Unsynchronized int access from threads

[aliak]

On Thursday, 18 June 2020 at 16:42:15 UTC, H. S. Teoh wrote:
> Suppose I have an int[] which may contain some zeroes, and 2 
> functions, one scans the array for zeroes but does not modify 
> it, and the other modifies array elements but never introduces 
> new zeroes (though it may write a 0 to an existing 0).  Is it 
> thread-safe to run the two functions in parallel without any 
> synchronization between them?
>
> I.e., will the first function always see zeroes where there are 
> zeroes, in spite of the 2nd function writing to the array 
> simultaneously?  Are there any hardware situations where the 
> 1st function may read a non-zero value if the 2nd function is 
> simultaneously overwriting an existing zero with another zero?  
> Or a situation where the 1st function may read a zero if the 
> 2nd function is simultaneously overwriting a non-zero value 
> with another non-zero value?
>
> Or am I playing with fire here?
>
>
> T

If there's no alignment hanky panky (e.g. struct packing), and 
you're on x86-64, you have your guarantee - as reading aligned 
words are atomic. If you want portability then there's no 
guarantee.

Steven's case could certainly happen if a value crosses a word 
boundary on a platform that doesn't guarantee atomic reads/writes.

I was confirming things and found this which was a fun little 
read: 
https://preshing.com/20130618/atomic-vs-non-atomic-operations/