LoopUtils.h

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//===- llvm/Transforms/Utils/LoopUtils.h - Loop utilities -------*- C++ -*-===//
//
// 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 defines some loop transformation utilities.
//
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_TRANSFORMS_UTILS_LOOPUTILS_H
#define LLVM_TRANSFORMS_UTILS_LOOPUTILS_H
 
#include "llvm/Analysis/IVDescriptors.h"
#include "llvm/Analysis/LoopAccessAnalysis.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
 
namespace llvm {
 
template <typename T> class DomTreeNodeBase;
using DomTreeNode = DomTreeNodeBase<BasicBlock>;
class AssumptionCache;
class StringRef;
class AnalysisUsage;
class TargetTransformInfo;
class AAResults;
class BasicBlock;
class ICFLoopSafetyInfo;
class IRBuilderBase;
class Loop;
class LoopInfo;
class MemoryAccess;
class MemorySSA;
class MemorySSAUpdater;
class OptimizationRemarkEmitter;
class PredIteratorCache;
class ScalarEvolution;
class SCEV;
class SCEVExpander;
class TargetLibraryInfo;
class LPPassManager;
class Instruction;
struct RuntimeCheckingPtrGroup;
typedef std::pair<const RuntimeCheckingPtrGroup *,
                  const RuntimeCheckingPtrGroup *>
    RuntimePointerCheck;
 
template <typename T> class Optional;
template <typename T, unsigned N> class SmallSetVector;
template <typename T, unsigned N> class SmallPriorityWorklist;
 
BasicBlock *InsertPreheaderForLoop(Loop *L, DominatorTree *DT, LoopInfo *LI,
                                   MemorySSAUpdater *MSSAU, bool PreserveLCSSA);
 
/// Ensure that all exit blocks of the loop are dedicated exits.
///
/// For any loop exit block with non-loop predecessors, we split the loop
/// predecessors to use a dedicated loop exit block. We update the dominator
/// tree and loop info if provided, and will preserve LCSSA if requested.
bool formDedicatedExitBlocks(Loop *L, DominatorTree *DT, LoopInfo *LI,
                             MemorySSAUpdater *MSSAU, bool PreserveLCSSA);
 
/// Ensures LCSSA form for every instruction from the Worklist in the scope of
/// innermost containing loop.
///
/// For the given instruction which have uses outside of the loop, an LCSSA PHI
/// node is inserted and the uses outside the loop are rewritten to use this
/// node.
///
/// LoopInfo and DominatorTree are required and, since the routine makes no
/// changes to CFG, preserved.
///
/// Returns true if any modifications are made.
///
/// This function may introduce unused PHI nodes. If \p PHIsToRemove is not
/// nullptr, those are added to it (before removing, the caller has to check if
/// they still do not have any uses). Otherwise the PHIs are directly removed.
bool formLCSSAForInstructions(
    SmallVectorImpl<Instruction *> &Worklist, const DominatorTree &DT,
    const LoopInfo &LI, ScalarEvolution *SE, IRBuilderBase &Builder,
    SmallVectorImpl<PHINode *> *PHIsToRemove = nullptr);
 
/// Put loop into LCSSA form.
///
/// Looks at all instructions in the loop which have uses outside of the
/// current loop. For each, an LCSSA PHI node is inserted and the uses outside
/// the loop are rewritten to use this node. Sub-loops must be in LCSSA form
/// already.
///
/// LoopInfo and DominatorTree are required and preserved.
///
/// If ScalarEvolution is passed in, it will be preserved.
///
/// Returns true if any modifications are made to the loop.
bool formLCSSA(Loop &L, const DominatorTree &DT, const LoopInfo *LI,
               ScalarEvolution *SE);
 
/// Put a loop nest into LCSSA form.
///
/// This recursively forms LCSSA for a loop nest.
///
/// LoopInfo and DominatorTree are required and preserved.
///
/// If ScalarEvolution is passed in, it will be preserved.
///
/// Returns true if any modifications are made to the loop.
bool formLCSSARecursively(Loop &L, const DominatorTree &DT, const LoopInfo *LI,
                          ScalarEvolution *SE);
 
