O2 Agressive Optimisation by GCC

[Umesh Kalappa]

Hi Richard,

making i unsigned still  the  optimization is effective ,no luck.
and yes test() is the threaded  routine and since i and j are global
,we need the side effects take place like assignment etc ,that are
observed by other threads .

By making volatile or thread safe or atomic operations ,the
optimization inhibited ,but still we  didn't  get  why its valid
optimization for UB and tried with -fno-strict-overflow too ,no luck
here .

Jakub and anyone can we inhibit these kind optimizations,that consider
the UB and optimize .

Thank you
~Umesh

On Fri, Jul 20, 2018 at 11:47 PM, Richard Biener
<richard.guenther@gmail.com> wrote:
> 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
>