/* consolidate backward */
if (!prev_inuse(p)) {
prevsize = prev_size (p);
size += prevsize;
p = chunk_at_offset(p, -((long) prevsize));
if (__glibc_unlikely (chunksize(p) != prevsize))
malloc_printerr ("corrupted size vs. prev_size while consolidating");
unlink_chunk (av, p);
}
 
if (nextchunk != av->top) {
/* get and clear inuse bit */
nextinuse = inuse_bit_at_offset(nextchunk, nextsize);
 
/* consolidate forward */
if (!nextinuse) {
unlink_chunk (av, nextchunk);
size += nextsize;
} else
clear_inuse_bit_at_offset(nextchunk, 0);

前向合并，合并物理相邻高地址空闲 chunk（除了 top chunk）。