/// Flags controlling how much is checked when sinking or hoisting
/// instructions.  The number of memory access in the loop (and whether there
/// are too many) is determined in the constructors when using MemorySSA.
class SinkAndHoistLICMFlags {
public:
  // Explicitly set limits.
  SinkAndHoistLICMFlags(unsigned LicmMssaOptCap,
                        unsigned LicmMssaNoAccForPromotionCap, bool IsSink,
                        Loop *L = nullptr, MemorySSA *MSSA = nullptr);
  // Use default limits.
  SinkAndHoistLICMFlags(bool IsSink, Loop *L = nullptr,
                        MemorySSA *MSSA = nullptr);
 
  void setIsSink(bool B) { IsSink = B; }
  bool getIsSink() { return IsSink; }
  bool tooManyMemoryAccesses() { return NoOfMemAccTooLarge; }
  bool tooManyClobberingCalls() { return LicmMssaOptCounter >= LicmMssaOptCap; }
  void incrementClobberingCalls() { ++LicmMssaOptCounter; }
 
protected:
  bool NoOfMemAccTooLarge = false;
  unsigned LicmMssaOptCounter = 0;
  unsigned LicmMssaOptCap;
  unsigned LicmMssaNoAccForPromotionCap;
  bool IsSink;
};
 
/// Walk the specified region of the CFG (defined by all blocks
/// dominated by the specified block, and that are in the current loop) in
/// reverse depth first order w.r.t the DominatorTree. This allows us to visit
/// uses before definitions, allowing us to sink a loop body in one pass without
/// iteration. Takes DomTreeNode, AAResults, LoopInfo, DominatorTree,
/// TargetLibraryInfo, Loop, AliasSet information for all
/// instructions of the loop and loop safety information as
/// arguments. Diagnostics is emitted via \p ORE. It returns changed status.
/// \p CurLoop is a loop to do sinking on. \p OutermostLoop is used only when
/// this function is called by \p sinkRegionForLoopNest.
bool sinkRegion(DomTreeNode *, AAResults *, LoopInfo *, DominatorTree *,
                TargetLibraryInfo *, TargetTransformInfo *, Loop *CurLoop,
                MemorySSAUpdater &, ICFLoopSafetyInfo *,
                SinkAndHoistLICMFlags &, OptimizationRemarkEmitter *,
                Loop *OutermostLoop = nullptr);
 
/// Call sinkRegion on loops contained within the specified loop
/// in order from innermost to outermost.
bool sinkRegionForLoopNest(DomTreeNode *, AAResults *, LoopInfo *,
                           DominatorTree *, TargetLibraryInfo *,
                           TargetTransformInfo *, Loop *, MemorySSAUpdater &,
                           ICFLoopSafetyInfo *, SinkAndHoistLICMFlags &,
                           OptimizationRemarkEmitter *);
 
/// Walk the specified region of the CFG (defined by all blocks
/// dominated by the specified block, and that are in the current loop) in depth
/// first order w.r.t the DominatorTree.  This allows us to visit definitions
/// before uses, allowing us to hoist a loop body in one pass without iteration.
/// Takes DomTreeNode, AAResults, LoopInfo, DominatorTree,
/// TargetLibraryInfo, Loop, AliasSet information for all
/// instructions of the loop and loop safety information as arguments.
/// Diagnostics is emitted via \p ORE. It returns changed status.
/// \p AllowSpeculation is whether values should be hoisted even if they are not
/// guaranteed to execute in the loop, but are safe to speculatively execute.
bool hoistRegion(DomTreeNode *, AAResults *, LoopInfo *, DominatorTree *,
                 AssumptionCache *, TargetLibraryInfo *, Loop *,
                 MemorySSAUpdater &, ScalarEvolution *, ICFLoopSafetyInfo *,
                 SinkAndHoistLICMFlags &, OptimizationRemarkEmitter *, bool,
                 bool AllowSpeculation);
 
/// This function deletes dead loops. The caller of this function needs to
/// guarantee that the loop is infact dead.
/// The function requires a bunch or prerequisites to be present:
///   - The loop needs to be in LCSSA form
///   - The loop needs to have a Preheader
///   - A unique dedicated exit block must exist
///
/// This also updates the relevant analysis information in \p DT, \p SE, \p LI
/// and \p MSSA if pointers to those are provided.
/// It also updates the loop PM if an updater struct is provided.
 
void deleteDeadLoop(Loop *L, DominatorTree *DT, ScalarEvolution *SE,
                    LoopInfo *LI, MemorySSA *MSSA = nullptr);
 
/// Remove the backedge of the specified loop.  Handles loop nests and general
/// loop structures subject to the precondition that the loop has no parent
/// loop and has a single latch block.  Preserves all listed analyses.
void breakLoopBackedge(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
                       LoopInfo &LI, MemorySSA *MSSA);
 
