O2 Agressive Optimisation by GCC

[Richard Biener]

On July 20, 2018 7:59:10 PM GMT+02:00, Martin Sebor <msebor@gmail.com> wrote:
>On 07/20/2018 06:19 AM, 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.
>>
>> Anyone from the community ,please share their insights why above
>> transformation is right ?
>
>The loop isn't necessarily undefined (and compilers don't look
>for undefined behavior as opportunities to optimize code), but

The variable i overflows.

>because it doesn't terminate it's not possible for a conforming
>C program to detect the side-effects in its body.  The only way
>to detect it is to examine the object code as you did.

I'm not sure we perform this kind of dead code elimination but yes, we could. Make i unsigned and check whether that changes behavior. 

>Compilers are allowed (and expected) to transform source code
>into efficient object code as long as the transformations don't
>change the observable effects of the program.  That's just what
>happens in this case.
>
>Martin