[llvm-dev] O2 Aggressive Optimization by Clang

[George Burgess IV via llvm-dev]

+1 for using either volatile or atomics, depending on precisely what your
goal is.

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

This
<https://stackoverflow.com/questions/15718262/what-exactly-is-the-as-if-rule>
looks like it might be helpful to you (there's a C-specific answer at the
bottom).

On Fri, Jul 20, 2018 at 5:45 AM mats petersson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>
> On 20 July 2018 at 13:21, Umesh Kalappa via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Edited the Subject.
>>
>> On Fri, Jul 20, 2018 at 5:50 PM, Umesh Kalappa <umesh.kalappa0 at gmail.com>
>> 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 :(clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)
>> > )
>> > clang -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 .
>>
>
> What are you actually trying to achieve? The sample code does not actually
> DO anything useful, even if it's technically possible to find ways to
> observe the value of `i` and `j` outside of `main`.
> What do you expect the compiler to do with this?
>
> If nothing else is using `i` and `j`, what's the purpose of changing their
> value?
>
> If you ARE using those values elsewhere, the using `volatile` is indeed
> the right thing to do. Or calling some functions inside `main`, that will
> cause the compiler to understand that "this does something more than just
> update some variables".
>
> --
> Mats
>
> >
>> >
>> > Thank you in advance.
>> > ~Umesh
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>
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