[cfe-dev] Ho to instruct the optimizer to not throw out certain constructs

[Compiler Dragon via cfe-dev]

Yep that's what I mean. As I said this is a very simplistic example. Reverse or bypass certain code areas is just one topic. The other equally imported is fault-tolerance.
The open question is how to define a set of objects and tell the optimizer
to leave them untouched.


> mats petersson <mats at planetcatfish.com> hat am 23. November 2016 um 12:15 geschrieben:
> 
>     Did you mean *res0 = 1, *res1 = 1?
> 
>     And you do realize that for anything that is worth reverse engineering, this will just add a few minutes to someones time when trying to reverse engineer/bypass your code?
> 
>     Obviously, if the compiler decides to inline `foo`, and then you haven't got a use for `*res0` and `*res1`, it will still just optimise it away.
> 
>     --
>     Mats
> 
>     On 23 November 2016 at 10:44, Compiler Dragon via cfe-dev <cfe-dev at lists.llvm.org mailto:cfe-dev at lists.llvm.org > wrote:
> 
>         > > Hi Mats,
> > 
> >         thanks for your reply. Basically I want to implement some defensive actions against unauthorized code changes. The artifical introduced variables are used in a specifc manner and should be left untouched
> >         by the optimizer.
> > 
> >         One very basic and simplistic example:
> > 
> >         Original code:
> > 
> >         res = foo(a);
> >         ---
> >         int foo(int a) {
> >           int b;
> >           ...
> >           b = a + 1;
> >           ...
> >           return(b);
> >         }
> > 
> >         Transformed code:
> > 
> >         foo(a0, a1, &res0, &res1);
> >         ...
> >         void foo(int a0, int a1, int *res0, int *res1) {
> >           int a0, a1;
> >           ...
> >           b0 = a0 + 1;
> >           b1 = a1 + 1;
> >           if ( b0 != b1 )
> >              throw_error();
> >           ...
> >           *b0 = 1;
> >           *b1 = 1;
> >           return;
> > 
> >         Thanks
> > 
> >         Marcel
> > 
> >         > If they are not used, why do you want them to remain? `volatile` can be
> >         > used to ensure that the compiler doesn't optimise something away, or
> >         > "explicit use in a way that is never true - but compiler doesn't know it is
> >         > so" (e.g. `if (sin(x) > 1.0) { do stuff that actually never happens }` ) -
> >         > calling `sin` is probably not the best choice, but there may be other ways
> >         > to achieve something similar that is less intrusive but still "unknown to
> >         > the compiler".
> >         >
> >         >
> >         > Can you give an example piece of code?
> >         >
> >         > --
> >         > Mats
> >         >
> >         > On 22 November 2016 at 10:37, Compiler Dragon via cfe-dev <
> >         > cfe-dev at lists.llvm.org mailto:cfe-dev at lists.llvm.org > wrote:
> >         >
> >         > > Hi *,
> >         > >
> >         > > I am currently writing a clang plugin, which add to specific functions for
> >         > > e.g. additional arguments.
> >         > >
> >         > > How can I instruct the optimizer not to throw out these arguments, because
> >         > > they are not used in the function body?
> >         > >
> >         > > With (optnone) we can instruct the optimizer to not optimize the whole
> >         > > function. But this ist not what I want. The function body should be
> >         > > optimized like before.
> >         > >
> >         > > Any idea or hints where to look?
> >         > >
> >         > > Thanks
> >         > >
> >         > > Marcel
> >         _______________________________________________
> >         cfe-dev mailing list
> >         cfe-dev at lists.llvm.org mailto:cfe-dev at lists.llvm.org
> >         http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> > 
> >     > 
> 
 
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