Kenichi Okuyama <okuyama(a)trl.ibm.co.jp&gt; writes in xemacs-beta(a)xemacs.org: There's more to it than that. It also moves the tagbit from the topmost bit to the lower most bit. This will work fine so long as malloc() returns memory that is aligned to at least 32 bit boundaries. SJ> We're getting somewhere. :-) We do this already. With the old GNU malloc in XEmacs, the mmapping version of the relocating allocator can be used. The new GNU malloc (Doug Lea malloc) in Linux libc5 and GNU libc uses mmapping for allocation under certain conditions. Note that FSF Emacs 20.2 can use the Doug Lea malloc, but it unconditionally turns off usage of mmap. I haven't checked 20.3 to see if they fixed that. What is PHKmalloc? If it's portable and faster, I have no problem replacing the old GNU malloc with it, or adding it as an additional option. Yup. DL malloc does that too. Fortunately Kyle has already fixed the XEmacs internals so that the tagbits are on the low side so all of the high bits are usable in pointer addresses. ... We have the entire addressing space with 32 bit alignment. We don't have more than 31 bits available for integers.