On Freitag, 20. Juli 2018 14:19:12 CEST Umesh Kalappa wrote:
> Hi All ,
>
> We are looking at the C sample i.e
>
> extern int i,j;
>
> int test()
> {
> while(1)
> { i++;
> j=20;
> }
> return 0;
> }
>
> command used :(gcc 8.1.0)
> gcc -S test.c -O2
>
> the generated asm for x86
>
> .L2:
> jmp .L2
>
> we understand that,the infinite loop is not deterministic ,compiler
> is free to treat as that as UB and do aggressive optimization ,but we
> need keep the side effects like j=20 untouched by optimization .
>
> Please note that using the volatile qualifier for i and j or empty
> asm("") in the while loop,will stop the optimizer ,but we don't want
> do that.
>
But you need to do that! If you want changes to a variable to be observable in
another thread, you need to use either volatile, atomic, or some kind of
memory barrier implicit or explicit. This is the same if the loop wasn't
infinite, the compiler would keep the value in register during the loop and
only write it to memory on exiting the test() function.
'Allan
