Adding extra fields to float_block and cons_block?

[Po Lu <luangruo@yahoo.com>]

I tried adding an extra array of bits_words to cons_block and
float_block, in alloc.c, defined as follows:

  bits_word gc_pushed_to_stack_ps[1 + FLOAT_BLOCK_SIZE / BITS_PER_BITS_WORD];

But now Emacs crashes with:

  alloc.c:1252: Emacs fatal error: assertion failed: nbytes <= BLOCK_BYTES
  Fatal error 6: Aborted

I can't seem to figure out the hairy mess surrounding that code, and I'd
appreciate it if someone could explain the proper procedure for adding
extra fields to those two structs.  Thanks.