//===- llvm/Analysis/LoopInfo.h - Natural Loop Calculator -------*- 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 the LoopInfo class that is used to identify natural loops
// and determine the loop depth of various nodes of the CFG. A natural loop
// has exactly one entry-point, which is called the header. Note that natural
// loops may actually be several loops that share the same header node.
//
// This analysis calculates the nesting structure of loops in a function. For
// each natural loop identified, this analysis identifies natural loops
// contained entirely within the loop and the basic blocks the make up the loop.
//
// It can calculate on the fly various bits of information, for example:
//
// * whether there is a preheader for the loop
// * the number of back edges to the header
// * whether or not a particular block branches out of the loop
// * the successor blocks of the loop
// * the loop depth
// * etc...
//
// Note that this analysis specifically identifies *Loops* not cycles or SCCs
// in the CFG. There can be strongly connected components in the CFG which
// this analysis will not recognize and that will not be represented by a Loop
// instance. In particular, a Loop might be inside such a non-loop SCC, or a
// non-loop SCC might contain a sub-SCC which is a Loop.
//
// For an overview of terminology used in this API (and thus all of our loop
// analyses or transforms), see docs/LoopTerminology.rst.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_ANALYSIS_LOOPINFO_H
#define LLVM_ANALYSIS_LOOPINFO_H
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/GraphTraits.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PassManager.h"
#include "llvm/Pass.h"
#include "llvm/Support/Allocator.h"
#include <algorithm>
#include <optional>
#include <utility>
namespace llvm {
class DominatorTree;
class InductionDescriptor;
class Instruction;
class LoopInfo;
class Loop;
class MDNode;
class MemorySSAUpdater;
class ScalarEvolution;
class raw_ostream;
template <class N, bool IsPostDom> class DominatorTreeBase;
template <class N, class M> class LoopInfoBase;
template <class N, class M> class LoopBase;
//===----------------------------------------------------------------------===//
/// Instances of this class are used to represent loops that are detected in the
/// flow graph.
///
template <class BlockT, class LoopT> class LoopBase {
LoopT *ParentLoop;
// Loops contained entirely within this one.
std::vector<LoopT *> SubLoops;
// The list of blocks in this loop. First entry is the header node.
std::vector<BlockT *> Blocks;
SmallPtrSet<const BlockT *, 8> DenseBlockSet;
#if LLVM_ENABLE_ABI_BREAKING_CHECKS
/// Indicator that this loop is no longer a valid loop.
bool IsInvalid = false;
#endif
LoopBase(const LoopBase<BlockT, LoopT> &) = delete;
const LoopBase<BlockT, LoopT> &
operator=(const LoopBase<BlockT, LoopT> &) = delete;
public:
/// Return the nesting level of this loop. An outer-most loop has depth 1,
/// for consistency with loop depth values used for basic blocks, where depth
/// 0 is used for blocks not inside any loops.
unsigned getLoopDepth() const {
assert(!isInvalid() && "Loop not in a valid state!");
unsigned D = 1;
for (const LoopT *CurLoop = ParentLoop; CurLoop;
CurLoop = CurLoop->ParentLoop)
++D;
return D;
}
BlockT *getHeader() const { return getBlocks().front(); }
/// Return the parent loop if it exists or nullptr for top
/// level loops.
/// A loop is either top-level in a function (that is, it is not
/// contained in any other loop) or it is entirely enclosed in
/// some other loop.
/// If a loop is top-level, it has no parent, otherwise its
/// parent is the innermost loop in which it is enclosed.
LoopT *getParentLoop() const { return ParentLoop; }
/// Get the outermost loop in which this loop is contained.
/// This may be the loop itself, if it already is the outermost loop.
const LoopT *getOutermostLoop() const {
const LoopT *L = static_cast<const LoopT *>(this);
while (L->ParentLoop)
L = L->ParentLoop;
return L;
}
LoopT *getOutermostLoop() {
LoopT *L = static_cast<LoopT *>(this);
while (L->ParentLoop)
L = L->ParentLoop;
return L;
}
/// This is a raw interface for bypassing addChildLoop.
void setParentLoop(LoopT *L) {
assert(!isInvalid() && "Loop not in a valid state!");
ParentLoop = L;
}
/// Return true if the specified loop is contained within in this loop.
bool contains(const LoopT *L) const {
assert(!isInvalid() && "Loop not in a valid state!");
if (L == this)
return true;
if (!L)
return false;
return contains(L->getParentLoop());
}
/// Return true if the specified basic block is in this loop.
bool contains(const BlockT *BB) const {
assert(!isInvalid() && "Loop not in a valid state!");
return DenseBlockSet.count(BB);
}
/// Return true if the specified instruction is in this loop.
template <class InstT> bool contains(const InstT *Inst) const {
return contains(Inst->getParent());
}
/// Return the loops contained entirely within this loop.
const std::vector<LoopT *> &getSubLoops() const {
assert(!isInvalid() && "Loop not in a valid state!");
return SubLoops;
}
std::vector<LoopT *> &getSubLoopsVector() {
assert(!isInvalid() && "Loop not in a valid state!");
return SubLoops;
}
typedef typename std::vector<LoopT *>::const_iterator iterator;
typedef
typename std::vector<LoopT *>::const_reverse_iterator reverse_iterator;
iterator begin() const { return getSubLoops().begin(); }
iterator end() const { return getSubLoops().end(); }
reverse_iterator rbegin() const { return getSubLoops().rbegin(); }
reverse_iterator rend() const { return getSubLoops().rend(); }
// LoopInfo does not detect irreducible control flow, just natural
// loops. That is, it is possible that there is cyclic control
// flow within the "innermost loop" or around the "outermost
// loop".
/// Return true if the loop does not contain any (natural) loops.
bool isInnermost() const { return getSubLoops().empty(); }
/// Return true if the loop does not have a parent (natural) loop
// (i.e. it is outermost, which is the same as top-level).
bool isOutermost() const { return getParentLoop() == nullptr; }
/// Get a list of the basic blocks which make up this loop.
ArrayRef<BlockT *> getBlocks() const {
assert(!isInvalid() && "Loop not in a valid state!");
return Blocks;
}
typedef typename ArrayRef<BlockT *>::const_iterator block_iterator;
block_iterator block_begin() const { return getBlocks().begin(); }
block_iterator block_end() const { return getBlocks().end(); }
inline iterator_range<block_iterator> blocks() const {
assert(!isInvalid() && "Loop not in a valid state!");
return make_range(block_begin(), block_end());
}
/// Get the number of blocks in this loop in constant time.
