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

Patch from Daniel Stutzbach

1 files changed, 17 insertions, 10 deletions

diff --git a/configure.in b/configure.in
index 0109367..6a69413 100644
--- a/configure.in
+++ b/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