Loads.h

Go to the documentation of this file.

//===- Loads.h - Local load analysis --------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file declares simple local analyses for load instructions.
//
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_ANALYSIS_LOADS_H
#define LLVM_ANALYSIS_LOADS_H
 
#include "llvm/ADT/APInt.h"
#include "llvm/Analysis/SimplifyQuery.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/GEPNoWrapFlags.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
 
namespace llvm {
 
class BatchAAResults;
class AssumptionCache;
class DataLayout;
class DominatorTree;
class Instruction;
class LoadInst;
struct LoadStoreInstProperties;
class Loop;
class MemoryLocation;
class SCEV;
class ScalarEvolution;
class SCEVPredicate;
class StoreInst;
template <typename T> class SmallVectorImpl;
class TargetLibraryInfo;
 
/// Returns true if V is always a dereferenceable pointer with alignment
/// greater or equal than requested. If the context instruction is specified
/// performs context-sensitive analysis and returns true if the pointer is
/// dereferenceable at the specified instruction.
/// If \p IgnoreFree is set, ignore potential frees of the object.
LLVM_ABI bool isDereferenceableAndAlignedPointer(const Value *V, Type *Ty,
                                                 Align Alignment,
                                                 const SimplifyQuery &Q,
                                                 bool IgnoreFree = false);
 
/// Returns true if V is always dereferenceable for Size byte with alignment
/// greater or equal than requested. If the context instruction is specified
/// performs context-sensitive analysis and returns true if the pointer is
/// dereferenceable at the specified instruction.
/// If \p IgnoreFree is set, ignore potential frees of the object.
LLVM_ABI bool isDereferenceableAndAlignedPointer(const Value *V,
                                                 Align Alignment,
                                                 const APInt &Size,
                                                 const SimplifyQuery &Q,
                                                 bool IgnoreFree = false);
 
/// Equivalent to isDereferenceableAndAlignedPointer with an alignment of 1.
LLVM_ABI bool isDereferenceablePointer(const Value *V, Type *Ty,
                                       const SimplifyQuery &Q,
                                       bool IgnoreFree = false);
 
/// Equivalent to isDereferenceableAndAlignedPointer with an alignment of 1.
LLVM_ABI bool isDereferenceablePointer(const Value *V, const APInt &Size,
                                       const SimplifyQuery &Q,
                                       bool IgnoreFree = false);
 
/// Return true if we know that executing a load from this value cannot trap.
///
/// If SQ.CtxI is specified this method performs context-sensitive analysis
/// and returns true if it is safe to load immediately before SQ.CtxI.
///
/// If it is not obviously safe to load from the specified pointer, we do a
/// quick local scan of the basic block containing SQ.CtxI, to determine if
/// the address is already accessed.
LLVM_ABI bool isSafeToLoadUnconditionally(Value *V, Align Alignment,
                                          const APInt &Size,
                                          const SimplifyQuery &SQ,
                                          bool IgnoreFree = false);
 
/// Return true if we can prove that the given load (which is assumed to be
/// within the specified loop) would access only dereferenceable memory, and
/// be properly aligned on every iteration of the specified loop regardless of
/// its placement within the loop. (i.e. does not require predication beyond
/// that required by the header itself and could be hoisted into the header
/// if desired.)  This is more powerful than the variants above when the
/// address loaded from is analyzeable by SCEV.
LLVM_ABI bool isDereferenceableAndAlignedInLoop(
    LoadInst *LI, Loop *L, ScalarEvolution &SE, DominatorTree &DT,
    AssumptionCache *AC = nullptr,
    SmallVectorImpl<const SCEVPredicate *> *Predicates = nullptr);
 
/// Overload for isDereferenceableAndAlignedInLoop taking the pointer and access
/// size directly as SCEVs.
LLVM_ABI bool isDereferenceableAndAlignedInLoop(
    const SCEV *PtrSCEV, Align Alignment, const SCEV *EltSizeSCEV, Loop *L,
    ScalarEvolution &SE, DominatorTree &DT, AssumptionCache *AC = nullptr,
    SmallVectorImpl<const SCEVPredicate *> *Predicates = nullptr);
 
/// Returns true if the loop contains read-only memory accesses and doesn't
/// throw. Puts loads that may fault into \p NonDereferenceableAndAlignedLoads.
LLVM_ABI bool
isReadOnlyLoop(Loop *L, ScalarEvolution *SE, DominatorTree *DT,
               AssumptionCache *AC,
               SmallVectorImpl<LoadInst *> &NonDereferenceableAndAlignedLoads,
               SmallVectorImpl<const SCEVPredicate *> *Predicates = nullptr);
 
/// Return true if we know that executing a load from this value cannot trap.
///
/// If SQ.CtxI is specified this method performs context-sensitive analysis
/// and returns true if it is safe to load immediately before SQ.CtxI.
///
/// If it is not obviously safe to load from the specified pointer, we do a
/// quick local scan of the basic block containing SQ.CtxI, to determine if
/// the address is already accessed.
LLVM_ABI bool isSafeToLoadUnconditionally(Value *V, Type *Ty, Align Alignment,
                                          const SimplifyQuery &SQ,
                                          bool IgnoreFree = false);
 
/// Return true if speculation of the given load must be suppressed to avoid
/// ordering or interfering with an active sanitizer.  If not suppressed,
/// dereferenceability and alignment must be proven separately.  Note: This
/// is only needed for raw reasoning; if you use the interface below
/// (isSafeToSpeculativelyExecute), this is handled internally.
LLVM_ABI bool mustSuppressSpeculation(const LoadInst &LI);
 
