build without -fno-strict-aliasing when gcc isn't buggy #3326

Patch from Daniel Stutzbach

Misc/NEWS

@@ -883,6 +883,9 @@ Extension Modules
 Build
 -----
 
+- Issue #3326: Build Python without -fno-strict-aliasing when the gcc does not
+  give false warnings.
+
 - Issue #1628484: The Makefile doesn't ignore the CFLAGS environment
   variable anymore.  It also forwards the LDFLAGS settings to the linker
   when building a shared library.

configure

 #! /bin/sh
-# From configure.in Revision: 78965 .
+# From configure.in Revision: 79401 .
 # Guess values for system-dependent variables and create Makefiles.
 # Generated by GNU Autoconf 2.61 for python 3.2.
 #
@@ -4569,14 +4569,16 @@ UNIVERSAL_ARCH_FLAGS=
 # tweak BASECFLAGS based on compiler and platform
 case $GCC in
 yes)
-    # Python violates C99 rules, by casting between incompatible
-    # pointer types. GCC may generate bad code as a result of that,
-    # so use -fno-strict-aliasing if supported.
-    { echo "$as_me:$LINENO: checking whether $CC accepts -fno-strict-aliasing" >&5
-echo $ECHO_N "checking whether $CC accepts -fno-strict-aliasing... $ECHO_C" >&6; }
+    # Python doesn't violate C99 aliasing rules, but older versions of
+    # GCC produce warnings for legal Python code.  Enable
+    # -fno-strict-aliasing on versions of GCC that support but produce
+    # warnings.  See Issue3326
+    { echo "$as_me:$LINENO: checking whether $CC accepts and needs -fno-strict-aliasing" >&5
+echo $ECHO_N "checking whether $CC accepts and needs -fno-strict-aliasing... $ECHO_C" >&6; }
      ac_save_cc="$CC"
      CC="$CC -fno-strict-aliasing"
-     if test "${ac_cv_no_strict_aliasing_ok+set}" = set; then
+     save_CFLAGS="$CFLAGS"
+     if test "${ac_cv_no_strict_aliasing+set}" = set; then
   echo $ECHO_N "(cached) $ECHO_C" >&6
 else
   cat >conftest.$ac_ext <<_ACEOF
@@ -4611,21 +4613,64 @@ eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
 	 test -z "$ac_c_werror_flag" ||
 	 test ! -s conftest.err
        } && test -s conftest.$ac_objext; then
-  ac_cv_no_strict_aliasing_ok=yes
+  CC="$ac_save_cc -fstrict-aliasing"
+         CFLAGS="$CFLAGS -Werror -Wstrict-aliasing"
+         cat >conftest.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+
+int
+main ()
+{
+void f(int **x) {} int main() { double *x; f((int **) &x); return 0; }
+  ;
+  return 0;
+}
+_ACEOF
+rm -f conftest.$ac_objext
+if { (ac_try="$ac_compile"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
+  (eval "$ac_compile") 2>conftest.er1
+  ac_status=$?
+  grep -v '^ *+' conftest.er1 >conftest.err
+  rm -f conftest.er1
+  cat conftest.err >&5
+  echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); } && {
+	 test -z "$ac_c_werror_flag" ||
+	 test ! -s conftest.err
+       } && test -s conftest.$ac_objext; then
+  ac_cv_no_strict_aliasing=no
+else
+  echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+	ac_cv_no_strict_aliasing=yes
+fi
+
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
 else
   echo "$as_me: failed program was:" >&5
 sed 's/^/| /' conftest.$ac_ext >&5
 
-	ac_cv_no_strict_aliasing_ok=no
+	ac_cv_no_strict_aliasing=no
 fi
 
 rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
 fi
 
+     CFLAGS="$save_CFLAGS"
      CC="$ac_save_cc"
-    { echo "$as_me:$LINENO: result: $ac_cv_no_strict_aliasing_ok" >&5
-echo "${ECHO_T}$ac_cv_no_strict_aliasing_ok" >&6; }
-    if test $ac_cv_no_strict_aliasing_ok = yes
+    { echo "$as_me:$LINENO: result: $ac_cv_no_strict_aliasing" >&5
+echo "${ECHO_T}$ac_cv_no_strict_aliasing" >&6; }
+    if test $ac_cv_no_strict_aliasing = yes
     then
       BASECFLAGS="$BASECFLAGS -fno-strict-aliasing"
     fi

configure.in

@@ -867,19 +867,26 @@ AC_SUBST(UNIVERSAL_ARCH_FLAGS)
 # tweak BASECFLAGS based on compiler and platform
 case $GCC in
 yes)
-    # Python violates C99 rules, by casting between incompatible
-    # pointer types. GCC may generate bad code as a result of that,
-    # so use -fno-strict-aliasing if supported.
-    AC_MSG_CHECKING(whether $CC accepts -fno-strict-aliasing)
+    # Python doesn't violate C99 aliasing rules, but older versions of
+    # GCC produce warnings for legal Python code.  Enable
+    # -fno-strict-aliasing on versions of GCC that support but produce
+    # warnings.  See Issue3326
+    AC_MSG_CHECKING(whether $CC accepts and needs -fno-strict-aliasing)
      ac_save_cc="$CC"
      CC="$CC -fno-strict-aliasing"
-     AC_CACHE_VAL(ac_cv_no_strict_aliasing_ok,
-     AC_TRY_COMPILE([],[int main() { return 0; }],
-     ac_cv_no_strict_aliasing_ok=yes,
-     ac_cv_no_strict_aliasing_ok=no))
+     save_CFLAGS="$CFLAGS"
+     AC_CACHE_VAL(ac_cv_no_strict_aliasing,
+       AC_TRY_COMPILE([],[int main() { return 0; }],
+         CC="$ac_save_cc -fstrict-aliasing"
+         CFLAGS="$CFLAGS -Werror -Wstrict-aliasing"
+         AC_TRY_COMPILE([],[void f(int **x) {} int main() { double *x; f((int **) &x); return 0; }],
+           ac_cv_no_strict_aliasing=no,
+           ac_cv_no_strict_aliasing=yes),
+         ac_cv_no_strict_aliasing=no))
+     CFLAGS="$save_CFLAGS"
      CC="$ac_save_cc"
-    AC_MSG_RESULT($ac_cv_no_strict_aliasing_ok)
-    if test $ac_cv_no_strict_aliasing_ok = yes
+    AC_MSG_RESULT($ac_cv_no_strict_aliasing)
+    if test $ac_cv_no_strict_aliasing = yes
     then
       BASECFLAGS="$BASECFLAGS -fno-strict-aliasing"
     fi