Ritual II: Sum an Array
Eight bytes mark each step.
A dwindling count guides the path.
One register remains.
Write an RV64 function named sum. It receives the address of an array of 64-bit integers in a0 and its element count in a1. Return their wrapping sum in a0.
An empty array must return zero. Use only caller-saved registers and do not modify memory.
Reveal solution
.globl sum
sum:
mv t0, a0
mv t1, a1
li a0, 0
.Lsum_loop:
beqz t1, .Lsum_done
ld t2, 0(t0)
add a0, a0, t2
addi t0, t0, 8
addi t1, t1, -1
j .Lsum_loop
.Lsum_done:
rett0 walks through memory while t1 counts down. Initializing a0 before the loop handles an empty array and leaves the result in the return register.