Reputation: 111
I am trying to create a recursive program in RISC-V but I can't get it to get me the right result. It looks like it is calling itself only two times max, but I tried running it on paper and everything seems correct:
addi x31, x0, 4
addi x30, x0, 2
addi x2, x0, 1600 //initialize the stack to 1600, x2= stackpointer
ecall x5, x0, 5 //read the input to x5
jal x1, rec_func
ecall x0, x10, 2 //print the result
beq x0, x0, end
rec_func:
addi x2, x2, -16 //make room in stack
sd x1, 0(x2) //store pointer and result in stack
sd x10, 8(x2)
bge x5, x31, true // if i > 3, then go to true branch
addi x10, x0, 1 // if i <= 3, then return 1
addi x2, x2, 16 // reset stack point
jalr x0, 0(x1)
true:
addi x5, x5, -2 // compute i-2
jal x1, rec_func // call recursive func for i-2
ld x1, 0(x2) // load the return address
ld x10, 8(x2) // load result from last function call
addi x2, x2, 16 // reset stack point
mul x10, x10, x30 // multiply by 2
addi x10, x10, 1 // add 1
jalr x0, 0(x1) // return
end:
This is the original program logic:
if i<= 3 return 1
else return 2 * rec_func(i-2) +1
Upvotes: 2
Views: 12169
Reputation: 111
I got it working now. The changes that I made were following:
The final code looks like this:
addi x31, x0, 4
addi x30, x0, 2
addi x2, x0, 1600 // initialize the stack to 1600, x2= stackpointer
ecall x5, x0, 5 // read the input to x5
jal x1, rec_func
ecall x0, x10, 2 // print the result
beq x0, x0, end
rec_func:
addi x2, x2, -8 // make room in stack
sd x1, 0(x2) // store pointer and result in stack
bge x5, x31, true // if i > 3, then go to true branch
ld x1, 0(x2)
addi x10, x0, 1 // if i <= 3, then return 1
addi x2, x2, 8 // reset stack point
jalr x0, 0(x1)
true:
addi x5, x5, -2 // compute i-2
jal x1, rec_func // call recursive func for i-2
ld x1, 0(x2) // load the return address
addi x2, x2, 8 // reset stack point
mul x10, x10, x30 // multiply by 2
addi x10, x10, 1 // add 1
jalr x0, 0(x1) // return
end:
Upvotes: 0
Reputation: 151
I don't have enough reputation to add a comment but have you tried running this with a debugger (GDB ?) instead of on paper? That should show what's actually in the registers and why it's not branching as you might expect. I'm not familiar enough with these instructions (learning x86 assembly) to figure the source out at the moment.
Upvotes: 1