Greater than, less than equal, greater than equal in MIPS

17
votes

Given two registers $s0, $s1, how can I convert the following pseudocode into MIPS assembly language using only the slt (set on less than) and beq and bne (branch if equal, branch if not equal) instructions.

   if ($s0 > $s1) { goto label1 }
   if ($s0 >= $s1) { goto label2 }
   if ($s0 <= $s1) { go to label3 }
2
assemblymips

2 Answers

35
votes

I'm assuming that the pseudocode executes sequentially, so an earlier condition being true means you go there and never reach the later if statements. This makes the last branch guaranteed taken if it's reached at all, so it doesn't even need to be conditional. (Also assuming that this is a MIPS without a branch-delay slot.)

slt  $t1,$s1,$s0      # checks if $s0 > $s1
bne  $t1,$zero,label1 # if $s0 >  $s1, goes to label1
beq  $s1,$s2,label2   # if $s0 == $s2, goes to label2 
# beq  $t1,$zero,label3 # if $s0 <  $s1, goes to label3
b    label3            # only possibility left

If that's not the case, you'll want to implement $s0 >= $s1 as
!($s0 < $s1) with slt $t1, $s0, $s1 / beqz $t1, target, for example,
as shown in Ahmed's answer.

3
votes

To implement each compare separately, without optimizing across multiple compares with earlier true conditions making later ones unreachable:

slt $at, $s1, $s0           # $s0 > $s1  as ($s1 < $s0) != 0
bne $at, $zero, label1

slt $t0, $s0, $s1           # $s0 >= $s1 as (s0<s1) == 0
beq $t0, $zero, label2

slt $t1, $s1, $s0           # $s0 <= $s1 the same but reversing the inputs
beq $t1, $zero, label3

label1:
label2:
label3:

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