Question

Consider the C code below which reverses the elements in the array A in place. Both array indexing and pointer versions are given. Assume that A holds doubleword integers and that size is the number of elements in the array. Translate to RISC-V using as few instructions as possible. For your RISC-V code, assume the base address of A is initially in x10 and that the number of elements in the array (i.e., size) is initially in x11.

long long int temp;

for (i = 0; i < size/2; i++) {

temp = A[i];

A[i] = A[size - i - 1];

A[size - i - 1] = temp;

}

Pointer version:

long long int *p = A;

long long int *q = A + size - 1;

while (p < q) {

temp = *p;

*p = *q;

*q = temp;

p++;

q--;

}

Answer 1

Greetings!!

Code:

#DATA SEGMENT

.data

array: .dword 0x1111111111111111,0x2222222222222222,0x3333333333333333,0x4444444444444444,0x5555555555555555,0x6666666666666666,0x7777777777777777,0x8888888888888888

#CODE SEGMENT

.text

main:

la a0,array         #loading address of the array

li a1,7                #load length-1

li t0,8                #word size

li t3,1                #constant

mul t1,a1,t0       #calculating the offset of the last element

add t1,a0,t1       #calculating the address of the last element

srl a1,a1,t3         #(length-1)/2 for number of iterations

addi a1,a1,1       #adjusting the number of iterations

loop:

beq a1,zero,end #whether the iteration count is 0?

lw a3,0(a0)        #else load the word from the dword from the starting

lw t2,0(t1)q       #load the word from the dword from the end of array

sw t2,0(a0)                 #store word from the end as word to start

sw a3,0(t1)                 #store word from the start as word to end

addi a0,a0,4       #increment for start index for next word

addi t1,t1,4        #increment for end index for next word

lw a3,0(a0)        #load the 2^nd word from dword from start

lw t2,0(t1)         # load the 2^nd word from dword from end

sw t2,0(a0)                 #store

sw a3,0(t1)                 #store

addi a1,a1,-1     #reduce loop count

addi a0,a0,4       #increment start index

addi t1,t1,-12     #decrement end index

j loop                 #repeat

end:

li a7,10              #parameter for termination ecal

ecall

Output screenshot:even length array

Before reversing:

After reversing:

Odd length array:

Before:

After:

Hope this helps