Re: O2 Agressive Optimisation by GCC

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On Fri, 20 Jul 2018 at 23:06, Allan Sandfeld Jensen wrote:
>
> 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,

No, volatile doesn't work for that.

http://www.isvolatileusefulwiththreads.in/C/

> 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
>
>