Use 0 for Qnil
Fixes Bug#15880. If USE_LSB_TAG, arrange for the representation of Qnil to be zero so that NILP (x) is equivalent to testing whether x is 0 at the machine level. The overall effects of this and the previous patch shrink the size of the text segment by 2.3% and speeds up compilation of all the .elc files by about 0.5% on my platform, which is Fedora 20 x86-64. * lib-src/make-docfile.c (compare_globals): * src/lisp.h (lisp_h_XPNTR, lisp_h_XSYMBOL, lisp_h_XUNTAG) (make_lisp_symbol) [USE_LSB_TAG]: Symbols now tag the difference from lispsym, not the pointer. (lisp_h_XUNTAGBASE, TAG_SYMPTR): New macros. (Lisp_Int0, Lisp_Int1, Lisp_Symbol, Lisp_Misc, Lisp_String, Lisp_Cons): Renumber so that Lisp_Symbol is 0, so that Qnil is zero. (XSYMBOL): New forward decl. (XUNTAGBASE): New function. (XUNTAG): Use it.
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