MIPS Assembly Language Programming

48 CHAPTER 3. ADVANCED MIPS TUTORIAL
sw $t1, 16($sp) # save $t1.
sub $a0, $t0, 2 # compute fib (n - 2)
jal fib
move $t2, $v0 # $t2 = fib (n - 2)
lw $t0, 20($sp) # restore n.
lw $t1, 16($sp) # restore $t1.
add $v0, $t1, $t2 # $v0 = fib (n - 1) + fib (n - 2).
b fib_return
fib_base_case: # in the base case, return 1.
li $v0, 1
fib_return:
lw $ra, 28($sp) # Restore the Return Address.
lw $fp, 24($sp) # restore the Frame Pointer.
addu $sp, $sp, 32 # restore the Stack Pointer.
jr $ra # return.
Once again, we can time the execution of this program in order to see if this
change has made any improvement:
% echo 20 | /bin/time spim -file fib-t.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
4.5 real 4.1 user 0.1 sys
In these tests, the user time is what we want to measure, and as we can see,
fib-s.asm is approximately 17% slower than fib-t.asm.
3.1.1.3 Optimization: fib-o.asm
Warning! Hacks ahead! There are still more tricks we can try in order to increase the
performance of this program. Of course, the best way to increase the performance of
this program would be to use a better algorithm, but for now we’ll concentrate on
optimizing our assembly implementation of the algorithm we’ve been using.
Starting with the observation that about half the calls to fib have an argument n
of 1 or 0, and therefore do not need to do anything except return a 1, we can simplify
the program considerably: this base case doesn’t require building a stack frame, or