[LLVMdev] question about PRE

Owen Anderson resistor at mac.com
Sun Apr 29 15:51:45 PDT 2007


The old PRE never really worked properly, or at least was never  
really finished.  We're planning to implement GVN-PRE (ftp:// 
ftp.cs.purdue.edu/pub/hosking/papers/vandrunen.pdf) as it is a more  
powerful algorithm.  You are right, of course, that the ability of  
the PRE to determine the equivalence of two expression.  This is an  
area that can always be improved, and it's mostly an issue of if it's  
worth the amount of work required to prove a given equivalence.  This  
is something that'll probably have to be determined empirically.

Hope this help,


On Apr 27, 2007, at 5:44 PM, Jiyu Zhang wrote:

> Hi, I am Jill, I'm new here and new with llvm. I find that its old
> PRE.cpp has been removed from llvm-1.9 and the project homepage says
> that a GVN-PRE pass is being implemented.
> Since many PRE algorithms have been suggested, and I think PRE further
> differs according to the specific defination of equivalence, whether
> it is an "optimal" algorithm and optimal at what metric, etc, can I
> ask what differences you're going to make between this one and the old
> pass, and based on which algorithm you are going to develop?
> Thanks:)
> -- 
> Jill
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