/// Try to promote memory values to scalars by sinking stores out of
/// the loop and moving loads to before the loop.  We do this by looping over
/// the stores in the loop, looking for stores to Must pointers which are
/// loop invariant. It takes a set of must-alias values, Loop exit blocks
/// vector, loop exit blocks insertion point vector, PredIteratorCache,
/// LoopInfo, DominatorTree, Loop, AliasSet information for all instructions
/// of the loop and loop safety information as arguments.
/// Diagnostics is emitted via \p ORE. It returns changed status.
/// \p AllowSpeculation is whether values should be hoisted even if they are not
/// guaranteed to execute in the loop, but are safe to speculatively execute.
bool promoteLoopAccessesToScalars(
    const SmallSetVector<Value *, 8> &, SmallVectorImpl<BasicBlock *> &,
    SmallVectorImpl<Instruction *> &, SmallVectorImpl<MemoryAccess *> &,
    PredIteratorCache &, LoopInfo *, DominatorTree *, AssumptionCache *AC,
    const TargetLibraryInfo *, TargetTransformInfo *, Loop *,
    MemorySSAUpdater &, ICFLoopSafetyInfo *, OptimizationRemarkEmitter *,
    bool AllowSpeculation);
 
/// Does a BFS from a given node to all of its children inside a given loop.
/// The returned vector of nodes includes the starting point.
SmallVector<DomTreeNode *, 16> collectChildrenInLoop(DomTreeNode *N,
                                                     const Loop *CurLoop);
 
/// Returns the instructions that use values defined in the loop.
SmallVector<Instruction *, 8> findDefsUsedOutsideOfLoop(Loop *L);
 
/// Find a combination of metadata ("llvm.loop.vectorize.width" and
/// "llvm.loop.vectorize.scalable.enable") for a loop and use it to construct a
/// ElementCount. If the metadata "llvm.loop.vectorize.width" cannot be found
/// then None is returned.
Optional<ElementCount>
getOptionalElementCountLoopAttribute(const Loop *TheLoop);
 
/// Create a new loop identifier for a loop created from a loop transformation.
///
/// @param OrigLoopID The loop ID of the loop before the transformation.
/// @param FollowupAttrs List of attribute names that contain attributes to be
///                      added to the new loop ID.
/// @param InheritOptionsAttrsPrefix Selects which attributes should be inherited
///                                  from the original loop. The following values
///                                  are considered:
///        nullptr   : Inherit all attributes from @p OrigLoopID.
///        ""        : Do not inherit any attribute from @p OrigLoopID; only use
///                    those specified by a followup attribute.
///        "<prefix>": Inherit all attributes except those which start with
///                    <prefix>; commonly used to remove metadata for the
///                    applied transformation.
/// @param AlwaysNew If true, do not try to reuse OrigLoopID and never return
///                  None.
///
/// @return The loop ID for the after-transformation loop. The following values
///         can be returned:
///         None         : No followup attribute was found; it is up to the
///                        transformation to choose attributes that make sense.
///         @p OrigLoopID: The original identifier can be reused.
///         nullptr      : The new loop has no attributes.
///         MDNode*      : A new unique loop identifier.
Optional<MDNode *>
makeFollowupLoopID(MDNode *OrigLoopID, ArrayRef<StringRef> FollowupAttrs,
                   const char *InheritOptionsAttrsPrefix = "",
                   bool AlwaysNew = false);
 
/// Look for the loop attribute that disables all transformation heuristic.
bool hasDisableAllTransformsHint(const Loop *L);
 
/// Look for the loop attribute that disables the LICM transformation heuristics.
bool hasDisableLICMTransformsHint(const Loop *L);
 
/// The mode sets how eager a transformation should be applied.
enum TransformationMode {
  /// The pass can use heuristics to determine whether a transformation should
  /// be applied.
  TM_Unspecified,
 
  /// The transformation should be applied without considering a cost model.
  TM_Enable,
 
  /// The transformation should not be applied.
  TM_Disable,
 
  /// Force is a flag and should not be used alone.
  TM_Force = 0x04,
 
  /// The transformation was directed by the user, e.g. by a #pragma in
  /// the source code. If the transformation could not be applied, a
  /// warning should be emitted.
  TM_ForcedByUser = TM_Enable | TM_Force,
 
