L6 due April 16.
 *40 points for performance
 *20 points for correctness
 *5  points for style

Get the correctness points this week.
 *Get a feel for how hard the lab is.
 *You'll probably need the time (starting a couple days before is BAD)

How to get the correctness points:
 We provide mm-helper.c which contains the code from the book.
  -malloc works
  -free works (with coalescing)
  -heapchecking doesn't work
  -realloc doesn't work

 Implement a dumb version of realloc
  -malloc new block, memcpy, free old block, return new block

 Implement heap checking
  -have to add a request id field to each allocated block (tricky)
   -hint: need padding to maintain 8 byte alignment of user pointer
  -in the book's code bp always the same as the user pointer

       ---------------------------------
       |size+a|   payload..     |footer|
       ---------------------------------
              ^
              |
              bp

  -the 4 bytes immediately before bp contain size of payload
  -3 lsb of size unused (because of alignment)
    -first bit indicates of the block is alloced or not

  -Need to change block layout to look like this:
       ----------------------------------------
       |  id  |size+a|   payload..     |footer|
       ----------------------------------------
                     ^
                     |
                     bp

    -This changes how the implicit list has to be traversed
     -but size is at same place relative to bp

  OR

       ----------------------------------------
       |size+a|  id  |   payload..     |footer|
       ----------------------------------------
                     ^
                     |
                     bp

    -All accesses to what was size now access id
     -but can be clever and make size 4 bytes larger
     -could even make bp point to id..
      -most code would just work

 -Once malloc, free, and realloc work with the id field, write heapcheck
  -iterate over the whole heap and print out allocated blocks
   -need to read the id field..

That's it for correctness.

Hints:
 -Remember that pointer arithematic behaves differently depending
  on type of pointer
 -Consider using structs/unions to eliminate some messy pointer code
 -Get things working with the short trace file first:
   ./mdriver -f short1-bal.rep