[llvm-commits] [LLVMdev] [polly] scev codegen (first step to remove the dependence on ivcanon pass)

[Sebastian Pop]

Hi Tobi,

here is the first part of removing the ivcanon pass: the two patches for llvm
are implementing the scev apply function and the parameter rewriter (based on
your previous code SCEVRewriter).  On the Polly side the two patches are first
collecting the LoopToScev map and using the apply function and the parameter
rewriter.

Ok to commit?

Thanks,
Sebastian

Sebastian Pop wrote:
> Hi Tobi,
> 
> I would like to remove the SCEVRewriter code and replace it with a call to
> SCEVAddRec::apply (see attached a patch that adds just this function).  More
> precisely I want to add another function called apply_map that applies a map
> (loop -> expr) on a given scev.  This is the apply function on a multi-variate
> polynomial.
> 
> So here is an overview of how I would like the scev code generator to work on an
> example: supposing that we have a Stmt_1 that gets code generated by either
> CLooG or ISL-codegen like this:
> 
>   Stmt_1(c1, c1+4, c1+c2);
> 
> we will construct a map that maps the old iteration domain with 3 dimensions
> (there are 3 arguments in Stmt_1 representing the original loop nest in which
> Stmt_1 was located, let's call the original loop nest loop_1, loop_2, loop_3) to
> the new expressions generated by cloog that are function of the new iteration
> domain with 2 dimensions (c1 and c2 are the new induction variables of the code
> generated by cloog).  So the content of that map is:
> 
> loop_1 -> c1
> loop_2 -> c1+7
> loop_3 -> c1+c2
> 
> Given an access function from the original program: 
>   Scev_1 = {{{0, +, 4}_1, +, 5}_2, +, 6}_3
> 
> we will apply the map on it, and we will get a symbolic expression function of
> the new induction variables c1, and c2:
> 
> Scev_2 = apply (loop_1 -> c1)    on Scev_1 = {{4*c1, +, 5}_2, +, 6}_3
> Scev_3 = apply (loop_2 -> c1+7)  on Scev_2 = {4*c1 + 5*(c1+7), +, 6}_3
> Scev_4 = apply (loop_3 -> c1+c2) on Scev_3 = 4*c1 + 5*(c1+7) + 6*(c1+c2)
> 
> We will then code generate Scev_4 and we will use this new expression for the
> array access in the new loop nest.
> 
> Remark that in all this process we have never referred to the original
> "canonical induction variable".  SCEV actually provides such a canonical form
> for the induction variables without having to transform the code.
> 

-- 
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