MIPS Assembly Language Programming

50 CHAPTER 3. ADVANCED MIPS TUTORIAL
lw $ra, 28($sp) # restore Return Address.
lw $fp, 24($sp) # restore Frame Pointer.
addu $sp, $sp, 32 # restore Stack Pointer.
jr $ra # return.
Let’s time this and see how it compares:
% echo 20 | /bin/time spim -file fib-o.asm
SPIM Version 5.4 of Jan. 17, 1994
Copyright 1990-1994 by James R. Larus (larus@cs.wisc.edu).
All Rights Reserved.
See the file README a full copyright notice.
Loaded: /home/usr6/cs51/de51/SPIM/lib/trap.handler
10946
3.1 real 2.8 user 0.2 sys
This is clearly much faster. In fact, it’s nearly twice as fast as the original
fib-s.asm. This makes sense, since we have eliminated building and destroying
about half of the stack frames, and a large percentage of the fib function does nothing
but set up and dismantle the stack frame.
Note that the reason that optimizing the base case of the recursion helps so much
with this algorithm is because it occurs about half of the time– but this is not characteristic
of all recursive algorithms. For example, in a recursive algorithm to compute
the factorial of n, the recursive case will occur about n − 1 times, while the base case
will only occur once. Therefore, it makes more sense to optimize the recursive case
in that situation.
There’s still more that can be done, however; see exercise 3.3.3 to pursue this
farther. A complete listing of a program that uses this implementation of the fib
function can be found in section 5.8 (on page 84).
3.2 Structures and sbrk: the treesort Program
Included in section 5.9 of this document is the source code for a SPIM program that
reads a list of numbers from the user and prints out the list in ascending order. The
input is read one number per line, using the read_int syscall, until the user types in
the sentinel value. The sentinel value is currently 0, but can be changed in the code
to any 32-bit integer.
The treesort algorithm should be familiar to anyone who has used ordered binary
trees. The general algorithm is shown in 3.1.