[LLVMdev] Loop transformations using LLVM

[Malveeka Tewari]

Thanks for the suggestions!
These were very extremely helpful.

Malveeka

On Fri, May 6, 2011 at 5:24 PM, Andrew Trick <atrick at apple.com> wrote:

>
> On May 5, 2011, at 5:11 AM, Rotem, Nadav wrote:
>
> > Malveeka,
> >
> > You can use the LoopInfo analysis to find the induction variable.
> >
> >
> http://llvm.org/docs/doxygen/html/classllvm_1_1Loop.html#a72bbf45d2e00971f56bf8cfe4e1df01c
> >
> > Cheers,
> > Nadav
>
> getCanonicalInductionVariable is a good example of finding IVs without
> iterating over instructions. But you may need to generalize it a bit for
> your purpose.
>
> A completely general approach to IV analysis is ScalarEvolution. You can
> query the expression for a loop header phi using ScalarEvolution::getSCEV.
> If it returns SCEVAddRecExpr with getLoop() == ThisLoop, then you've found
> an IV. You can build a new IV by replacing the "step" with SCEVAddExpr(1 +
> OrigStep). To materialize it in IR, you need to use a SCEVExpander.
> ...probably more complicated than you need.
>
> -Andy
>
>
> >
> >
> > From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Malveeka Tewari
> > Sent: Thursday, May 05, 2011 14:51
> > To: llvmdev at cs.uiuc.edu
> > Subject: [LLVMdev] Loop transformations using LLVM
> >
> > Hi
> >
> > I am trying to write up a transformation for the following code where
> instead of incrementing the loop variable by 1, I increment it by 2 ie.
> >
> > for (i=0; i < THRES; i++) {
> >   //do something
> > }
> >
> > gets transformed to
> >
> > for (i=0; i < THRES; i+=2) {
> >   //do something
> > }
> >
> > I am thinking of transforming the llvm  bit-code in the following way.
> > Iterate over the function for the original code till I get the basicBlock
> for the for loop, then iterate over the instructions till I get the
> increment instruction and then add the same increment instruction so that
> the increment happens twice.
> >
> > I am still getting familiar with the llvm infrastructure and I am not
> sure if the above approach is even going to work. It'll be great if someone
> can suggest a better or more concrete way of doing the above transformation
> ? Is there already something similar in the source tree  that can I look up
> for ideas ?
> >
> > Thanks!
> > Malveeka
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