only use -fno-strict-aliasing when needed by compiler

diff --git a/configure b/configure
index 2eb16b94eda6cdae9787c6fb5f3ccb36d9642a9e..d7cffff8af8c72dfe9e4802dd067ece4e23b45e8 100755 (executable)
--- a/configure
+++ b/configure
@@ -5295,33 +5295,72 @@ 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.
-    { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC accepts -fno-strict-aliasing" >&5
-$as_echo_n "checking whether $CC accepts -fno-strict-aliasing... " >&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
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC accepts and needs -fno-strict-aliasing" >&5
+$as_echo_n "checking whether $CC accepts and needs -fno-strict-aliasing... " >&6; }
      ac_save_cc="$CC"
      CC="$CC -fno-strict-aliasing"
-     if test "$cross_compiling" = yes; then :
-  ac_cv_no_strict_aliasing_ok=no
+     save_CFLAGS="$CFLAGS"
+     if test "${ac_cv_no_strict_aliasing+set}" = set; then :
+  $as_echo_n "(cached) " >&6
 else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-int main() { return 0; }
+
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+
 _ACEOF
-if ac_fn_c_try_run "$LINENO"; then :
-  ac_cv_no_strict_aliasing_ok=yes
+if ac_fn_c_try_compile "$LINENO"; then :
+
+          CC="$ac_save_cc -fstrict-aliasing"
+           CFLAGS="$CFLAGS -Werror -Wstrict-aliasing"
+           cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+              void f(int **x) {}
+int
+main ()
+{
+double *x; f((int **) &x);
+  ;
+  return 0;
+}
+
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+
+              ac_cv_no_strict_aliasing=no
+
 else
-  ac_cv_no_strict_aliasing_ok=no
+
+               ac_cv_no_strict_aliasing=yes
+
 fi
-rm -f core *.core core.conftest.* gmon.out bb.out conftest$ac_exeext \
-  conftest.$ac_objext conftest.beam conftest.$ac_ext
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+
+else
+
+          ac_cv_no_strict_aliasing=no
+
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
 fi
 
+     CFLAGS="$save_CFLAGS"
      CC="$ac_save_cc"
-    { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_no_strict_aliasing_ok" >&5
-$as_echo "$ac_cv_no_strict_aliasing_ok" >&6; }
-    if test $ac_cv_no_strict_aliasing_ok = yes
+    { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_no_strict_aliasing" >&5
+$as_echo "$ac_cv_no_strict_aliasing" >&6; }
+    if test $ac_cv_no_strict_aliasing = yes
     then
       BASECFLAGS="$BASECFLAGS -fno-strict-aliasing"
     fi

diff --git a/configure.in b/configure.in
index f499fccf8833df003ad9f396891ed30f338dc9bb..0520ea4444fabc263d0e79382ba6bc87bb5fad6e 100644 (file)
--- a/configure.in
+++ b/configure.in
@@ -878,19 +878,37 @@ 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="$CC -fno-strict-aliasing"
-     AC_TRY_RUN([int main() { return 0; }],
-     ac_cv_no_strict_aliasing_ok=yes,
-     ac_cv_no_strict_aliasing_ok=no,
-     ac_cv_no_strict_aliasing_ok=no)
+     save_CFLAGS="$CFLAGS"
+     AC_CACHE_VAL(ac_cv_no_strict_aliasing,
+       AC_COMPILE_IFELSE(
+         [
+          AC_LANG_PROGRAM([[]], [[]])
+        ],[
+          CC="$ac_save_cc -fstrict-aliasing"
+           CFLAGS="$CFLAGS -Werror -Wstrict-aliasing"
+           AC_COMPILE_IFELSE(
+            [
+              AC_LANG_PROGRAM([[void f(int **x) {}]],
+                [[double *x; f((int **) &x);]])
+            ],[
+              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