Modular•11mo ago

SIMD produces weird results without print statement at the end

The code below produces weird results:

from algorithm import vectorize
from tensor import Tensor

alias type = DType.int64
alias nelts = simdwidthof[type]()

def main():
    let size = 10
    var a = Tensor[type](size)
    var b = Tensor[type](size)
    var c = Tensor[type](size)

    for i in range(size):
        a[i] = i + 1
        b[i] = i
    
    @parameter
    fn diff[nelts : Int](x : Int):
        #print(a.simd_load[nelts](x))
        c.simd_store[nelts](x, a.simd_load[nelts](x) - b.simd_load[nelts](x))
        #print(nelts, x)
    vectorize[nelts, diff](size)

    for i in range(size):
        print(c[i])

    #print(a.simd_load[nelts](0))

from algorithm import vectorize
from tensor import Tensor

alias type = DType.int64
alias nelts = simdwidthof[type]()

def main():
    let size = 10
    var a = Tensor[type](size)
    var b = Tensor[type](size)
    var c = Tensor[type](size)

    for i in range(size):
        a[i] = i + 1
        b[i] = i
    
    @parameter
    fn diff[nelts : Int](x : Int):
        #print(a.simd_load[nelts](x))
        c.simd_store[nelts](x, a.simd_load[nelts](x) - b.simd_load[nelts](x))
        #print(nelts, x)
    vectorize[nelts, diff](size)

    for i in range(size):
        print(c[i])

    #print(a.simd_load[nelts](0))

For example:

2305843009213693952
4611686018780248321
3
1
1
1
1
1
1
1407374883553281

2305843009213693952
4611686018780248321
3
1
1
1
1
1
1
1407374883553281

If I uncomment last print statement, suddenly results are correct:

Can someone please explain this behaviour? Am I doing something wrong with SIMD?

12 Replies

vmois•11mo ago

I experience the same behaviour on both arm64 and x86

Helehex•11mo ago

I get similar results i think uncommenting #print(nelts, x) should give something like 4,4,2

vmois•11mo ago

To make it work, I uncommeted last print print(a.simd_load[nelts](0)) I have M1, so it gives nelts = 2. Indexes provided by vectorize look fine. 0 2 4 6 8

Helehex•11mo ago

ah yeah i get 4, 4, 4, 4 for the one i mentioned and uncommenting the last line doesnt work for me

vmois•11mo ago

Hmm. Interesting. Try to change size to multiple of 4, like 16?

Helehex•11mo ago

using 16: uncommenting the last print doesnt change it for me, other than the 2 undefined elements at the beginning which are still there

vmois•11mo ago

I see. Surprisingly, my other SIMD implementation for euclidian distance (from Modular blog) is working fine. Maybe I am missing something. When I will have time, I will double check code and maybe look at assembly Thanks for looking

Helehex•11mo ago

the inner nelts looks correct for me actually- 4,4,1,1, and uncommenting does solve the issue in a certain sense my bad but still getting the 2 randos at the beginning: happens with simdload, inside of a function decorated with @parameter, using the tensor after the load solves the issue, so it may have to do with the lifetime to solve, you can do: ` = a` for each tensor (at the end of main in this case)

vmois•11mo ago

What do you mean by simd_load inside the function? Should it still display a correct results after vectorize? But I see where you are pointing. Cause in all examples provided by Modular they returned from function at this point.

ModularBot•11mo ago

Congrats @vmois, you just advanced to level 3!

vmois•11mo ago

_ = b also works. _ = c doesn't 🙂 Oh wait. Just saw your edir So by using _ = a I force compiler to provide a visibility for my tensor after vectorize? Anyway. That is quite interesting. Thank you for the help.

Helehex•11mo ago

no problem

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SIMD produces weird results without print statement at the end