$4\mathrm{.}8.$ But some existing optimizing compilers $($including gcc$,$ which tends to be relatively conservative$)$ will $$optimize$$ count$\_$positives to something similar to

The code reads as follows:

Line $1$: void count underscore positives open parenthesis list $1$ close parenthesis at indentation level $0.$

Line $2$: open curly bracket at indentation level $0.$

Line $3$: list p semi colon at indentation level $1.$

Line $4$: register int r semi colon at indentation level $1.$

Line $5$: r equals global underscore positives semi colon at indentation level $1.$

Line $6$: for open parenthesis p equals $1$ semi colon p semi colon p equals p arrow operator next close parenthesis at indentation level $1.$

Line $7$: if open parenthesis p arrow operator val greaterthan $0\mathrm{.}0$ close parenthesis pre increment r semi colon at indentation level $2.$

Line $8$: global underscore positives equals r semi colon at indentation level $1.$

Line $9$: close curly bracket at indentation level $0.$

What problem or potential problem occurs with this compiled version of the program if threads A and B are executed concurrently?