[LLVMdev] Loop transformations using LLVM

[Andrew Trick]

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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