  /// The transformation must not be applied. For instance, `#pragma clang loop
  /// unroll(disable)` explicitly forbids any unrolling to take place. Unlike
  /// general loop metadata, it must not be dropped. Most passes should not
  /// behave differently under TM_Disable and TM_SuppressedByUser.
  TM_SuppressedByUser = TM_Disable | TM_Force
};
 
/// @{
/// Get the mode for LLVM's supported loop transformations.
TransformationMode hasUnrollTransformation(const Loop *L);
TransformationMode hasUnrollAndJamTransformation(const Loop *L);
TransformationMode hasVectorizeTransformation(const Loop *L);
TransformationMode hasDistributeTransformation(const Loop *L);
TransformationMode hasLICMVersioningTransformation(const Loop *L);
/// @}
 
/// Set input string into loop metadata by keeping other values intact.
/// If the string is already in loop metadata update value if it is
/// different.
void addStringMetadataToLoop(Loop *TheLoop, const char *MDString,
                             unsigned V = 0);
 
/// Returns a loop's estimated trip count based on branch weight metadata.
/// In addition if \p EstimatedLoopInvocationWeight is not null it is
/// initialized with weight of loop's latch leading to the exit.
/// Returns 0 when the count is estimated to be 0, or None when a meaningful
/// estimate can not be made.
Optional<unsigned>
getLoopEstimatedTripCount(Loop *L,
                          unsigned *EstimatedLoopInvocationWeight = nullptr);
 
/// Set a loop's branch weight metadata to reflect that loop has \p
/// EstimatedTripCount iterations and \p EstimatedLoopInvocationWeight exits
/// through latch. Returns true if metadata is successfully updated, false
/// otherwise. Note that loop must have a latch block which controls loop exit
/// in order to succeed.
bool setLoopEstimatedTripCount(Loop *L, unsigned EstimatedTripCount,
                               unsigned EstimatedLoopInvocationWeight);
 
/// Check inner loop (L) backedge count is known to be invariant on all
/// iterations of its outer loop. If the loop has no parent, this is trivially
/// true.
bool hasIterationCountInvariantInParent(Loop *L, ScalarEvolution &SE);
 
/// Helper to consistently add the set of standard passes to a loop pass's \c
/// AnalysisUsage.
///
/// All loop passes should call this as part of implementing their \c
/// getAnalysisUsage.
void getLoopAnalysisUsage(AnalysisUsage &AU);
 
/// Returns true if is legal to hoist or sink this instruction disregarding the
/// possible introduction of faults.  Reasoning about potential faulting
/// instructions is the responsibility of the caller since it is challenging to
/// do efficiently from within this routine.
/// \p TargetExecutesOncePerLoop is true only when it is guaranteed that the
/// target executes at most once per execution of the loop body.  This is used
/// to assess the legality of duplicating atomic loads.  Generally, this is
/// true when moving out of loop and not true when moving into loops.
/// If \p ORE is set use it to emit optimization remarks.
bool canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
                        Loop *CurLoop, MemorySSAUpdater &MSSAU,
                        bool TargetExecutesOncePerLoop,
                        SinkAndHoistLICMFlags &LICMFlags,
                        OptimizationRemarkEmitter *ORE = nullptr);
 
/// Returns the comparison predicate used when expanding a min/max reduction.
CmpInst::Predicate getMinMaxReductionPredicate(RecurKind RK);
 
/// See RecurrenceDescriptor::isSelectCmpPattern for a description of the
/// pattern we are trying to match. In this pattern we are only ever selecting
/// between two values: 1) an initial PHI start value, and 2) a loop invariant
/// value. This function uses \p LoopExitInst to determine 2), which we then use
/// to select between \p Left and \p Right. Any lane value in \p Left that
/// matches 2) will be merged into \p Right.
Value *createSelectCmpOp(IRBuilderBase &Builder, Value *StartVal, RecurKind RK,
                         Value *Left, Value *Right);
 
/// Returns a Min/Max operation corresponding to MinMaxRecurrenceKind.
/// The Builder's fast-math-flags must be set to propagate the expected values.
Value *createMinMaxOp(IRBuilderBase &Builder, RecurKind RK, Value *Left,
                      Value *Right);
 
/// Generates an ordered vector reduction using extracts to reduce the value.
Value *getOrderedReduction(IRBuilderBase &Builder, Value *Acc, Value *Src,
                           unsigned Op, RecurKind MinMaxKind = RecurKind::None);
 
