[LLVMdev] Questions about trip count

Douglas do Couto Teixeira douglasdocouto at gmail.com
Sat Aug 14 09:16:20 PDT 2010


On Thu, Aug 12, 2010 at 5:22 PM, Tobias Grosser
<grosser at fim.uni-passau.de>wrote:

> On 08/12/2010 09:41 PM, Douglas do Couto Teixeira wrote:
>
>> Dear guys,
>>
>>     I am having problems to obtain good information from the LoopInfo.
>> I am always getting a trip count of 0, even though I am clearly passing
>> a loop with a constant bound. I am using this pass below:
>>
> Hi,
>
> I would propose to first check if the trip count is calculated correctly. I
> would do this with opt by calling:
>
> opt -mem2reg -loopsimplify -indvars -scalar-evolution -analyze prog.ll
>
> At the end of the output the tripcount of every loop should be written.
>
> To get the trip count in your pass you should ask ScalarEvolution.
>
> ScalarEvolution *SE;
> Loop *L;
> SE->getBackedgeTakenCount(L);
>
> If you need help trying this please ask back.
>
> Cheers
> Tobi
>

Thanks Tobias. Your answer helped me a lot.  Now I can get the trip count of
the loop like this:

#define DEBUG_TYPE "hello"
#include "llvm/Pass.h"
#include "llvm/Function.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolution.h"

using namespace llvm;

STATISTIC(HelloCounter, "Counts number of functions greeted");

namespace {

struct Hello: public FunctionPass {
static char ID; // Pass identification, replacement for typeid
Hello() : FunctionPass(&ID) {}

void getLoopInfo(const Function& F) const {

const LoopInfo *LI = &getAnalysis<LoopInfo> ();
ScalarEvolution *SE = &getAnalysis<ScalarEvolution> ();
Function::const_iterator BB = F.begin(), E = F.end();

for (; BB != E; ++BB) {
const Loop* L = LI->getLoopFor(&*BB);
if (L != NULL) {
errs() << "Trip count: " << *SE->getBackedgeTakenCount(L);
L->dump();
}
}
}

virtual bool runOnFunction(Function &F) {

HelloCounter++;
errs() << "Hello: ";
errs().write_escaped(F.getName()) << '\n';
getLoopInfo(F);
return false;
}

// We don't modify the program, so we preserve all analyses
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
AU.addRequired<LoopInfo> ();
AU.addRequired<ScalarEvolution> ();
}
};
}

char Hello::ID = 0;
static RegisterPass<Hello> Y("hello",
"Hello World Pass (with getAnalysisUsage implemented)");


But when I use that same function within a class I get some compiler errors.
I'm trying to use this function like this:


#define DEBUG_TYPE "hello"
#include "llvm/Pass.h"
#include "llvm/Function.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolution.h"

using namespace llvm;

STATISTIC(HelloCounter, "Counts number of functions greeted");

namespace {

struct Hello: public FunctionPass {
static char ID; // Pass identification, replacement for typeid
Hello() : FunctionPass(&ID) {}

class testLoopInfo {

void getLoopInfo(const Function& F) const {

const LoopInfo *LI = &getAnalysis<LoopInfo> ();
ScalarEvolution *SE = &getAnalysis<ScalarEvolution> ();
Function::const_iterator BB = F.begin(), E = F.end();
for (; BB != E; ++BB) {

const Loop* L = LI->getLoopFor(&*BB);
if (L != NULL) {
errs() << "Trip count: " << *SE->getBackedgeTakenCount(L);
L->dump();
}
}
}

};

virtual bool runOnFunction(Function &F) {

HelloCounter++;
errs() << "Hello: ";
errs().write_escaped(F.getName()) << '\n';
//getLoopInfo(F);
return false;
}

// We don't modify the program, so we preserve all analyses
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
AU.addRequired<LoopInfo> ();
AU.addRequired<ScalarEvolution> ();
}
};
}

char Hello::ID = 0;
static RegisterPass<Hello> Y("hello",
"Hello World Pass (with getAnalysisUsage implemented)");

I get the following errors:

**** Build of configuration Debug for project HelloLLVM ****

make all
llvm[0]: Compiling Hello.cpp for Release build (PIC)
Hello.cpp: In member function
‘void<unnamed>::Hello::testLoopInfo::getLoopInfo(const llvm::Function&)
const’:
Hello.cpp:37: error: no matching function for call to
‘llvm::Pass::getAnalysis()’
/home/douglas/Programas/llvm-2.7/include/llvm/PassAnalysisSupport.h:209:
note: candidates are: AnalysisType& llvm::Pass::getAnalysis() const [with
AnalysisType = llvm::LoopInfo] <near match>
Hello.cpp:38: error: no matching function for call to
‘llvm::Pass::getAnalysis()’
/home/douglas/Programas/llvm-2.7/include/llvm/PassAnalysisSupport.h:209:
note: candidates are: AnalysisType& llvm::Pass::getAnalysis() const [with
AnalysisType = llvm::ScalarEvolution] <near match>
Hello.cpp: In member function ‘virtual
bool<unnamed>::Hello::runOnFunction(llvm::Function&)’:
Hello.cpp:57: error: ‘getLoopInfo’ was not declared in this scope
make: **
[/home/douglas/Programas/llvm-2.7/lib/Transforms/Hello/Release/Hello.o] Erro
1


Anyone know how can I fix it?


Thanks a lot,

Douglas
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