/// Invalidate the loop, indicating that it is no longer a loop.
unsigned getNumBlocks() const {
assert(!isInvalid() && "Loop not in a valid state!");
return Blocks.size();
}
/// Return a direct, mutable handle to the blocks vector so that we can
/// mutate it efficiently with techniques like `std::remove`.
std::vector<BlockT *> &getBlocksVector() {
assert(!isInvalid() && "Loop not in a valid state!");
return Blocks;
}
/// Return a direct, mutable handle to the blocks set so that we can
/// mutate it efficiently.
SmallPtrSetImpl<const BlockT *> &getBlocksSet() {
assert(!isInvalid() && "Loop not in a valid state!");
return DenseBlockSet;
}
/// Return a direct, immutable handle to the blocks set.
const SmallPtrSetImpl<const BlockT *> &getBlocksSet() const {
assert(!isInvalid() && "Loop not in a valid state!");
return DenseBlockSet;
}
/// Return true if this loop is no longer valid. The only valid use of this
/// helper is "assert(L.isInvalid())" or equivalent, since IsInvalid is set to
/// true by the destructor. In other words, if this accessor returns true,
/// the caller has already triggered UB by calling this accessor; and so it
/// can only be called in a context where a return value of true indicates a
/// programmer error.
bool isInvalid() const {
#if LLVM_ENABLE_ABI_BREAKING_CHECKS
return IsInvalid;
#else
return false;
#endif
}
/// True if terminator in the block can branch to another block that is
/// outside of the current loop. \p BB must be inside the loop.
bool isLoopExiting(const BlockT *BB) const {
assert(!isInvalid() && "Loop not in a valid state!");
assert(contains(BB) && "Exiting block must be part of the loop");
for (const auto *Succ : children<const BlockT *>(BB)) {
if (!contains(Succ))
return true;
}
return false;
}
/// Returns true if \p BB is a loop-latch.
/// A latch block is a block that contains a branch back to the header.
/// This function is useful when there are multiple latches in a loop
/// because \fn getLoopLatch will return nullptr in that case.
bool isLoopLatch(const BlockT *BB) const {
assert(!isInvalid() && "Loop not in a valid state!");
assert(contains(BB) && "block does not belong to the loop");
BlockT *Header = getHeader();
auto PredBegin = GraphTraits<Inverse<BlockT *>>::child_begin(Header);
auto PredEnd = GraphTraits<Inverse<BlockT *>>::child_end(Header);
return std::find(PredBegin, PredEnd, BB) != PredEnd;
}
/// Calculate the number of back edges to the loop header.
unsigned getNumBackEdges() const {
assert(!isInvalid() && "Loop not in a valid state!");
unsigned NumBackEdges = 0;
BlockT *H = getHeader();
for (const auto Pred : children<Inverse<BlockT *>>(H))
if (contains(Pred))
++NumBackEdges;
return NumBackEdges;
}
//===--------------------------------------------------------------------===//
// APIs for simple analysis of the loop.
//
// Note that all of these methods can fail on general loops (ie, there may not
// be a preheader, etc). For best success, the loop simplification and
// induction variable canonicalization pass should be used to normalize loops
// for easy analysis. These methods assume canonical loops.
/// Return all blocks inside the loop that have successors outside of the
/// loop. These are the blocks _inside of the current loop_ which branch out.
/// The returned list is always unique.
void getExitingBlocks(SmallVectorImpl<BlockT *> &ExitingBlocks) const;
/// If getExitingBlocks would return exactly one block, return that block.
/// Otherwise return null.
BlockT *getExitingBlock() const;
/// Return all of the successor blocks of this loop. These are the blocks
/// _outside of the current loop_ which are branched to.
void getExitBlocks(SmallVectorImpl<BlockT *> &ExitBlocks) const;
/// If getExitBlocks would return exactly one block, return that block.
/// Otherwise return null.
BlockT *getExitBlock() const;
/// Return true if no exit block for the loop has a predecessor that is
/// outside the loop.
bool hasDedicatedExits() const;
/// Return all unique successor blocks of this loop.
/// These are the blocks _outside of the current loop_ which are branched to.
void getUniqueExitBlocks(SmallVectorImpl<BlockT *> &ExitBlocks) const;
/// Return all unique successor blocks of this loop except successors from
/// Latch block are not considered. If the exit comes from Latch has also
/// non Latch predecessor in a loop it will be added to ExitBlocks.
/// These are the blocks _outside of the current loop_ which are branched to.
void getUniqueNonLatchExitBlocks(SmallVectorImpl<BlockT *> &ExitBlocks) const;
/// If getUniqueExitBlocks would return exactly one block, return that block.
/// Otherwise return null.
BlockT *getUniqueExitBlock() const;
/// Return true if this loop does not have any exit blocks.
bool hasNoExitBlocks() const;
/// Edge type.
typedef std::pair<BlockT *, BlockT *> Edge;
/// Return all pairs of (_inside_block_,_outside_block_).
void getExitEdges(SmallVectorImpl<Edge> &ExitEdges) const;
/// If there is a preheader for this loop, return it. A loop has a preheader
/// if there is only one edge to the header of the loop from outside of the
/// loop. If this is the case, the block branching to the header of the loop
/// is the preheader node.
///
/// This method returns null if there is no preheader for the loop.
BlockT *getLoopPreheader() const;
/// If the given loop's header has exactly one unique predecessor outside the
/// loop, return it. Otherwise return null.
/// This is less strict that the loop "preheader" concept, which requires
/// the predecessor to have exactly one successor.
BlockT *getLoopPredecessor() const;
/// If there is a single latch block for this loop, return it.
/// A latch block is a block that contains a branch back to the header.
BlockT *getLoopLatch() const;
/// Return all loop latch blocks of this loop. A latch block is a block that
/// contains a branch back to the header.
void getLoopLatches(SmallVectorImpl<BlockT *> &LoopLatches) const {
assert(!isInvalid() && "Loop not in a valid state!");
BlockT *H = getHeader();
for (const auto Pred : children<Inverse<BlockT *>>(H))
if (contains(Pred))
LoopLatches.push_back(Pred);
}
/// Return all inner loops in the loop nest rooted by the loop in preorder,
/// with siblings in forward program order.
template <class Type>
static void getInnerLoopsInPreorder(const LoopT &L,
SmallVectorImpl<Type> &PreOrderLoops) {
SmallVector<LoopT *, 4> PreOrderWorklist;
PreOrderWorklist.append(L.rbegin(), L.rend());
while (!PreOrderWorklist.empty()) {
LoopT *L = PreOrderWorklist.pop_back_val();
// Sub-loops are stored in forward program order, but will process the
// worklist backwards so append them in reverse order.
