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

[mats petersson via cfe-dev]

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> 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> 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
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20161123/355d8e7f/attachment.html>