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List:       gcc
Subject:    Re: Importance of transformations that turn data dependencies into control dependencies?
From:       Michael Matz <matz () suse ! de>
Date:       2016-03-01 16:55:07
Message-ID: alpine.LSU.2.20.1603011748130.20277 () wotan ! suse ! de
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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)


Ciao,
Michael.
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