Temporary fix for bug 4254 - a _lot_ of strict aliasing warnings

Ozkan Sezer

A horde of strict aliasing violation warnings are emitted from joystick
layer, and also from a few other places. This happens with gcc-4.4.7 on
Linux CentOS 6.10.  Some other sysjoystick would possibly have the same
warnings.

Attached my full log here. Example entry:
src/joystick/SDL_joystick.c: In function 'SDL_GetJoystickGUIDInfo':
src/joystick/SDL_joystick.c:1094: warning: dereferencing pointer '({anonymous})' does break strict-aliasing rules

CMakeLists.txt

@@ -418,6 +418,11 @@ if(USE_GCC OR USE_CLANG)
     endif()
   endif()
 
+  check_c_compiler_flag(-fno-strict-aliasing HAVE_GCC_NO_STRICT_ALIASING)
+  if(HAVE_GCC_NO_STRICT_ALIASING)
+    list(APPEND EXTRA_CFLAGS "-fno-strict-aliasing")
+  endif()
+
   check_c_compiler_flag(-Wdeclaration-after-statement HAVE_GCC_WDECLARATION_AFTER_STATEMENT)
   if(HAVE_GCC_WDECLARATION_AFTER_STATEMENT)
     check_c_compiler_flag(-Werror=declaration-after-statement HAVE_GCC_WERROR_DECLARATION_AFTER_STATEMENT)

configure

@@ -19396,6 +19396,43 @@ $as_echo "$have_gcc_fvisibility" >&6; }
     fi
 }
 
+CheckNoStrictAliasing()
+{
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking for GCC -fno-strict-aliasing option" >&5
+$as_echo_n "checking for GCC -fno-strict-aliasing option... " >&6; }
+    have_gcc_no_strict_aliasing=no
+
+    save_CFLAGS="$CFLAGS"
+    CFLAGS="$save_CFLAGS -fno-strict-aliasing"
+    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+    int x = 0;
+
+int
+main ()
+{
+
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+
+    have_gcc_no_strict_aliasing=yes
+
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+    { $as_echo "$as_me:${as_lineno-$LINENO}: result: $have_gcc_no_strict_aliasing" >&5
+$as_echo "$have_gcc_no_strict_aliasing" >&6; }
+    CFLAGS="$save_CFLAGS"
+
+    if test x$have_gcc_no_strict_aliasing = xyes; then
+        EXTRA_CFLAGS="$EXTRA_CFLAGS -fno-strict-aliasing"
+    fi
+}
+
 CheckStackBoundary()
 {
     { $as_echo "$as_me:${as_lineno-$LINENO}: checking for GCC -mpreferred-stack-boundary option" >&5
@@ -24334,6 +24371,7 @@ fi
 
 
 CheckWarnAll
+CheckNoStrictAliasing
 
 CheckEventSignals
 
@@ -24402,8 +24440,8 @@ case "$host" in
         CheckOpenGLESX11
         CheckVulkan
         CheckWayland
-        CheckLibUDev
         CheckInputEvents
+        CheckLibUDev
         CheckDBus
         CheckIME
         CheckIBus

configure.ac

@@ -1329,6 +1329,29 @@ CheckVisibilityHidden()
     fi
 }
 
+dnl See if GCC's -fno-strict-aliasingis supported.
+dnl  Reference: https://bugzilla.libsdl.org/show_bug.cgi?id=4254
+CheckNoStrictAliasing()
+{
+    AC_MSG_CHECKING(for GCC -fno-strict-aliasing option)
+    have_gcc_no_strict_aliasing=no
+
+    save_CFLAGS="$CFLAGS"
+    CFLAGS="$save_CFLAGS -fno-strict-aliasing"
+    AC_TRY_COMPILE([
+    int x = 0;
+    ],[
+    ],[
+    have_gcc_no_strict_aliasing=yes
+    ])
+    AC_MSG_RESULT($have_gcc_no_strict_aliasing)
+    CFLAGS="$save_CFLAGS"
+
+    if test x$have_gcc_no_strict_aliasing = xyes; then
+        EXTRA_CFLAGS="$EXTRA_CFLAGS -fno-strict-aliasing"
+    fi
+}
+
 dnl See if GCC's -mpreferred-stack-boundary is supported.
 dnl  Reference: http://bugzilla.libsdl.org/show_bug.cgi?id=1296
 CheckStackBoundary()
@@ -3263,6 +3286,7 @@ AS_HELP_STRING([--enable-foregrounding-signal], [number to use for magic foregro
 
 dnl Do this on all platforms, before everything else (other things might want to override it).
 CheckWarnAll
+CheckNoStrictAliasing
 
 dnl Do this for every platform, but for some it doesn't mean anything, but better to catch it here anyhow.
 CheckEventSignals