The right operand is negative, but I don't think it should be
the right operand has all kinds of odd values
You are always shifting 32 bits right, which I think in practice will be identity on x86.
(That's why Futhark says "undefined behaviour", because it would be expensive to make x86 do something else.)
and got confused
I am still confused as to why it fails now I've used this code for a very long time now
The only used of
rotluse constants below 32 though...
-- xoshiro128**
type PRNG = {state: (u32,u32,u32,u32)}
let rotl (x:u32) (k:u32) = x << k | x >> 32-k
let next (g:PRNG) : (u32, PRNG) =
let (a,b,c,d) = g.state
let res = (rotl (b * 5) 7) * 9
let t = b << 9
let c = c ^ a
let d = d ^ b
let b = b ^ c
let a = a ^ d
let c = c ^ t
let d = rotl d 11
in (res, {state = (a,b,c,d)})
Uh, wait, no.
I wonder if this is some other undefined behaviour thing in C that the code generator doesn't guard properly against.
Sometimes C compilers are like evil genies that try to interpret your wishes in the most destructive way.
while (loop_while_150581) {
int32_t defunc_0_f_res_150583 = add32(1, x_150582);
int32_t x_150584 = lshr32(defunc_2_reduce_res_150569,
defunc_0_f_res_150583);
bool defunc_0_p_res_150585 = x_150584 == 0;
bool loop_cond_150586 = !defunc_0_p_res_150585;
bool loop_while_tmp_162843 = loop_cond_150586;
int32_t x_tmp_162844 = defunc_0_f_res_150583;
loop_while_150581 = loop_while_tmp_162843;
x_150582 = x_tmp_162844;
}
starting at 0
This loop will never terminate if its most significant bit is set.
At that point you will be saying
x>>32, which is undefined behaviour (and just returns x on x86).
So while I am pretty sure this is an instance of undefined behaviour, you should consider using this function to replace the loop anyway: https://futhark-lang.org/docs/prelude/doc/prelude/math.html#1217
Now I wonder which piece of code this is part of, but th I can look for myself
Thanks
That explains why it's a new problem. I used a radix sort for some data generation
assuming you think of undefined behavoiur that is
and now it works
adding a warning in the interpreter would indeed be nice though
Just like division by zero.