Use native alignment to access Lisp object data
Instead of using __builtin_assume_aligned (P, GCALIGNMENT) to tell GCC that P has alignment 8, use (T *) P where T is the type of the pointed-to object, to tell GCC that P has native alignment. This is simpler, matches the intent better, and should help simplify future improvements. Some of these changes are to pacify gcc -Wnull-dereference, since GCC is smarter about pointers now that Emacs no longer uses __builtin_assume_aligned; these minor changes should improve code efficiency slightly. On Fedora 28 x86-64 with default optimization this patch shrinks the size of the Emacs text segment by 0.36%. * src/conf_post.h (__has_builtin, __builtin_assume_aligned): Remove; no longer used. * src/dbusbind.c (XD_OBJECT_TO_DBUS_TYPE): Pacify -Wnull-dereference by using XCAR instead of CAR_SAFE and XCDR instead of CDR_SAFE when this is safe. * src/fileio.c (Fexpand_file_name): * src/font.c (clear_font_cache): Pacify -Wnull-dereference by removing unnecessary NILP test. * src/keyboard.c (xevent_start): New function. (read_char, read_key_sequence): Pacify -Wnull-dereference by using xevent_start instead of EVENT_START. * src/lisp.h (lisp_h_XUNTAG): Remove; XUNTAG is always a macro now, since it can no longer be implemented as a function. (XUNTAG): New third argument CTYPE. All uses changed. Cast result to CTYPE * instead of using __builtin_assume_aligned. Simplify by using LISP_WORD_TAG. (LISP_WORD_TAG): New macro. (TAG_PTR): Use it. * src/menu.c (x_popup_menu_1): Pacify -Wnull-dereference by using XCAR instead of Fcar and XCDR instead of Fcdr where this is safe.
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