replace "and" with shift and rotate registers with the result and input staying the same

Question

.text       
.globl __start 

__start:            
li $t1,9        
li $t2,7        
        
addi $t1,$t1,3

addi $t2,$t2,3

li $t3,0xFFFFFFFF    
srl $t3,$t3,$t1    
sll $t3,$t3,$t1    
sll $t3,$t3,$t2   
srl $t3,$t3,$t2    
not $t3,$t3
    
li $s1,0x12345678   # input data in $s1 register

and $s2,$s1,$t3     # output data in $s2 register

li $v0,10      
syscall         # exit

Answer 1

It looks like that sequence ending with not $t3,$t3 creates an AND mask with 1 bits at the edges but zeros in the middle. So you could do the same thing of knocking out the middle bits by rotating then to one end, shifting them out, and then rotating back.

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But MIPS doesn't have hardware rotate instructions. ror and rol are only pseudo-instructions that shift twice and OR, so this would not be more efficient.

Creating and using an AND mask probably is the best way to zero a range of bits between two inputs. But you can probably construct the mask more efficiently. Like ~((1<<high) - (1<<low)) or something like that, which takes one li reg,1, two sllv, one subu, and one not. There might be an off-by-one in that expression; I didn't check since this isn't what you were really asking about.

Or of course if the bit positions are known constants, you should just do the math at assemble time and run one li with the mask. (1 or 2 hardware instructions, or even just an andi if the resulting mask has set bits only in the low 16.) A good assembler will let you write expressions involving constants and evaluate it, like li $t1, ~((1<<9) - (1<<7)). But MARS for example doesn't let you do that. You'd have to hard-code the 0xfffffe7f