Re:Emacs internals: Deconstructing Lisp_Object in C (Part 2)
by birdgoose on Sunday March 08, 2026 @01:03PM
(#47298902)
The sibling comment explains why we prefer to use the lower bits as a tag (these are guaranteed to be zero if the value is a pointer on a 64-bit system).
Another reason why we wouldn’t want to use the top bit is that, as the parent comment suggested, the tagged pointer representation of a fixnum integer isn’t a pointer at all but is instead twice the number it represents. Generally speaking, we represent integers in twos-complement representation which uses that top bit to determine if the value is positive or negative.
Another reason why we wouldn’t want to use the top bit is that, as the parent comment suggested, the tagged pointer representation of a fixnum integer isn’t a pointer at all but is instead twice the number it represents. Generally speaking, we represent integers in twos-complement representation which uses that top bit to determine if the value is positive or negative.