[LLVMdev] Detect if a basicblock is part of a loop

Arnaud ALLARD DE GRANDMAISON
Thu, 26 Apr 2012 20:21:19 -0000

Hi,

Depending on what have run before your pass, the loop may have been unrolled or
simplified if the computation inside the loop is too simple.

Cheers,
--
Arnaud de Grandmaison

________________________________________
From: llvmdev-bounces at cs.uiuc.edu [llvmdev-bounces at cs.uiuc.edu] On Behalf
Of Cristianno Martins [cristiannomartins at gmail.com]
Sent: Thursday, April 26, 2012 5:52 PM
To: Hal Finkel
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Detect if a basicblock is part of a loop

Hi Rinaldini,

In order to find information about loops inside a given function you should use
something like "LoopInfo *LI = P->getAnalysis<LoopInfo>()",
remembering to add "AU.addRequired<LoopInfo>();" to your
getAnalysisUsage method.

If the function you are interested to is not located in the module being
compiled (if you created it as an auxiliary function, for example), you could
create this information by simply creating a DominatorTreeBase of your function
and using it to calculate the LoopInfo, as below:
    DominatorTreeBase<BasicBlock> *DTB;
    DTB = new DominatorTreeBase<BasicBlock>(false);
    DTB->recalculate(*AuxFunction);

    LoopInfoBase<BasicBlock, Loop> LIB;
    LIB.Calculate(*DTB);

Cheers,

--
Cristianno Martins
PhD Student of Computer Science
University of Campinas
cmartins at ic.unicamp.br


On Thursday, 26 de April de 2012 at 11:57, Hal Finkel wrote:

Rinaldini,

What exactly did you run? Specifically, you may be missing some
analysis passes that are necessary for LoopInfo to have the loop
information you desire.

-Hal

On Thu, 26 Apr 2012 14:02:04 +0000
Rinaldini Julien <julien.rinaldini at heig-vd.ch<mailto:julien.rinaldini
at heig-vd.ch>> wrote:

Hi,

I'm trying to detect if a basicblock is part of a loop or not.

I tried the llvm::LoopInfo like that
(http://llvm.org/docs/doxygen/html/classllvm_1_1LoopInfo.html#a4abca289c73cd09487e05d11d9f7d877):

LoopInfo *loop = new LoopInfo();
bool isLoop = loop->getLoopFor(myBB); // getLoopFor - Return the
inner most loop that BB lives in. If a basic block is in no loop (for
example the entry node), null is returned. See doxygen

But getLoopFor() always return me NULL even if my basicblock is
inside a for loop... I tried this:
http://comments.gmane.org/gmane.comp.compilers.llvm.devel/38490 but
it didn't work too :(

Any advice?
Thx

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--
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory
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