GlobalStatus.h

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//===- GlobalStatus.h - Compute status info for globals ---------*- 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
//
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_TRANSFORMS_UTILS_GLOBALSTATUS_H
#define LLVM_TRANSFORMS_UTILS_GLOBALSTATUS_H
 
#include "llvm/IR/Instructions.h"
#include "llvm/Support/AtomicOrdering.h"
 
namespace llvm {
 
class Constant;
class Function;
class Value;
 
/// It is safe to destroy a constant iff it is only used by constants itself.
/// Note that constants cannot be cyclic, so this test is pretty easy to
/// implement recursively.
///
bool isSafeToDestroyConstant(const Constant *C);
 
/// As we analyze each global or thread-local variable, keep track of some
/// information about it.  If we find out that the address of the global is
/// taken, none of this info will be accurate.
struct GlobalStatus {
  /// True if the global's address is used in a comparison.
  bool IsCompared = false;
 
  /// True if the global is ever loaded.  If the global isn't ever loaded it
  /// can be deleted.
  bool IsLoaded = false;
 
  /// Number of stores to the global.
  unsigned NumStores = 0;
 
  /// Keep track of what stores to the global look like.
  enum StoredType {
    /// There is no store to this global.  It can thus be marked constant.
    NotStored,
 
    /// This global is stored to, but the only thing stored is the constant it
    /// was initialized with. This is only tracked for scalar globals.
    InitializerStored,
 
    /// This global is stored to, but only its initializer and one other value
    /// is ever stored to it.  If this global isStoredOnce, we track the value
    /// stored to it via StoredOnceStore below.  This is only tracked for scalar
    /// globals.
    StoredOnce,
 
    /// This global is stored to by multiple values or something else that we
    /// cannot track.
    Stored
  } StoredType = NotStored;
 
  /// If only one value (besides the initializer constant) is ever stored to
  /// this global, keep track of what value it is via the store instruction.
  const StoreInst *StoredOnceStore = nullptr;
 
  /// If only one value (besides the initializer constant) is ever stored to
  /// this global return the stored value.
  Value *getStoredOnceValue() const {
    return (StoredType == StoredOnce && StoredOnceStore)
               ? StoredOnceStore->getOperand(0)
               : nullptr;
  }
 
  /// These start out null/false.  When the first accessing function is noticed,
  /// it is recorded. When a second different accessing function is noticed,
  /// HasMultipleAccessingFunctions is set to true.
  const Function *AccessingFunction = nullptr;
  bool HasMultipleAccessingFunctions = false;
 
  /// Set to the strongest atomic ordering requirement.
  AtomicOrdering Ordering = AtomicOrdering::NotAtomic;
 
  GlobalStatus();
 
  /// Look at all uses of the global and fill in the GlobalStatus structure.  If
  /// the global has its address taken, return true to indicate we can't do
  /// anything with it.
  static bool analyzeGlobal(const Value *V, GlobalStatus &GS);
};
 
} // end namespace llvm
 
#endif // LLVM_TRANSFORMS_UTILS_GLOBALSTATUS_H