LLVM 17.0.0git
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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 "llvm/Analysis/LoopAccessAnalysis.h"
Classes | |
struct | Dependence |
Dependece between memory access instructions. More... | |
Public Types | |
enum class | VectorizationSafetyStatus { Safe , PossiblySafeWithRtChecks , Unsafe } |
Type to keep track of the status of the dependence check. More... | |
typedef PointerIntPair< Value *, 1, bool > | MemAccessInfo |
typedef SmallVector< MemAccessInfo, 8 > | MemAccessInfoList |
typedef EquivalenceClasses< MemAccessInfo > | DepCandidates |
Set of potential dependent memory accesses. | |
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. | |
void | addAccess (LoadInst *LI) |
Register the location (instructions are given increasing numbers) of a write access. | |
bool | areDepsSafe (DepCandidates &AccessSets, MemAccessInfoList &CheckDeps, const DenseMap< Value *, const SCEV * > &Strides) |
Check whether the dependencies between the accesses are safe. | |
bool | isSafeForVectorization () const |
No memory dependence was encountered that would inhibit vectorization. | |
bool | isSafeForAnyVectorWidth () const |
Return true if the number of elements that are safe to operate on simultaneously is not bounded. | |
uint64_t | getMaxSafeDepDistBytes () |
The maximum number of bytes of a vector register we can vectorize the accesses safely with. | |
uint64_t | getMaxSafeVectorWidthInBits () const |
Return the number of elements that are safe to operate on simultaneously, multiplied by the size of the element in bits. | |
bool | shouldRetryWithRuntimeCheck () const |
In same cases when the dependency check fails we can still vectorize the loop with a dynamic array access check. | |
const SmallVectorImpl< Dependence > * | getDependences () const |
Returns the memory dependences. | |
void | clearDependences () |
const SmallVectorImpl< Instruction * > & | getMemoryInstructions () const |
The vector of memory access instructions. | |
DenseMap< Instruction *, unsigned > | generateInstructionOrderMap () const |
Generate a mapping between the memory instructions and their indices according to program order. | |
SmallVector< Instruction *, 4 > | getInstructionsForAccess (Value *Ptr, bool isWrite) const |
Find the set of instructions that read or write via Ptr . | |
ArrayRef< unsigned > | getOrderForAccess (Value *Ptr, bool IsWrite) const |
Return the program order indices for the access location (Ptr, IsWrite). | |
const Loop * | getInnermostLoop () const |
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 87 of file LoopAccessAnalysis.h.
Set of potential dependent memory accesses.
Definition at line 92 of file LoopAccessAnalysis.h.
typedef PointerIntPair<Value *, 1, bool> llvm::MemoryDepChecker::MemAccessInfo |
Definition at line 89 of file LoopAccessAnalysis.h.
Definition at line 90 of file LoopAccessAnalysis.h.
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Type to keep track of the status of the dependence check.
The order of the elements is important and has to be from most permissive to least permissive.
Enumerator | |
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Safe | |
PossiblySafeWithRtChecks | |
Unsafe |
Definition at line 97 of file LoopAccessAnalysis.h.
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Definition at line 172 of file LoopAccessAnalysis.h.
void MemoryDepChecker::addAccess | ( | LoadInst * | LI | ) |
Register the location (instructions are given increasing numbers) of a write access.
Definition at line 1602 of file LoopAccessAnalysis.cpp.
References llvm::LoadInst::getPointerOperand(), Ptr, and visitPointers().
void MemoryDepChecker::addAccess | ( | StoreInst * | SI | ) |
Register the location (instructions are given increasing numbers) of a write access.
Definition at line 1593 of file LoopAccessAnalysis.cpp.
References Ptr, SI, and visitPointers().
bool MemoryDepChecker::areDepsSafe | ( | DepCandidates & | AccessSets, |
MemAccessInfoList & | CheckDeps, | ||
const DenseMap< Value *, const SCEV * > & | Strides | ||
) |
Check whether the dependencies between the accesses are safe.
Only checks sets with elements in CheckDeps
.
Definition at line 2012 of file LoopAccessAnalysis.cpp.
References A, assert(), B, llvm::SmallPtrSetImpl< PtrType >::count(), llvm::dbgs(), llvm::sys::path::const_iterator::end, llvm::EquivalenceClasses< ElemTy, Compare >::findValue(), llvm::EquivalenceClasses< ElemTy, Compare >::getLeaderValue(), I, llvm::SmallPtrSetImpl< PtrType >::insert(), isSafeForVectorization(), llvm::MemoryDepChecker::Dependence::isSafeForVectorization(), LLVM_DEBUG, MaxDependences, llvm::EquivalenceClasses< ElemTy, Compare >::member_begin(), llvm::EquivalenceClasses< ElemTy, Compare >::member_end(), llvm::MemoryDepChecker::Dependence::NoDep, and std::swap().
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Definition at line 225 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 235 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 221 of file LoopAccessAnalysis.h.
Definition at line 257 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 2090 of file LoopAccessAnalysis.cpp.
References Idx, Ptr, and 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 203 of file LoopAccessAnalysis.h.
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Return the number of elements that are safe to operate on simultaneously, multiplied by the size of the element in bits.
Definition at line 207 of file LoopAccessAnalysis.h.
Referenced by llvm::LoopVectorizationLegality::getMaxSafeVectorWidthInBits().
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The vector of memory access instructions.
The indices are used as instruction identifiers in the Dependence class.
Definition at line 229 of file LoopAccessAnalysis.h.
Referenced by llvm::LoopVersioning::annotateLoopWithNoAlias(), llvm::MemoryDepChecker::Dependence::getDestination(), and llvm::MemoryDepChecker::Dependence::getSource().
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Return the program order indices for the access location (Ptr, IsWrite).
Returns an empty ArrayRef if there are no accesses for the location.
Definition at line 250 of file LoopAccessAnalysis.h.
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Return true if the number of elements that are safe to operate on simultaneously is not bounded.
Definition at line 197 of file LoopAccessAnalysis.h.
Referenced by llvm::LoopVectorizationLegality::isSafeForAnyVectorWidth().
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No memory dependence was encountered that would inhibit vectorization.
Definition at line 191 of file LoopAccessAnalysis.h.
References Safe.
Referenced by areDepsSafe().
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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 213 of file LoopAccessAnalysis.h.
References PossiblySafeWithRtChecks.