DependenceInfo - This class is the main dependence-analysis driver. More...

#include "llvm/Analysis/DependenceAnalysis.h"

Public Member Functions
	DependenceInfo (Function F, AliasAnalysis AA, ScalarEvolution SE, LoopInfo LI)

std::unique_ptr< Dependence >	depends (Instruction Src, Instruction Dst, bool PossiblyLoopIndependent)
	depends - Tests for a dependence between the Src and Dst instructions. More...

const SCEV *	getSplitIteration (const Dependence &Dep, unsigned Level)
	getSplitIteration - Give a dependence that's splittable at some particular level, return the iteration that should be used to split the loop. More...

Function *	getFunction () const

Detailed Description

DependenceInfo - This class is the main dependence-analysis driver.

Definition at line 272 of file DependenceAnalysis.h.

Constructor & Destructor Documentation

◆ DependenceInfo()

llvm::DependenceInfo::DependenceInfo	(	Function *	F,
		AliasAnalysis *	AA,
		ScalarEvolution *	SE,
		LoopInfo *	LI
	)

inline

Definition at line 274 of file DependenceAnalysis.h.

Member Function Documentation

◆ depends()

std::unique_ptr< Dependence > DependenceInfo::depends	(	Instruction *	Src,
		Instruction *	Dst,
		bool	PossiblyLoopIndependent
	)

depends - Tests for a dependence between the Src and Dst instructions.

Returns NULL if no dependence; otherwise, returns a Dependence (or a FullDependence) with as much information as can be gleaned. The flag PossiblyLoopIndependent should be set by the caller if it appears that control flow can reach from Src to Dst without traversing a loop back edge.

Definition at line 3384 of file DependenceAnalysis.cpp.

References llvm::SmallBitVector::any(), assert(), llvm::SmallBitVector::count(), llvm::dbgs(), Delinearize, dumpSmallBitVector(), llvm::Dependence::DVEntry::EQ, F(), llvm::MemoryLocation::get(), llvm::FullDependence::getDirection(), llvm::getLoadStorePointerOperand(), isLoadOrStore(), LLVM_DEBUG, llvm_unreachable, llvm::MayAlias, llvm::Instruction::mayReadFromMemory(), llvm::Instruction::mayWriteToMemory(), llvm::MustAlias, llvm::NoAlias, llvm::Dependence::DVEntry::NONE, P, llvm::PartialAlias, propagate(), llvm::SmallVectorTemplateBase< T >::push_back(), llvm::SmallBitVector::reset(), llvm::SmallVectorImpl< T >::resize(), llvm::SmallBitVector::set(), llvm::SmallBitVector::set_bits(), SI, llvm::SmallVectorBase::size(), and underlyingObjectsAlias().

Referenced by checkDependencies(), dumpExampleDependence(), and populateDependencyMatrix().

◆ getFunction()

Function* llvm::DependenceInfo::getFunction ( ) const

inline

Definition at line 330 of file DependenceAnalysis.h.

References B, C, D, llvm::Dependence::DVEntry::Direction, llvm::Dependence::DVEntry::Distance, llvm::Dependence::dump(), llvm::Empty, F(), llvm::isKnownNonNegative(), isLoopInvariant(), Kind, Loops, llvm::Lower, propagate(), Size, llvm::Upper, X, and Y.

Referenced by dumpExampleDependence().

◆ getSplitIteration()

const SCEV * DependenceInfo::getSplitIteration	(	const Dependence &	Dep,
		unsigned	Level
	)

getSplitIteration - Give a dependence that's splittable at some particular level, return the iteration that should be used to split the loop.

Generally, the dependence analyzer will be used to build a dependence graph for a function (basically a map from instructions to dependences). Looking for cycles in the graph shows us loops that cannot be trivially vectorized/parallelized.

We can try to improve the situation by examining all the dependences that make up the cycle, looking for ones we can break. Sometimes, peeling the first or last iteration of a loop will break dependences, and there are flags for those possibilities. Sometimes, splitting a loop at some other iteration will do the trick, and we've got a flag for that case. Rather than waste the space to record the exact iteration (since we rarely know), we provide a method that calculates the iteration. It's a drag that it must work from scratch, but wonderful in that it's possible.

Here's an example:

for (i = 0; i < 10; i++) A[i] = ... ... = A[11 - i]

There's a loop-carried flow dependence from the store to the load, found by the weak-crossing SIV test. The dependence will have a flag, indicating that the dependence can be broken by splitting the loop. Calling getSplitIteration will return 5. Splitting the loop breaks the dependence, like so:

for (i = 0; i <= 5; i++) A[i] = ... ... = A[11 - i] for (i = 6; i < 10; i++) A[i] = ... ... = A[11 - i]

breaks the dependence and allows us to vectorize/parallelize both loops.

Definition at line 3811 of file DependenceAnalysis.cpp.

References llvm::SmallBitVector::any(), assert(), llvm::SmallBitVector::count(), llvm::dbgs(), Delinearize, F(), llvm::MemoryLocation::get(), llvm::Dependence::getDst(), llvm::getLoadStorePointerOperand(), llvm::Dependence::getSrc(), isLoadOrStore(), llvm::Dependence::isSplitable(), LLVM_DEBUG, llvm_unreachable, llvm::Instruction::mayReadFromMemory(), llvm::Instruction::mayWriteToMemory(), llvm::MustAlias, P, propagate(), llvm::SmallBitVector::reset(), llvm::SmallVectorImpl< T >::resize(), llvm::SmallBitVector::set(), llvm::SmallBitVector::set_bits(), SI, llvm::SmallVectorBase::size(), and underlyingObjectsAlias().