[llvm] [SandboxIR] IR Tracker (PR #99238)

[Arthur Eubanks via llvm-commits]

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+//===- Tracker.h ------------------------------------------------*- 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 is the component of SandboxIR that tracks all changes made to its
+// state, such that we can revert the state when needed.
+//
+// Tracking changes
+// ----------------
+// The user needs to call `Tracker::save()` to enable tracking changes
+// made to SandboxIR. From that point on, any change made to SandboxIR, will
+// automatically create a change tracking object and register it with the
+// tracker. IR-change objects are subclasses of `IRChangeBase` and get
+// registered with the `Tracker::track()` function. The change objects
+// are saved in the order they are registered with the tracker and are stored in
+// the `Tracker::Changes` vector. All of this is done transparently to
+// the user.
+//
+// Reverting changes
+// -----------------
+// Calling `Tracker::revert()` will restore the state saved when
+// `Tracker::save()` was called. Internally this goes through the
+// change objects in `Tracker::Changes` in reverse order, calling their
+// `IRChangeBase::revert()` function one by one.
+//
+// Accepting changes
+// -----------------
+// The user needs to either revert or accept changes before the tracker object
+// is destroyed. This is enforced in the tracker's destructor.
+// This is the job of `Tracker::accept()`. Internally this will go
+// through the change objects in `Tracker::Changes` in order, calling
+// `IRChangeBase::accept()`.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SANDBOXIR_TRACKER_H
+#define LLVM_SANDBOXIR_TRACKER_H
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/IR/Module.h"
+#include "llvm/SandboxIR/Use.h"
+#include "llvm/Support/Debug.h"
+#include <memory>
+#include <regex>
+
+namespace llvm::sandboxir {
+
+class BasicBlock;
+class Tracker;
+
+/// The base class for IR Change classes.
+class IRChangeBase {
+protected:
+  Tracker &Parent;
+
+public:
+  IRChangeBase(Tracker &Parent);
+  /// This runs when changes get reverted.
+  virtual void revert() = 0;
+  /// This runs when changes get accepted.
+  virtual void accept() = 0;
+  virtual ~IRChangeBase() = default;
+#ifndef NDEBUG
+  /// \Returns the index of this change by iterating over all changes in the
+  /// tracker. This is only used for debugging.
+  unsigned getIdx() const;
+  void dumpCommon(raw_ostream &OS) const { OS << getIdx() << ". "; }
+  virtual void dump(raw_ostream &OS) const = 0;
+  LLVM_DUMP_METHOD virtual void dump() const = 0;
+  friend raw_ostream &operator<<(raw_ostream &OS, const IRChangeBase &C) {
+    C.dump(OS);
+    return OS;
+  }
+#endif
+};
+
+/// Tracks the change of the source Value of a sandboxir::Use.
+class UseSet : public IRChangeBase {
+  Use U;
+  Value *OrigV = nullptr;
+
+public:
+  UseSet(const Use &U, Tracker &Tracker)
+      : IRChangeBase(Tracker), U(U), OrigV(U.get()) {}
+  void revert() final { U.set(OrigV); }
+  void accept() final {}
+#ifndef NDEBUG
+  void dump(raw_ostream &OS) const final {
+    dumpCommon(OS);
+    OS << "UseSet";
+  }
+  LLVM_DUMP_METHOD void dump() const final;
+#endif
+};
+
+/// The tracker collects all the change objects and implements the main API for
+/// saving / reverting / accepting.
+class Tracker {
+public:
+  enum class TrackerState {
+    Disabled, ///> Tracking is disabled
+    Record,   ///> Tracking changes
+    Revert,   ///> Undoing changes
+    Accept,   ///> Accepting changes
----------------
aeubanks wrote:

are these states ever used for anything? is there anything to gain from setting the state to `Revert`/`Accept` when reverting/accepting (like `InMiddleOfCreatingChange`)?

https://github.com/llvm/llvm-project/pull/99238