/// Generates a vector reduction using shufflevectors to reduce the value.
/// Fast-math-flags are propagated using the IRBuilder's setting.
Value *getShuffleReduction(IRBuilderBase &Builder, Value *Src, unsigned Op,
                           RecurKind MinMaxKind = RecurKind::None);
 
/// Create a target reduction of the given vector. The reduction operation
/// is described by the \p Opcode parameter. min/max reductions require
/// additional information supplied in \p RdxKind.
/// The target is queried to determine if intrinsics or shuffle sequences are
/// required to implement the reduction.
/// Fast-math-flags are propagated using the IRBuilder's setting.
Value *createSimpleTargetReduction(IRBuilderBase &B,
                                   const TargetTransformInfo *TTI, Value *Src,
                                   RecurKind RdxKind);
 
/// Create a target reduction of the given vector \p Src for a reduction of the
/// kind RecurKind::SelectICmp or RecurKind::SelectFCmp. The reduction operation
/// is described by \p Desc.
Value *createSelectCmpTargetReduction(IRBuilderBase &B,
                                      const TargetTransformInfo *TTI,
                                      Value *Src,
                                      const RecurrenceDescriptor &Desc,
                                      PHINode *OrigPhi);
 
/// Create a generic target reduction using a recurrence descriptor \p Desc
/// The target is queried to determine if intrinsics or shuffle sequences are
/// required to implement the reduction.
/// Fast-math-flags are propagated using the RecurrenceDescriptor.
Value *createTargetReduction(IRBuilderBase &B, const TargetTransformInfo *TTI,
                             const RecurrenceDescriptor &Desc, Value *Src,
                             PHINode *OrigPhi = nullptr);
 
/// Create an ordered reduction intrinsic using the given recurrence
/// descriptor \p Desc.
Value *createOrderedReduction(IRBuilderBase &B,
                              const RecurrenceDescriptor &Desc, Value *Src,
                              Value *Start);
 
/// Get the intersection (logical and) of all of the potential IR flags
/// of each scalar operation (VL) that will be converted into a vector (I).
/// If OpValue is non-null, we only consider operations similar to OpValue
/// when intersecting.
/// Flag set: NSW, NUW (if IncludeWrapFlags is true), exact, and all of
/// fast-math.
void propagateIRFlags(Value *I, ArrayRef<Value *> VL, Value *OpValue = nullptr,
                      bool IncludeWrapFlags = true);
 
/// Returns true if we can prove that \p S is defined and always negative in
/// loop \p L.
bool isKnownNegativeInLoop(const SCEV *S, const Loop *L, ScalarEvolution &SE);
 
/// Returns true if we can prove that \p S is defined and always non-negative in
/// loop \p L.
bool isKnownNonNegativeInLoop(const SCEV *S, const Loop *L,
                              ScalarEvolution &SE);
 
/// Returns true if \p S is defined and never is equal to signed/unsigned max.
bool cannotBeMaxInLoop(const SCEV *S, const Loop *L, ScalarEvolution &SE,
                       bool Signed);
 
/// Returns true if \p S is defined and never is equal to signed/unsigned min.
bool cannotBeMinInLoop(const SCEV *S, const Loop *L, ScalarEvolution &SE,
                       bool Signed);
 
enum ReplaceExitVal {
  NeverRepl,
  OnlyCheapRepl,
  NoHardUse,
  UnusedIndVarInLoop,
  AlwaysRepl
};
 
/// If the final value of any expressions that are recurrent in the loop can
/// be computed, substitute the exit values from the loop into any instructions
/// outside of the loop that use the final values of the current expressions.
/// Return the number of loop exit values that have been replaced, and the
/// corresponding phi node will be added to DeadInsts.
int rewriteLoopExitValues(Loop *L, LoopInfo *LI, TargetLibraryInfo *TLI,
                          ScalarEvolution *SE, const TargetTransformInfo *TTI,
                          SCEVExpander &Rewriter, DominatorTree *DT,
                          ReplaceExitVal ReplaceExitValue,
                          SmallVector<WeakTrackingVH, 16> &DeadInsts);
 