PreOrderWorklist.append(L->rbegin(), L->rend());
PreOrderLoops.push_back(L);
}
}
/// Return all loops in the loop nest rooted by the loop in preorder, with
/// siblings in forward program order.
SmallVector<const LoopT *, 4> getLoopsInPreorder() const {
SmallVector<const LoopT *, 4> PreOrderLoops;
const LoopT *CurLoop = static_cast<const LoopT *>(this);
PreOrderLoops.push_back(CurLoop);
getInnerLoopsInPreorder(*CurLoop, PreOrderLoops);
return PreOrderLoops;
}
SmallVector<LoopT *, 4> getLoopsInPreorder() {
SmallVector<LoopT *, 4> PreOrderLoops;
LoopT *CurLoop = static_cast<LoopT *>(this);
PreOrderLoops.push_back(CurLoop);
getInnerLoopsInPreorder(*CurLoop, PreOrderLoops);
return PreOrderLoops;
}
//===--------------------------------------------------------------------===//
// APIs for updating loop information after changing the CFG
//
/// This method is used by other analyses to update loop information.
/// NewBB is set to be a new member of the current loop.
/// Because of this, it is added as a member of all parent loops, and is added
/// to the specified LoopInfo object as being in the current basic block. It
/// is not valid to replace the loop header with this method.
void addBasicBlockToLoop(BlockT *NewBB, LoopInfoBase<BlockT, LoopT> &LI);
/// This is used when splitting loops up. It replaces the OldChild entry in
/// our children list with NewChild, and updates the parent pointer of
/// OldChild to be null and the NewChild to be this loop.
/// This updates the loop depth of the new child.
void replaceChildLoopWith(LoopT *OldChild, LoopT *NewChild);
/// Add the specified loop to be a child of this loop.
/// This updates the loop depth of the new child.
void addChildLoop(LoopT *NewChild) {
assert(!isInvalid() && "Loop not in a valid state!");
assert(!NewChild->ParentLoop && "NewChild already has a parent!");
NewChild->ParentLoop = static_cast<LoopT *>(this);
SubLoops.push_back(NewChild);
}
/// This removes the specified child from being a subloop of this loop. The
/// loop is not deleted, as it will presumably be inserted into another loop.
LoopT *removeChildLoop(iterator I) {
assert(!isInvalid() && "Loop not in a valid state!");
assert(I != SubLoops.end() && "Cannot remove end iterator!");
LoopT *Child = *I;
assert(Child->ParentLoop == this && "Child is not a child of this loop!");
SubLoops.erase(SubLoops.begin() + (I - begin()));
Child->ParentLoop = nullptr;
return Child;
}
/// This removes the specified child from being a subloop of this loop. The
/// loop is not deleted, as it will presumably be inserted into another loop.
LoopT *removeChildLoop(LoopT *Child) {
return removeChildLoop(llvm::find(*this, Child));
}
/// This adds a basic block directly to the basic block list.
/// This should only be used by transformations that create new loops. Other
/// transformations should use addBasicBlockToLoop.
void addBlockEntry(BlockT *BB) {
assert(!isInvalid() && "Loop not in a valid state!");
Blocks.push_back(BB);
DenseBlockSet.insert(BB);
}
/// interface to reverse Blocks[from, end of loop] in this loop
void reverseBlock(unsigned from) {
assert(!isInvalid() && "Loop not in a valid state!");
std::reverse(Blocks.begin() + from, Blocks.end());
}
/// interface to do reserve() for Blocks
void reserveBlocks(unsigned size) {
assert(!isInvalid() && "Loop not in a valid state!");
Blocks.reserve(size);
}
/// This method is used to move BB (which must be part of this loop) to be the
/// loop header of the loop (the block that dominates all others).
void moveToHeader(BlockT *BB) {
assert(!isInvalid() && "Loop not in a valid state!");
if (Blocks[0] == BB)
return;
for (unsigned i = 0;; ++i) {
assert(i != Blocks.size() && "Loop does not contain BB!");
if (Blocks[i] == BB) {
Blocks[i] = Blocks[0];
Blocks[0] = BB;
return;
}
}
}
/// This removes the specified basic block from the current loop, updating the
/// Blocks as appropriate. This does not update the mapping in the LoopInfo
/// class.
void removeBlockFromLoop(BlockT *BB) {
assert(!isInvalid() && "Loop not in a valid state!");
auto I = find(Blocks, BB);
assert(I != Blocks.end() && "N is not in this list!");
Blocks.erase(I);
DenseBlockSet.erase(BB);
}
/// Verify loop structure
void verifyLoop() const;
/// Verify loop structure of this loop and all nested loops.
void verifyLoopNest(DenseSet<const LoopT *> *Loops) const;
/// Returns true if the loop is annotated parallel.
///
/// Derived classes can override this method using static template
/// polymorphism.
bool isAnnotatedParallel() const { return false; }
/// Print loop with all the BBs inside it.
void print(raw_ostream &OS, bool Verbose = false, bool PrintNested = true,
unsigned Depth = 0) const;
protected:
friend class LoopInfoBase<BlockT, LoopT>;
/// This creates an empty loop.
LoopBase() : ParentLoop(nullptr) {}
explicit LoopBase(BlockT *BB) : ParentLoop(nullptr) {
Blocks.push_back(BB);
DenseBlockSet.insert(BB);
}
// Since loop passes like SCEV are allowed to key analysis results off of
// `Loop` pointers, we cannot re-use pointers within a loop pass manager.
// This means loop passes should not be `delete` ing `Loop` objects directly
// (and risk a later `Loop` allocation re-using the address of a previous one)
// but should be using LoopInfo::markAsRemoved, which keeps around the `Loop`
// pointer till the end of the lifetime of the `LoopInfo` object.
//
// To make it easier to follow this rule, we mark the destructor as
// non-public.
