Hi Paul,

The usage of "map" in "steps" is invalid because the two statements create a read-after-write dependency on "arg". Specifically, the neighbor() call in the second statement reads the output of the first statement in another map() invocation instance. And this is explicitly disallowed in the API reference manual.

All we need to know about map() is documented here (link). The paragraph that disallows this type of usage is copied below:

The arbb::neighbor() function does not allow reading the outputs of other arbb::map()
invocation instances, because it always returns the element as it was
before the function began executing. This allows such invocations to
execute in parallel, because they remain independent.

Hope this helps.

Zhang

It is curious that amodifed example, swapping the linesbut still violating the reference manual paragraph (above), runs...

void steps(f32 &val, f32 &arg)
{
val = (arg + neighbor(arg, 1))*0.5;
arg = (arg + neighbor(arg, 1))*0.5;
};

...and no error is generated during execution.Howeverthe"val" resultschange if the 2nd statement is commented out, which is unexpected.

Best regards,
- paul

I could not exactly reproduce the problem. When I ran the code I saw the "val" results to be the same, commenting out the 2nd statement or not. Would you please send me or post here your code that produces unexpected results?

Your revised example can be equivalently written as:

void steps(f32& val, f32& arg)
{
f32 temp = (arg + neighbor(arg, 1))*0.5;
val = temp;
arg = temp;
}

This does not violate the restrictions stated in the reference manual. However, the original example (where the two statements are not swapped) cannot be written this way. I guess this makes the difference.

Thanks,
Zhang