Andy Wingo <[EMAIL PROTECTED]> writes: > Hi, > > On Mon, 2006-05-08 at 00:29 +0300, Marius Vollmer wrote: > > 137c137 > > < #define CELL_P(x) (SCM_ITAG3 (x) == scm_tc3_cons) > > --- > > > #define CELL_P(x) ((SCM_UNPACK(x) & (sizeof(scm_t_cell)-1)) == > > > scm_tc3_cons) > > I don't really understand how the region can give you unaligned > pointers, but I do confirm that this patch allows me to build guile.
Excellent! Here is what is going on: the CELL_P predicate is used during the conservative scanning of the GC to decide whether a random word can possibly be a non-immediate SCM value. Non-immediate values are the ones that point into the heap. The type tag for such a non-immediate value is "lower three bits zero". On 32-bit architectures, a cell is 8 bytes, which means that a non-immediate value is always aligned to a cell. On 64-bit machines, a cell is 16 bytes, and that means that a word with "lower three bits zero" can still be invalid because it points into the middle of a cell. (We have similar check already for double-cells, which are 16 bytes on 32-bit machines.) -- GPG: D5D4E405 - 2F9B BCCC 8527 692A 04E3 331E FAF8 226A D5D4 E405 _______________________________________________ Guile-devel mailing list Guile-devel@gnu.org http://lists.gnu.org/mailman/listinfo/guile-devel
