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//===- llvm/Use.h - Definition of the Use class -----------------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
/// \file
/// This defines the Use class. The Use class represents the operand of an
/// instruction or some other User instance which refers to a Value. The Use
/// class keeps the "use list" of the referenced value up to date.
/// Pointer tagging is used to efficiently find the User corresponding to a Use
/// without having to store a User pointer in every Use. A User is preceded in
/// memory by all the Uses corresponding to its operands, and the low bits of
/// one of the fields (Prev) of the Use class are used to encode offsets to be
/// able to find that User given a pointer to any Use. For details, see:
#ifndef LLVM_IR_USE_H
#define LLVM_IR_USE_H
#include "llvm-c/Types.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/Support/CBindingWrapping.h"
#include "llvm/Support/Compiler.h"
namespace llvm {
template <typename> struct simplify_type;
class User;
class Value;
/// A Use represents the edge between a Value definition and its users.
/// This is notionally a two-dimensional linked list. It supports traversing
/// all of the uses for a particular value definition. It also supports jumping
/// directly to the used value when we arrive from the User's operands, and
/// jumping directly to the User when we arrive from the Value's uses.
/// The pointer to the used Value is explicit, and the pointer to the User is
/// implicit. The implicit pointer is found via a waymarking algorithm
/// described in the programmer's manual:
/// This is essentially the single most memory intensive object in LLVM because
/// of the number of uses in the system. At the same time, the constant time
/// operations it allows are essential to many optimizations having reasonable
/// time complexity.
class Use {
Use(const Use &U) = delete;
/// Provide a fast substitute to std::swap<Use>
/// that also works with less standard-compliant compilers
void swap(Use &RHS);
/// Pointer traits for the UserRef PointerIntPair. This ensures we always
/// use the LSB regardless of pointer alignment on different targets.
struct UserRefPointerTraits {
static inline void *getAsVoidPointer(User *P) { return P; }
static inline User *getFromVoidPointer(void *P) {
return (User *)P;
enum { NumLowBitsAvailable = 1 };
// A type for the word following an array of hung-off Uses in memory, which is
// a pointer back to their User with the bottom bit set.
using UserRef = PointerIntPair<User *, 1, unsigned, UserRefPointerTraits>;
/// Pointer traits for the Prev PointerIntPair. This ensures we always use
/// the two LSBs regardless of pointer alignment on different targets.
struct PrevPointerTraits {
static inline void *getAsVoidPointer(Use **P) { return P; }
static inline Use **getFromVoidPointer(void *P) {
return (Use **)P;
enum { NumLowBitsAvailable = 2 };
/// Destructor - Only for zap()
~Use() {
if (Val)
enum PrevPtrTag { zeroDigitTag, oneDigitTag, stopTag, fullStopTag };
/// Constructor
Use(PrevPtrTag tag) { Prev.setInt(tag); }
friend class Value;
operator Value *() const { return Val; }
Value *get() const { return Val; }
/// Returns the User that contains this Use.
/// For an instruction operand, for example, this will return the
/// instruction.
User *getUser() const LLVM_READONLY;
inline void set(Value *Val);
inline Value *operator=(Value *RHS);
inline const Use &operator=(const Use &RHS);
Value *operator->() { return Val; }
const Value *operator->() const { return Val; }
Use *getNext() const { return Next; }
/// Return the operand # of this use in its User.
unsigned getOperandNo() const;
/// Initializes the waymarking tags on an array of Uses.
/// This sets up the array of Uses such that getUser() can find the User from
/// any of those Uses.
static Use *initTags(Use *Start, Use *Stop);
/// Destroys Use operands when the number of operands of
/// a User changes.
static void zap(Use *Start, const Use *Stop, bool del = false);
const Use *getImpliedUser() const LLVM_READONLY;
Value *Val = nullptr;
Use *Next;
PointerIntPair<Use **, 2, PrevPtrTag, PrevPointerTraits> Prev;
void setPrev(Use **NewPrev) { Prev.setPointer(NewPrev); }
void addToList(Use **List) {
Next = *List;
if (Next)
*List = this;
void removeFromList() {
Use **StrippedPrev = Prev.getPointer();
*StrippedPrev = Next;
if (Next)
/// Allow clients to treat uses just like values when using
/// casting operators.
template <> struct simplify_type<Use> {
using SimpleType = Value *;
static SimpleType getSimplifiedValue(Use &Val) { return Val.get(); }
template <> struct simplify_type<const Use> {
using SimpleType = /*const*/ Value *;
static SimpleType getSimplifiedValue(const Use &Val) { return Val.get(); }
// Create wrappers for C Binding types (see CBindingWrapping.h).
} // end namespace llvm
#endif // LLVM_IR_USE_H