/// The default number of maximum instructions to scan in the block, used by
/// FindAvailableLoadedValue().
LLVM_ABI extern cl::opt<unsigned> DefMaxInstsToScan;
 
/// Scan backwards to see if we have the value of the given load available
/// locally within a small number of instructions.
///
/// You can use this function to scan across multiple blocks: after you call
/// this function, if ScanFrom points at the beginning of the block, it's safe
/// to continue scanning the predecessors.
///
/// Note that performing load CSE requires special care to make sure the
/// metadata is set appropriately.  In particular, aliasing metadata needs
/// to be merged.  (This doesn't matter for store-to-load forwarding because
/// the only relevant load gets deleted.)
///
/// \param Load The load we want to replace.
/// \param ScanBB The basic block to scan.
/// \param [in,out] ScanFrom The location to start scanning from. When this
/// function returns, it points at the last instruction scanned.
/// \param MaxInstsToScan The maximum number of instructions to scan. If this
/// is zero, the whole block will be scanned.
/// \param AA Optional pointer to alias analysis, to make the scan more
/// precise.
/// \param [out] IsLoadCSE Whether the returned value is a load from the same
/// location in memory, as opposed to the value operand of a store.
///
/// \returns The found value, or nullptr if no value is found.
LLVM_ABI Value *FindAvailableLoadedValue(
    LoadInst *Load, BasicBlock *ScanBB, BasicBlock::iterator &ScanFrom,
    unsigned MaxInstsToScan = DefMaxInstsToScan, BatchAAResults *AA = nullptr,
    bool *IsLoadCSE = nullptr, unsigned *NumScanedInst = nullptr);
 
/// This overload provides a more efficient implementation of
/// FindAvailableLoadedValue() for the case where we are not interested in
/// finding the closest clobbering instruction if no available load is found.
/// This overload cannot be used to scan across multiple blocks.
LLVM_ABI Value *
FindAvailableLoadedValue(LoadInst *Load, BatchAAResults &AA, bool *IsLoadCSE,
                         unsigned MaxInstsToScan = DefMaxInstsToScan);
 
/// Check whether \p SI, which may alias \p MemLoc, can be safely skipped.
/// This is possible when \p SI does only MustAlias or NoAlias \p MemLoc (no
/// partial overlap possible), and it stores the value \p MemLoc currently
/// holds (loaded before the store and not modified in between).
LLVM_ABI bool isStorePreservingMemoryLocation(const StoreInst *SI,
                                              const MemoryLocation &MemLoc,
                                              Align MemLocAlign,
                                              BatchAAResults &AA,
                                              unsigned ScanLimit);
 
/// Scan backwards to see if we have the value of the given pointer available
/// locally within a small number of instructions.
///
/// You can use this function to scan across multiple blocks: after you call
/// this function, if ScanFrom points at the beginning of the block, it's safe
/// to continue scanning the predecessors.
///
/// \param Loc The location we want the load and store to originate from.
/// \param AccessTy The access type of the pointer.
/// \param AccessProps The properties of the load we want to replace.
/// \param ScanBB The basic block to scan.
/// \param [in,out] ScanFrom The location to start scanning from. When this
/// function returns, it points at the last instruction scanned.
/// \param MaxInstsToScan The maximum number of instructions to scan. If this
/// is zero, the whole block will be scanned.
/// \param AA Optional pointer to alias analysis, to make the scan more
/// precise.
/// \param [out] IsLoadCSE Whether the returned value is a load from the same
/// location in memory, as opposed to the value operand of a store.
///
/// \returns The found value, or nullptr if no value is found.
LLVM_ABI Value *
findAvailablePtrLoadStore(const MemoryLocation &Loc, Type *AccessTy,
                          const LoadStoreInstProperties &AccessProps,
                          BasicBlock *ScanBB, BasicBlock::iterator &ScanFrom,
                          unsigned MaxInstsToScan, BatchAAResults *AA,
                          bool *IsLoadCSE, unsigned *NumScanedInst);
 
/// Returns true if a pointer value \p From can be replaced with another pointer
/// value \To if they are deemed equal through some means (e.g. information from
/// conditions).
/// NOTE: The current implementation allows replacement in Icmp and PtrToInt
/// instructions, as well as when we are replacing with a null pointer.
/// Additionally it also allows replacement of pointers when both pointers have
/// the same underlying object.
LLVM_ABI bool canReplacePointersIfEqual(const Value *From, const Value *To,
                                        const DataLayout &DL);
LLVM_ABI bool canReplacePointersInUseIfEqual(const Use &U, const Value *To,
                                             const DataLayout &DL);
 
/// Linear expression BasePtr + Index * Scale + Offset.
/// Index, Scale and Offset all have the same bit width, which matches the
/// pointer index size of BasePtr.
/// Index may be nullptr if Scale is 0.
struct LinearExpression {
  Value *BasePtr;
  Value *Index = nullptr;
  APInt Scale;
  APInt Offset;
  GEPNoWrapFlags Flags = GEPNoWrapFlags::all();
 
  LinearExpression(Value *BasePtr, unsigned BitWidth)
      : BasePtr(BasePtr), Scale(BitWidth, 0), Offset(BitWidth, 0) {}
};
 
/// Decompose a pointer into a linear expression. This may look through
/// multiple GEPs.
LLVM_ABI LinearExpression decomposeLinearExpression(const DataLayout &DL,
                                                    Value *Ptr);
}
 
#endif