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LLVM
4.0.0
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Checks memory dependences among accesses to the same underlying object to determine whether there vectorization is legal or not (and at which vectorization factor). More...
#include <LoopAccessAnalysis.h>
Classes | |
| struct | Dependence |
| Dependece between memory access instructions. More... | |
Public Types | |
| typedef PointerIntPair< Value *, 1, bool > | MemAccessInfo |
| typedef SmallPtrSet < MemAccessInfo, 8 > | MemAccessInfoSet |
| typedef EquivalenceClasses < MemAccessInfo > | DepCandidates |
| Set of potential dependent memory accesses. More... | |
Public Member Functions | |
| MemoryDepChecker (PredicatedScalarEvolution &PSE, const Loop *L) | |
| void | addAccess (StoreInst *SI) |
| Register the location (instructions are given increasing numbers) of a write access. More... | |
| void | addAccess (LoadInst *LI) |
| Register the location (instructions are given increasing numbers) of a write access. More... | |
| bool | areDepsSafe (DepCandidates &AccessSets, MemAccessInfoSet &CheckDeps, const ValueToValueMap &Strides) |
| Check whether the dependencies between the accesses are safe. More... | |
| bool | isSafeForVectorization () const |
| No memory dependence was encountered that would inhibit vectorization. More... | |
| uint64_t | getMaxSafeDepDistBytes () |
| The maximum number of bytes of a vector register we can vectorize the accesses safely with. More... | |
| bool | shouldRetryWithRuntimeCheck () |
| In same cases when the dependency check fails we can still vectorize the loop with a dynamic array access check. More... | |
| const SmallVectorImpl < Dependence > * | getDependences () const |
| Returns the memory dependences. More... | |
| void | clearDependences () |
| const SmallVectorImpl < Instruction * > & | getMemoryInstructions () const |
| The vector of memory access instructions. More... | |
| DenseMap< Instruction *, unsigned > | generateInstructionOrderMap () const |
| Generate a mapping between the memory instructions and their indices according to program order. More... | |
| SmallVector< Instruction *, 4 > | getInstructionsForAccess (Value *Ptr, bool isWrite) const |
Find the set of instructions that read or write via Ptr. More... | |
Checks memory dependences among accesses to the same underlying object to determine whether there vectorization is legal or not (and at which vectorization factor).
Note: This class will compute a conservative dependence for access to different underlying pointers. Clients, such as the loop vectorizer, will sometimes deal these potential dependencies by emitting runtime checks.
We use the ScalarEvolution framework to symbolically evalutate access functions pairs. Since we currently don't restructure the loop we can rely on the program order of memory accesses to determine their safety. At the moment we will only deem accesses as safe for:
A negative constant distance assuming program order.
Safe: tmp = a[i + 1]; OR a[i + 1] = x; a[i] = tmp; y = a[i];
The latter case is safe because later checks guarantuee that there can't be a cycle through a phi node (that is, we check that "x" and "y" is not the same variable: a header phi can only be an induction or a reduction, a reduction can't have a memory sink, an induction can't have a memory source). This is important and must not be violated (or we have to resort to checking for cycles through memory).
A positive constant distance assuming program order that is bigger than the biggest memory access.
tmp = a[i] OR b[i] = x a[i+2] = tmp y = b[i+2];
Safe distance: 2 x sizeof(a[0]), and 2 x sizeof(b[0]), respectively.
Definition at line 126 of file LoopAccessAnalysis.h.
Set of potential dependent memory accesses.
Definition at line 131 of file LoopAccessAnalysis.h.
| typedef PointerIntPair<Value *, 1, bool> llvm::MemoryDepChecker::MemAccessInfo |
Definition at line 128 of file LoopAccessAnalysis.h.
Definition at line 129 of file LoopAccessAnalysis.h.
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Definition at line 198 of file LoopAccessAnalysis.h.
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Register the location (instructions are given increasing numbers) of a write access.
Definition at line 205 of file LoopAccessAnalysis.h.
References llvm::StoreInst::getPointerOperand(), and Ptr.
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Register the location (instructions are given increasing numbers) of a write access.
Definition at line 214 of file LoopAccessAnalysis.h.
References llvm::LoadInst::getPointerOperand(), and Ptr.
| bool MemoryDepChecker::areDepsSafe | ( | DepCandidates & | AccessSets, |
| MemAccessInfoSet & | CheckDeps, | ||
| const ValueToValueMap & | Strides | ||
| ) |
Check whether the dependencies between the accesses are safe.
Only checks sets with elements in CheckDeps.
Definition at line 1429 of file LoopAccessAnalysis.cpp.
References assert(), llvm::sys::path::begin(), llvm::SmallPtrSetImpl< PtrType >::begin(), llvm::dbgs(), DEBUG, llvm::SmallPtrSetImplBase::empty(), llvm::sys::path::end(), llvm::SmallPtrSetImpl< PtrType >::erase(), llvm::EquivalenceClasses< ElemTy >::findValue(), llvm::EquivalenceClasses< ElemTy >::getLeaderValue(), I, isDependent(), MaxDependences, llvm::EquivalenceClasses< ElemTy >::member_begin(), llvm::EquivalenceClasses< ElemTy >::member_end(), and std::swap().
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Definition at line 246 of file LoopAccessAnalysis.h.
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Generate a mapping between the memory instructions and their indices according to program order.
Definition at line 256 of file LoopAccessAnalysis.h.
References I.
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Returns the memory dependences.
If null is returned we exceeded the MaxDependences threshold and this information is not available.
Definition at line 242 of file LoopAccessAnalysis.h.
| SmallVector< Instruction *, 4 > MemoryDepChecker::getInstructionsForAccess | ( | Value * | Ptr, |
| bool | isWrite | ||
| ) | const |
Find the set of instructions that read or write via Ptr.
Definition at line 1496 of file LoopAccessAnalysis.cpp.
References llvm::transform().
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The maximum number of bytes of a vector register we can vectorize the accesses safely with.
Definition at line 233 of file LoopAccessAnalysis.h.
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The vector of memory access instructions.
The indices are used as instruction identifiers in the Dependence class.
Definition at line 250 of file LoopAccessAnalysis.h.
Referenced by llvm::LoopVersioning::annotateLoopWithNoAlias(), llvm::MemoryDepChecker::Dependence::getDestination(), and llvm::MemoryDepChecker::Dependence::getSource().
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No memory dependence was encountered that would inhibit vectorization.
Definition at line 229 of file LoopAccessAnalysis.h.
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In same cases when the dependency check fails we can still vectorize the loop with a dynamic array access check.
Definition at line 237 of file LoopAccessAnalysis.h.
1.8.6