~LoopBase() {
for (auto *SubLoop : SubLoops)
SubLoop->~LoopT();
#if LLVM_ENABLE_ABI_BREAKING_CHECKS
IsInvalid = true;
#endif
SubLoops.clear();
Blocks.clear();
DenseBlockSet.clear();
ParentLoop = nullptr;
}
};
template <class BlockT, class LoopT>
raw_ostream &operator<<(raw_ostream &OS, const LoopBase<BlockT, LoopT> &Loop) {
Loop.print(OS);
return OS;
}
// Implementation in LoopInfoImpl.h
extern template class LoopBase<BasicBlock, Loop>;
/// Represents a single loop in the control flow graph. Note that not all SCCs
/// in the CFG are necessarily loops.
class LLVM_EXTERNAL_VISIBILITY Loop : public LoopBase<BasicBlock, Loop> {
public:
/// A range representing the start and end location of a loop.
class LocRange {
DebugLoc Start;
DebugLoc End;
public:
LocRange() = default;
LocRange(DebugLoc Start) : Start(Start), End(Start) {}
LocRange(DebugLoc Start, DebugLoc End)
: Start(std::move(Start)), End(std::move(End)) {}
const DebugLoc &getStart() const { return Start; }
const DebugLoc &getEnd() const { return End; }
/// Check for null.
///
explicit operator bool() const { return Start && End; }
};
/// Return true if the specified value is loop invariant.
bool isLoopInvariant(const Value *V) const;
/// Return true if all the operands of the specified instruction are loop
/// invariant.
bool hasLoopInvariantOperands(const Instruction *I) const;
/// If the given value is an instruction inside of the loop and it can be
/// hoisted, do so to make it trivially loop-invariant.
/// Return true if \c V is already loop-invariant, and false if \c V can't
/// be made loop-invariant. If \c V is made loop-invariant, \c Changed is
/// set to true. This function can be used as a slightly more aggressive
/// replacement for isLoopInvariant.
///
/// If InsertPt is specified, it is the point to hoist instructions to.
/// If null, the terminator of the loop preheader is used.
///
bool makeLoopInvariant(Value *V, bool &Changed,
Instruction *InsertPt = nullptr,
MemorySSAUpdater *MSSAU = nullptr,
ScalarEvolution *SE = nullptr) const;
/// If the given instruction is inside of the loop and it can be hoisted, do
/// so to make it trivially loop-invariant.
/// Return true if \c I is already loop-invariant, and false if \c I can't
/// be made loop-invariant. If \c I is made loop-invariant, \c Changed is
/// set to true. This function can be used as a slightly more aggressive
/// replacement for isLoopInvariant.
///
/// If InsertPt is specified, it is the point to hoist instructions to.
/// If null, the terminator of the loop preheader is used.
///
bool makeLoopInvariant(Instruction *I, bool &Changed,
Instruction *InsertPt = nullptr,
MemorySSAUpdater *MSSAU = nullptr,
ScalarEvolution *SE = nullptr) const;
/// Check to see if the loop has a canonical induction variable: an integer
/// recurrence that starts at 0 and increments by one each time through the
/// loop. If so, return the phi node that corresponds to it.
///
/// The IndVarSimplify pass transforms loops to have a canonical induction
/// variable.
///
PHINode *getCanonicalInductionVariable() const;
/// Get the latch condition instruction.
ICmpInst *getLatchCmpInst() const;
/// Obtain the unique incoming and back edge. Return false if they are
/// non-unique or the loop is dead; otherwise, return true.
bool getIncomingAndBackEdge(BasicBlock *&Incoming,
BasicBlock *&Backedge) const;
/// Below are some utilities to get the loop guard, loop bounds and induction
/// variable, and to check if a given phinode is an auxiliary induction
/// variable, if the loop is guarded, and if the loop is canonical.
///
/// Here is an example:
/// \code
/// for (int i = lb; i < ub; i+=step)
/// <loop body>
/// --- pseudo LLVMIR ---
/// beforeloop:
/// guardcmp = (lb < ub)
/// if (guardcmp) goto preheader; else goto afterloop
/// preheader:
/// loop:
/// i_1 = phi[{lb, preheader}, {i_2, latch}]
/// <loop body>
/// i_2 = i_1 + step
/// latch:
/// cmp = (i_2 < ub)
/// if (cmp) goto loop
/// exit:
/// afterloop:
/// \endcode
///
/// - getBounds
/// - getInitialIVValue --> lb
/// - getStepInst --> i_2 = i_1 + step
/// - getStepValue --> step
/// - getFinalIVValue --> ub
/// - getCanonicalPredicate --> '<'
/// - getDirection --> Increasing
///
/// - getInductionVariable --> i_1
/// - isAuxiliaryInductionVariable(x) --> true if x == i_1
/// - getLoopGuardBranch()
/// --> `if (guardcmp) goto preheader; else goto afterloop`
/// - isGuarded() --> true
/// - isCanonical --> false
struct LoopBounds {
/// Return the LoopBounds object if
/// - the given \p IndVar is an induction variable
/// - the initial value of the induction variable can be found
/// - the step instruction of the induction variable can be found
/// - the final value of the induction variable can be found
///
/// Else None.
static std::optional<Loop::LoopBounds>
getBounds(const Loop &L, PHINode &IndVar, ScalarEvolution &SE);
/// Get the initial value of the loop induction variable.
Value &getInitialIVValue() const { return InitialIVValue; }
/// Get the instruction that updates the loop induction variable.
Instruction &getStepInst() const { return StepInst; }
/// Get the step that the loop induction variable gets updated by in each
/// loop iteration. Return nullptr if not found.
Value *getStepValue() const { return StepValue; }
/// Get the final value of the loop induction variable.
Value &getFinalIVValue() const { return FinalIVValue; }
/// Return the canonical predicate for the latch compare instruction, if
/// able to be calcuated. Else BAD_ICMP_PREDICATE.
///
/// A predicate is considered as canonical if requirements below are all
/// satisfied:
/// 1. The first successor of the latch branch is the loop header
/// If not, inverse the predicate.
/// 2. One of the operands of the latch comparison is StepInst
/// If not, and
/// - if the current calcuated predicate is not ne or eq, flip the
/// predicate.
