WRAP -O3 -Wall"
+		OPT="-g -O3 -Wall"
 	    fi
 	    ;;
 	*)
@@ -2246,6 +2268,10 @@ AC_DEFUN([PY_CHECK_CC_WARNING], [
 ])
 
 # tweak BASECFLAGS based on compiler and platform
+AS_VAR_IF([with_strict_overflow], [yes],
+          [BASECFLAGS="$BASECFLAGS $STRICT_OVERFLOW_CFLAGS"],
+          [BASECFLAGS="$BASECFLAGS $NO_STRICT_OVERFLOW_CFLAGS"])
+
 case $GCC in
 yes)
     CFLAGS_NODIST="$CFLAGS_NODIST -std=c11"
@@ -7307,7 +7333,7 @@ PY_STDLIB_MOD([_crypt],
   [$LIBCRYPT_CFLAGS], [$LIBCRYPT_LIBS])
 PY_STDLIB_MOD([_ctypes],
   [], [test "$have_libffi" = yes],
-  [$LIBFFI_CFLAGS], [$LIBFFI_LIBS])
+  [$NO_STRICT_OVERFLOW_CFLAGS $LIBFFI_CFLAGS], [$LIBFFI_LIBS])
 PY_STDLIB_MOD([_curses],
   [], [test "$have_curses" != "no"],
   [$CURSES_CFLAGS], [$CURSES_LIBS]