/// Set weights for \p UnrolledLoop and \p RemainderLoop based on weights for
/// \p OrigLoop and the following distribution of \p OrigLoop iteration among \p
/// UnrolledLoop and \p RemainderLoop. \p UnrolledLoop receives weights that
/// reflect TC/UF iterations, and \p RemainderLoop receives weights that reflect
/// the remaining TC%UF iterations.
///
/// Note that \p OrigLoop may be equal to either \p UnrolledLoop or \p
/// RemainderLoop in which case weights for \p OrigLoop are updated accordingly.
/// Note also behavior is undefined if \p UnrolledLoop and \p RemainderLoop are
/// equal. \p UF must be greater than zero.
/// If \p OrigLoop has no profile info associated nothing happens.
///
/// This utility may be useful for such optimizations as unroller and
/// vectorizer as it's typical transformation for them.
void setProfileInfoAfterUnrolling(Loop *OrigLoop, Loop *UnrolledLoop,
                                  Loop *RemainderLoop, uint64_t UF);
 
/// Utility that implements appending of loops onto a worklist given a range.
/// We want to process loops in postorder, but the worklist is a LIFO data
/// structure, so we append to it in *reverse* postorder.
/// For trees, a preorder traversal is a viable reverse postorder, so we
/// actually append using a preorder walk algorithm.
template <typename RangeT>
void appendLoopsToWorklist(RangeT &&, SmallPriorityWorklist<Loop *, 4> &);
/// Utility that implements appending of loops onto a worklist given a range.
/// It has the same behavior as appendLoopsToWorklist, but assumes the range of
/// loops has already been reversed, so it processes loops in the given order.
template <typename RangeT>
void appendReversedLoopsToWorklist(RangeT &&,
                                   SmallPriorityWorklist<Loop *, 4> &);
 
/// Utility that implements appending of loops onto a worklist given LoopInfo.
/// Calls the templated utility taking a Range of loops, handing it the Loops
/// in LoopInfo, iterated in reverse. This is because the loops are stored in
/// RPO w.r.t. the control flow graph in LoopInfo. For the purpose of unrolling,
/// loop deletion, and LICM, we largely want to work forward across the CFG so
/// that we visit defs before uses and can propagate simplifications from one
/// loop nest into the next. Calls appendReversedLoopsToWorklist with the
/// already reversed loops in LI.
/// FIXME: Consider changing the order in LoopInfo.
void appendLoopsToWorklist(LoopInfo &, SmallPriorityWorklist<Loop *, 4> &);
 
/// Recursively clone the specified loop and all of its children,
/// mapping the blocks with the specified map.
Loop *cloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM,
                LoopInfo *LI, LPPassManager *LPM);
 
/// Add code that checks at runtime if the accessed arrays in \p PointerChecks
/// overlap. Returns the final comparator value or NULL if no check is needed.
Value *
addRuntimeChecks(Instruction *Loc, Loop *TheLoop,
                 const SmallVectorImpl<RuntimePointerCheck> &PointerChecks,
                 SCEVExpander &Expander);
 
Value *addDiffRuntimeChecks(
    Instruction *Loc, ArrayRef<PointerDiffInfo> Checks, SCEVExpander &Expander,
    function_ref<Value *(IRBuilderBase &, unsigned)> GetVF, unsigned IC);
 
/// Struct to hold information about a partially invariant condition.
struct IVConditionInfo {
  /// Instructions that need to be duplicated and checked for the unswitching
  /// condition.
  SmallVector<Instruction *> InstToDuplicate;
 
  /// Constant to indicate for which value the condition is invariant.
  Constant *KnownValue = nullptr;
 
  /// True if the partially invariant path is no-op (=does not have any
  /// side-effects and no loop value is used outside the loop).
  bool PathIsNoop = true;
 
  /// If the partially invariant path reaches a single exit block, ExitForPath
  /// is set to that block. Otherwise it is nullptr.
  BasicBlock *ExitForPath = nullptr;
};
 
/// Check if the loop header has a conditional branch that is not
/// loop-invariant, because it involves load instructions. If all paths from
/// either the true or false successor to the header or loop exists do not
/// modify the memory feeding the condition, perform 'partial unswitching'. That
/// is, duplicate the instructions feeding the condition in the pre-header. Then
/// unswitch on the duplicated condition. The condition is now known in the
/// unswitched version for the 'invariant' path through the original loop.
///
/// If the branch condition of the header is partially invariant, return a pair
/// containing the instructions to duplicate and a boolean Constant to update
/// the condition in the loops created for the true or false successors.
Optional<IVConditionInfo> hasPartialIVCondition(const Loop &L,
                                                unsigned MSSAThreshold,
                                                const MemorySSA &MSSA,
                                                AAResults &AA);
 
} // end namespace llvm
 
#endif // LLVM_TRANSFORMS_UTILS_LOOPUTILS_H