/// - else if the loop is increasing, return slt
/// (notice that it is safe to change from ne or eq to sign compare)
/// - else if the loop is decreasing, return sgt
/// (notice that it is safe to change from ne or eq to sign compare)
///
/// Here is an example when both (1) and (2) are not satisfied:
/// \code
/// loop.header:
/// %iv = phi [%initialiv, %loop.preheader], [%inc, %loop.header]
/// %inc = add %iv, %step
/// %cmp = slt %iv, %finaliv
/// br %cmp, %loop.exit, %loop.header
/// loop.exit:
/// \endcode
/// - The second successor of the latch branch is the loop header instead
/// of the first successor (slt -> sge)
/// - The first operand of the latch comparison (%cmp) is the IndVar (%iv)
/// instead of the StepInst (%inc) (sge -> sgt)
///
/// The predicate would be sgt if both (1) and (2) are satisfied.
/// getCanonicalPredicate() returns sgt for this example.
/// Note: The IR is not changed.
ICmpInst::Predicate getCanonicalPredicate() const;
/// An enum for the direction of the loop
/// - for (int i = 0; i < ub; ++i) --> Increasing
/// - for (int i = ub; i > 0; --i) --> Descresing
/// - for (int i = x; i != y; i+=z) --> Unknown
enum class Direction { Increasing, Decreasing, Unknown };
/// Get the direction of the loop.
Direction getDirection() const;
private:
LoopBounds(const Loop &Loop, Value &I, Instruction &SI, Value *SV, Value &F,
ScalarEvolution &SE)
: L(Loop), InitialIVValue(I), StepInst(SI), StepValue(SV),
FinalIVValue(F), SE(SE) {}
const Loop &L;
// The initial value of the loop induction variable
Value &InitialIVValue;
// The instruction that updates the loop induction variable
Instruction &StepInst;
// The value that the loop induction variable gets updated by in each loop
// iteration
Value *StepValue;
// The final value of the loop induction variable
Value &FinalIVValue;
ScalarEvolution &SE;
};
/// Return the struct LoopBounds collected if all struct members are found,
/// else std::nullopt.
std::optional<LoopBounds> getBounds(ScalarEvolution &SE) const;
/// Return the loop induction variable if found, else return nullptr.
/// An instruction is considered as the loop induction variable if
/// - it is an induction variable of the loop; and
/// - it is used to determine the condition of the branch in the loop latch
///
/// Note: the induction variable doesn't need to be canonical, i.e. starts at
/// zero and increments by one each time through the loop (but it can be).
PHINode *getInductionVariable(ScalarEvolution &SE) const;
/// Get the loop induction descriptor for the loop induction variable. Return
/// true if the loop induction variable is found.
bool getInductionDescriptor(ScalarEvolution &SE,
InductionDescriptor &IndDesc) const;
/// Return true if the given PHINode \p AuxIndVar is
/// - in the loop header
/// - not used outside of the loop
/// - incremented by a loop invariant step for each loop iteration
/// - step instruction opcode should be add or sub
/// Note: auxiliary induction variable is not required to be used in the
/// conditional branch in the loop latch. (but it can be)
bool isAuxiliaryInductionVariable(PHINode &AuxIndVar,
ScalarEvolution &SE) const;
/// Return the loop guard branch, if it exists.
///
/// This currently only works on simplified loop, as it requires a preheader
/// and a latch to identify the guard. It will work on loops of the form:
/// \code
/// GuardBB:
/// br cond1, Preheader, ExitSucc <== GuardBranch
/// Preheader:
/// br Header
/// Header:
/// ...
/// br Latch
/// Latch:
/// br cond2, Header, ExitBlock
/// ExitBlock:
/// br ExitSucc
/// ExitSucc:
/// \endcode
BranchInst *getLoopGuardBranch() const;
/// Return true iff the loop is
/// - in simplify rotated form, and
/// - guarded by a loop guard branch.
bool isGuarded() const { return (getLoopGuardBranch() != nullptr); }
/// Return true if the loop is in rotated form.
///
/// This does not check if the loop was rotated by loop rotation, instead it
/// only checks if the loop is in rotated form (has a valid latch that exists
/// the loop).
bool isRotatedForm() const {
assert(!isInvalid() && "Loop not in a valid state!");
BasicBlock *Latch = getLoopLatch();
return Latch && isLoopExiting(Latch);
}
/// Return true if the loop induction variable starts at zero and increments
/// by one each time through the loop.
bool isCanonical(ScalarEvolution &SE) const;
/// Return true if the Loop is in LCSSA form. If \p IgnoreTokens is set to
/// true, token values defined inside loop are allowed to violate LCSSA form.
bool isLCSSAForm(const DominatorTree &DT, bool IgnoreTokens = true) const;
/// Return true if this Loop and all inner subloops are in LCSSA form. If \p
/// IgnoreTokens is set to true, token values defined inside loop are allowed
/// to violate LCSSA form.
bool isRecursivelyLCSSAForm(const DominatorTree &DT, const LoopInfo &LI,
bool IgnoreTokens = true) const;
/// Return true if the Loop is in the form that the LoopSimplify form
/// transforms loops to, which is sometimes called normal form.
bool isLoopSimplifyForm() const;
/// Return true if the loop body is safe to clone in practice.
bool isSafeToClone() const;
/// Returns true if the loop is annotated parallel.
///
/// A parallel loop can be assumed to not contain any dependencies between
/// iterations by the compiler. That is, any loop-carried dependency checking
/// can be skipped completely when parallelizing the loop on the target
/// machine. Thus, if the parallel loop information originates from the
/// programmer, e.g. via the OpenMP parallel for pragma, it is the
/// programmer's responsibility to ensure there are no loop-carried
/// dependencies. The final execution order of the instructions across
/// iterations is not guaranteed, thus, the end result might or might not
/// implement actual concurrent execution of instructions across multiple
/// iterations.
bool isAnnotatedParallel() const;
/// Return the llvm.loop loop id metadata node for this loop if it is present.
///
/// If this loop contains the same llvm.loop metadata on each branch to the
/// header then the node is returned. If any latch instruction does not
/// contain llvm.loop or if multiple latches contain different nodes then
/// 0 is returned.
MDNode *getLoopID() const;
/// Set the llvm.loop loop id metadata for this loop.
///
/// The LoopID metadata node will be added to each terminator instruction in
/// the loop that branches to the loop header.
///
/// The LoopID metadata node should have one or more operands and the first
/// operand should be the node itself.
void setLoopID(MDNode *LoopID) const;
/// Add llvm.loop.unroll.disable to this loop's loop id metadata.
///
/// Remove existing unroll metadata and add unroll disable metadata to
/// indicate the loop has already been unrolled. This prevents a loop
/// from being unrolled more than is directed by a pragma if the loop
/// unrolling pass is run more than once (which it generally is).
void setLoopAlreadyUnrolled();
/// Add llvm.loop.mustprogress to this loop's loop id metadata.
void setLoopMustProgress();
void dump() const;
void dumpVerbose() const;
/// Return the debug location of the start of this loop.
/// This looks for a BB terminating instruction with a known debug
/// location by looking at the preheader and header blocks. If it
/// cannot find a terminating instruction with location information,
/// it returns an unknown location.
DebugLoc getStartLoc() const;
/// Return the source code span of the loop.
LocRange getLocRange() const;
StringRef getName() const {
if (BasicBlock *Header = getHeader())
if (Header->hasName())
return Header->getName();
return "<unnamed loop>";
}
private:
Loop() = default;
friend class LoopInfoBase<BasicBlock, Loop>;
friend class LoopBase<BasicBlock, Loop>;
explicit Loop(BasicBlock *BB) : LoopBase<BasicBlock, Loop>(BB) {}
~Loop() = default;
};
//===----------------------------------------------------------------------===//
/// This class builds and contains all of the top-level loop
/// structures in the specified function.
///
template <class BlockT, class LoopT> class LoopInfoBase {
// BBMap - Mapping of basic blocks to the inner most loop they occur in
DenseMap<const BlockT *, LoopT *> BBMap;
std::vector<LoopT *> TopLevelLoops;
BumpPtrAllocator LoopAllocator;
friend class LoopBase<BlockT, LoopT>;
friend class LoopInfo;
void operator=(const LoopInfoBase &) = delete;
LoopInfoBase(const LoopInfoBase &) = delete;
public:
LoopInfoBase() = default;
~LoopInfoBase() { releaseMemory(); }
LoopInfoBase(LoopInfoBase &&Arg)
: BBMap(std::move(Arg.BBMap)),
TopLevelLoops(std::move(Arg.TopLevelLoops)),
LoopAllocator(std::move(Arg.LoopAllocator)) {
// We have to clear the arguments top level loops as we've taken ownership.
Arg.TopLevelLoops.clear();
}
LoopInfoBase &operator=(LoopInfoBase &&RHS) {
BBMap = std::move(RHS.BBMap);
for (auto *L : TopLevelLoops)
L->~LoopT();
TopLevelLoops = std::move(RHS.TopLevelLoops);
LoopAllocator = std::move(RHS.LoopAllocator);
RHS.TopLevelLoops.clear();
return *this;
}
void releaseMemory() {
BBMap.clear();
for (auto *L : TopLevelLoops)
L->~LoopT();
TopLevelLoops.clear();
LoopAllocator.Reset();
}
template <typename... ArgsTy> LoopT *AllocateLoop(ArgsTy &&... Args) {
LoopT *Storage = LoopAllocator.Allocate<LoopT>();
return new (Storage) LoopT(std::forward<ArgsTy>(Args)...);
}
/// iterator/begin/end - The interface to the top-level loops in the current
/// function.
///
typedef typename std::vector<LoopT *>::const_iterator iterator;
typedef
typename std::vector<LoopT *>::const_reverse_iterator reverse_iterator;
iterator begin() const { return TopLevelLoops.begin(); }
iterator end() const { return TopLevelLoops.end(); }
reverse_iterator rbegin() const { return TopLevelLoops.rbegin(); }
reverse_iterator rend() const { return TopLevelLoops.rend(); }
bool empty() const { return TopLevelLoops.empty(); }
/// Return all of the loops in the function in preorder across the loop
/// nests, with siblings in forward program order.
///
/// Note that because loops form a forest of trees, preorder is equivalent to
/// reverse postorder.
SmallVector<LoopT *, 4> getLoopsInPreorder() const;
/// Return all of the loops in the function in preorder across the loop
/// nests, with siblings in *reverse* program order.
///
/// Note that because loops form a forest of trees, preorder is equivalent to
/// reverse postorder.
///
/// Also note that this is *not* a reverse preorder. Only the siblings are in
/// reverse program order.
SmallVector<LoopT *, 4> getLoopsInReverseSiblingPreorder() const;
/// Return the inner most loop that BB lives in. If a basic block is in no
/// loop (for example the entry node), null is returned.
LoopT *getLoopFor(const BlockT *BB) const { return BBMap.lookup(BB); }
/// Same as getLoopFor.
const LoopT *operator[](const BlockT *BB) const { return getLoopFor(BB); }
/// Return the loop nesting level of the specified block. A depth of 0 means
/// the block is not inside any loop.
unsigned getLoopDepth(const BlockT *BB) const {
const LoopT *L = getLoopFor(BB);
return L ? L->getLoopDepth() : 0;
}
// True if the block is a loop header node
bool isLoopHeader(const BlockT *BB) const {
const LoopT *L = getLoopFor(BB);
return L && L->getHeader() == BB;
}
/// Return the top-level loops.
const std::vector<LoopT *> &getTopLevelLoops() const { return TopLevelLoops; }
/// Return the top-level loops.
std::vector<LoopT *> &getTopLevelLoopsVector() { return TopLevelLoops; }
/// This removes the specified top-level loop from this loop info object.
/// The loop is not deleted, as it will presumably be inserted into
/// another loop.
LoopT *removeLoop(iterator I) {
assert(I != end() && "Cannot remove end iterator!");
LoopT *L = *I;
assert(L->isOutermost() && "Not a top-level loop!");
TopLevelLoops.erase(TopLevelLoops.begin() + (I - begin()));
return L;
}
/// Change the top-level loop that contains BB to the specified loop.
/// This should be used by transformations that restructure the loop hierarchy
/// tree.
void changeLoopFor(BlockT *BB, LoopT *L) {
if (!L) {
BBMap.erase(BB);
return;
}
BBMap[BB] = L;
}
/// Replace the specified loop in the top-level loops list with the indicated
/// loop.
void changeTopLevelLoop(LoopT *OldLoop, LoopT *NewLoop) {
auto I = find(TopLevelLoops, OldLoop);
assert(I != TopLevelLoops.end() && "Old loop not at top level!");
*I = NewLoop;
assert(!NewLoop->ParentLoop && !OldLoop->ParentLoop &&
"Loops already embedded into a subloop!");
}
/// This adds the specified loop to the collection of top-level loops.
void addTopLevelLoop(LoopT *New) {
assert(New->isOutermost() && "Loop already in subloop!");
TopLevelLoops.push_back(New);
}
/// This method completely removes BB from all data structures,
/// including all of the Loop objects it is nested in and our mapping from
/// BasicBlocks to loops.
void removeBlock(BlockT *BB) {
auto I = BBMap.find(BB);
if (I != BBMap.end()) {
for (LoopT *L = I->second; L; L = L->getParentLoop())
L->removeBlockFromLoop(BB);
BBMap.erase(I);
}
}
// Internals
static bool isNotAlreadyContainedIn(const LoopT *SubLoop,
const LoopT *ParentLoop) {
if (!SubLoop)
return true;
if (SubLoop == ParentLoop)
return false;
return isNotAlreadyContainedIn(SubLoop->getParentLoop(), ParentLoop);
}
/// Create the loop forest using a stable algorithm.
void analyze(const DominatorTreeBase<BlockT, false> &DomTree);
// Debugging
void print(raw_ostream &OS) const;
void verify(const DominatorTreeBase<BlockT, false> &DomTree) const;
/// Destroy a loop that has been removed from the `LoopInfo` nest.
///
/// This runs the destructor of the loop object making it invalid to
/// reference afterward. The memory is retained so that the *pointer* to the
/// loop remains valid.
///
/// The caller is responsible for removing this loop from the loop nest and
/// otherwise disconnecting it from the broader `LoopInfo` data structures.
/// Callers that don't naturally handle this themselves should probably call
/// `erase' instead.
void destroy(LoopT *L) {
L->~LoopT();
// Since LoopAllocator is a BumpPtrAllocator, this Deallocate only poisons
// \c L, but the pointer remains valid for non-dereferencing uses.
LoopAllocator.Deallocate(L);
}
};
// Implementation in LoopInfoImpl.h
extern template class LoopInfoBase<BasicBlock, Loop>;
class LoopInfo : public LoopInfoBase<BasicBlock, Loop> {
typedef LoopInfoBase<BasicBlock, Loop> BaseT;
friend class LoopBase<BasicBlock, Loop>;
void operator=(const LoopInfo &) = delete;
LoopInfo(const LoopInfo &) = delete;
public:
LoopInfo() = default;
explicit LoopInfo(const DominatorTreeBase<BasicBlock, false> &DomTree);
LoopInfo(LoopInfo &&Arg) : BaseT(std::move(static_cast<BaseT &>(Arg))) {}
LoopInfo &operator=(LoopInfo &&RHS) {
BaseT::operator=(std::move(static_cast<BaseT &>(RHS)));
return *this;
}
/// Handle invalidation explicitly.
bool invalidate(Function &F, const PreservedAnalyses &PA,
FunctionAnalysisManager::Invalidator &);
// Most of the public interface is provided via LoopInfoBase.
/// Update LoopInfo after removing the last backedge from a loop. This updates
/// the loop forest and parent loops for each block so that \c L is no longer
/// referenced, but does not actually delete \c L immediately. The pointer
/// will remain valid until this LoopInfo's memory is released.
void erase(Loop *L);
/// Returns true if replacing From with To everywhere is guaranteed to
/// preserve LCSSA form.
bool replacementPreservesLCSSAForm(Instruction *From, Value *To) {
// Preserving LCSSA form is only problematic if the replacing value is an
// instruction.
Instruction *I = dyn_cast<Instruction>(To);
if (!I)
return true;
// If both instructions are defined in the same basic block then replacement
// cannot break LCSSA form.
if (I->getParent() == From->getParent())
return true;
// If the instruction is not defined in a loop then it can safely replace
// anything.
Loop *ToLoop = getLoopFor(I->getParent());
if (!ToLoop)
return true;
// If the replacing instruction is defined in the same loop as the original
// instruction, or in a loop that contains it as an inner loop, then using
// it as a replacement will not break LCSSA form.
return ToLoop->contains(getLoopFor(From->getParent()));
}
/// Checks if moving a specific instruction can break LCSSA in any loop.
///
/// Return true if moving \p Inst to before \p NewLoc will break LCSSA,
/// assuming that the function containing \p Inst and \p NewLoc is currently
/// in LCSSA form.
bool movementPreservesLCSSAForm(Instruction *Inst, Instruction *NewLoc) {
assert(Inst->getFunction() == NewLoc->getFunction() &&
"Can't reason about IPO!");
auto *OldBB = Inst->getParent();
auto *NewBB = NewLoc->getParent();
// Movement within the same loop does not break LCSSA (the equality check is
// to avoid doing a hashtable lookup in case of intra-block movement).
if (OldBB == NewBB)
return true;
auto *OldLoop = getLoopFor(OldBB);
auto *NewLoop = getLoopFor(NewBB);
if (OldLoop == NewLoop)
return true;
// Check if Outer contains Inner; with the null loop counting as the
// "outermost" loop.
auto Contains = [](const Loop *Outer, const Loop *Inner) {
return !Outer || Outer->contains(Inner);
};
// To check that the movement of Inst to before NewLoc does not break LCSSA,
// we need to check two sets of uses for possible LCSSA violations at
// NewLoc: the users of NewInst, and the operands of NewInst.
// If we know we're hoisting Inst out of an inner loop to an outer loop,
// then the uses *of* Inst don't need to be checked.
if (!Contains(NewLoop, OldLoop)) {
for (Use &U : Inst->uses()) {
auto *UI = cast<Instruction>(U.getUser());
auto *UBB = isa<PHINode>(UI) ? cast<PHINode>(UI)->getIncomingBlock(U)
: UI->getParent();
if (UBB != NewBB && getLoopFor(UBB) != NewLoop)
return false;
}
}
// If we know we're sinking Inst from an outer loop into an inner loop, then
// the *operands* of Inst don't need to be checked.
if (!Contains(OldLoop, NewLoop)) {
// See below on why we can't handle phi nodes here.
if (isa<PHINode>(Inst))
return false;
for (Use &U : Inst->operands()) {
auto *DefI = dyn_cast<Instruction>(U.get());
if (!DefI)
return false;
// This would need adjustment if we allow Inst to be a phi node -- the
// new use block won't simply be NewBB.
auto *DefBlock = DefI->getParent();
if (DefBlock != NewBB && getLoopFor(DefBlock) != NewLoop)
return false;
}
}
return true;
}
// Return true if a new use of V added in ExitBB would require an LCSSA PHI
// to be inserted at the begining of the block. Note that V is assumed to
// dominate ExitBB, and ExitBB must be the exit block of some loop. The
// IR is assumed to be in LCSSA form before the planned insertion.
bool wouldBeOutOfLoopUseRequiringLCSSA(const Value *V,
const BasicBlock *ExitBB) const;
};
/// Enable verification of loop info.
///
/// The flag enables checks which are expensive and are disabled by default
/// unless the `EXPENSIVE_CHECKS` macro is defined. The `-verify-loop-info`
/// flag allows the checks to be enabled selectively without re-compilation.
extern bool VerifyLoopInfo;
// Allow clients to walk the list of nested loops...
template <> struct GraphTraits<const Loop *> {
typedef const Loop *NodeRef;
typedef LoopInfo::iterator ChildIteratorType;
static NodeRef getEntryNode(const Loop *L) { return L; }
static ChildIteratorType child_begin(NodeRef N) { return N->begin(); }
static ChildIteratorType child_end(NodeRef N) { return N->end(); }
};
template <> struct GraphTraits<Loop *> {
typedef Loop *NodeRef;
typedef LoopInfo::iterator ChildIteratorType;
static NodeRef getEntryNode(Loop *L) { return L; }
static ChildIteratorType child_begin(NodeRef N) { return N->begin(); }
static ChildIteratorType child_end(NodeRef N) { return N->end(); }
};
/// Analysis pass that exposes the \c LoopInfo for a function.
class LoopAnalysis : public AnalysisInfoMixin<LoopAnalysis> {
friend AnalysisInfoMixin<LoopAnalysis>;
static AnalysisKey Key;
public:
typedef LoopInfo Result;
LoopInfo run(Function &F, FunctionAnalysisManager &AM);
};
/// Printer pass for the \c LoopAnalysis results.
class LoopPrinterPass : public PassInfoMixin<LoopPrinterPass> {
raw_ostream &OS;
public:
explicit LoopPrinterPass(raw_ostream &OS) : OS(OS) {}
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
};
/// Verifier pass for the \c LoopAnalysis results.
struct LoopVerifierPass : public PassInfoMixin<LoopVerifierPass> {
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
};
/// The legacy pass manager's analysis pass to compute loop information.
class LoopInfoWrapperPass : public FunctionPass {
LoopInfo LI;
public:
static char ID; // Pass identification, replacement for typeid
LoopInfoWrapperPass();
LoopInfo &getLoopInfo() { return LI; }
const LoopInfo &getLoopInfo() const { return LI; }
/// Calculate the natural loop information for a given function.
bool runOnFunction(Function &F) override;
void verifyAnalysis() const override;
void releaseMemory() override { LI.releaseMemory(); }
void print(raw_ostream &O, const Module *M = nullptr) const override;
void getAnalysisUsage(AnalysisUsage &AU) const override;
};
/// Function to print a loop's contents as LLVM's text IR assembly.
void printLoop(Loop &L, raw_ostream &OS, const std::string &Banner = "");
/// Find and return the loop attribute node for the attribute @p Name in
/// @p LoopID. Return nullptr if there is no such attribute.
MDNode *findOptionMDForLoopID(MDNode *LoopID, StringRef Name);
/// Find string metadata for a loop.
///
/// Returns the MDNode where the first operand is the metadata's name. The
/// following operands are the metadata's values. If no metadata with @p Name is
/// found, return nullptr.
MDNode *findOptionMDForLoop(const Loop *TheLoop, StringRef Name);
std::optional<bool> getOptionalBoolLoopAttribute(const Loop *TheLoop,
StringRef Name);
/// Returns true if Name is applied to TheLoop and enabled.
bool getBooleanLoopAttribute(const Loop *TheLoop, StringRef Name);
/// Find named metadata for a loop with an integer value.
std::optional<int> getOptionalIntLoopAttribute(const Loop *TheLoop,
StringRef Name);
/// Find named metadata for a loop with an integer value. Return \p Default if
/// not set.
int getIntLoopAttribute(const Loop *TheLoop, StringRef Name, int Default = 0);
/// Find string metadata for loop
///
/// If it has a value (e.g. {"llvm.distribute", 1} return the value as an
/// operand or null otherwise. If the string metadata is not found return
/// Optional's not-a-value.
std::optional<const MDOperand *> findStringMetadataForLoop(const Loop *TheLoop,
StringRef Name);
/// Look for the loop attribute that requires progress within the loop.
/// Note: Most consumers probably want "isMustProgress" which checks
/// the containing function attribute too.
bool hasMustProgress(const Loop *L);
/// Return true if this loop can be assumed to make progress. (i.e. can't
/// be infinite without side effects without also being undefined)
bool isMustProgress(const Loop *L);
/// Return true if this loop can be assumed to run for a finite number of
/// iterations.
bool isFinite(const Loop *L);
/// Return whether an MDNode might represent an access group.
///
/// Access group metadata nodes have to be distinct and empty. Being
/// always-empty ensures that it never needs to be changed (which -- because
/// MDNodes are designed immutable -- would require creating a new MDNode). Note
/// that this is not a sufficient condition: not every distinct and empty NDNode
/// is representing an access group.
bool isValidAsAccessGroup(MDNode *AccGroup);
/// Create a new LoopID after the loop has been transformed.
///
/// This can be used when no follow-up loop attributes are defined
/// (llvm::makeFollowupLoopID returning None) to stop transformations to be
/// applied again.
///
/// @param Context The LLVMContext in which to create the new LoopID.
/// @param OrigLoopID The original LoopID; can be nullptr if the original
/// loop has no LoopID.
/// @param RemovePrefixes Remove all loop attributes that have these prefixes.
/// Use to remove metadata of the transformation that has
/// been applied.
/// @param AddAttrs Add these loop attributes to the new LoopID.
///
/// @return A new LoopID that can be applied using Loop::setLoopID().
llvm::MDNode *
makePostTransformationMetadata(llvm::LLVMContext &Context, MDNode *OrigLoopID,
llvm::ArrayRef<llvm::StringRef> RemovePrefixes,
llvm::ArrayRef<llvm::MDNode *> AddAttrs);
} // End llvm namespace
#endif