Confused by the interaction of the stack and set_local

[pierrec]

I am wondering why the following does not work (invalid index in set_local):

.code;
push 0;
set_local 0;

push_0;
exit;

but this does:

.code;
push 0;
push 0;
set_local 0;

push_0;
exit;

[pierrec]

And this comes from trying to make the READ_INT function in the fibonacci example working with the stack only without using locals, but failing miserably:

#
# Recursive computation of Fibonacci numbers
#

.data;
PROMPT_STR: .stringz "Input an integer:\n";

# Code section
.code;

push PROMPT_STR;
syscall print_str;

# Read input number
call READ_INT, 0;

# Call FIB with 1 argument
call FIB, 1;

# Print the result
syscall print_i64;
syscall print_endl;

push 0;
exit;

#
# u64 fib(u64 n)
#
FIB:
get_arg 0;
push 2;
lt_i64;
jz _if_false_0;
get_arg 0;
ret;
_if_false_0:
get_arg 0;
push 1;
sub_u64;
call FIB, 1;
get_arg 0;
push 2;
sub_u64;
call FIB, 1;
add_u64;
ret;

#
# Read a positive integer from stdlin
#
READ_INT:

push 0; # Current integer value

LOOP:
    # Read one character
    syscall getchar;

    # If < 0 done
    dup;
    push 48;
    lt_i64;
    jnz DONE;

    # If > 9 done
    dup;
    push 57;
    gt_i64;
    jnz DONE;

    # Convert to integer digit
    push 48;
    sub_u64;

    # int_val * 10;
    #COMMENTED get_local 0;
    swap;
    push 10;
    mul_u64;

    # int_val + 10;
    add_u64;
    #COMMENTED set_local 0;

    jmp LOOP;

DONE:

#COMMENTED get_local 0;
# shouldnt the stack have the result by now?
ret;

[maximecb]

Hello @pierrec

I tried your example above and was also surprised for a moment that it does not work 😆

The reason is that set_local will pop the value you just pushed, and then you end up with stack size 0.

push 0; # Stack size is 1
set_local 0; # Pops the value 0, stack size is then zero, error!

push_0;
exit;

To make it work, you would need to allocate another stack slot:

push 0; # Stack size is 1
push 77; # Stack size is now 2
set_local 0; # Pops the value 77

# After this point, you're left with 77 on the stack.

get_local 0; # This would read stack slot 0
exit; # Exits with value 77

[pierrec]

Oh so a stack of size zero is not allowed?

[maximecb]

There's nothing wrong with stack size 0, but set_local is trying to mutate the stack slot at index 0, and so the stack size has to be at least 1 for that.

[pierrec]

Ah I finally get it: set_local actually saves on the stack... I couldnt see that after reading https://github.com/maximecb/uvm/blob/main/doc/design.md, but I must have missed it.
Thanks!

[maximecb]

No problem. Let me know if there is anything else you need help with :)