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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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
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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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, > NadavgetCanonicalInductionVariable 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 > --------------------------------------------------------------------- > Intel Israel (74) Limited > > This e-mail and any attachments may contain confidential material for > the sole use of the intended recipient(s). Any review or distribution > by others is strictly prohibited. If you are not the intended > recipient, please contact the sender and delete all copies. > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev