O2 Agressive Optimisation by GCC

[Umesh Kalappa]

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.

Anyone from the community ,please share their insights why above
transformation is right ?

and without using volatile or memory barrier ,how we can stop the
above transformation .


Thank you in advance.
~Umesh