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Re: Importance of transformations that turn data dependencies into control dependencies?


On Tue, Mar 01, 2016 at 05:55:07PM +0100, Michael Matz wrote:
> Hi,
> 
> On Tue, 1 Mar 2016, Richard Biener wrote:
> 
> > > What about the example I gave above?  Is it unrealistic for compilers 
> > > do ever do something like this, or is it just unlikely to gain much 
> > > performance, or is it just that GCC does not do this today?
> > 
> > GCC does not do this today with the exception of value-profiling.  GCC 
> > in other cases does not establish equivalences but only relations (a < 
> > b, etc.) that are not a problem as far as I can see because those do not 
> > allow to change expressions using a to use b.
> 
> Made up example using relations:
> 
> int32 a, b;
> a = (b >> 31) & 1;
> 
> -->
> 
> if (b < 0)
>   a = 1;
> else
>   a = 0;
> 
> data-dep to control-dep and only relations :)  (I think this is taken care 
> of by Pauls wording, ignoring the fact that these aren't pointers anyway 
> and hence don't carry a dependency through them, only onto them at max)

Agreed, these are not pointers, and therefore do not carry dependencies.

							Thanx, Paul


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