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thirdparty/capstone/suite/synctools/tablegen/include/llvm/CodeGen/LoopTraversal.h

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+//==------ llvm/CodeGen/LoopTraversal.h - Loop Traversal -*- 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
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file Loop Traversal logic.
+///
+/// This class provides the basic blocks traversal order used by passes like
+/// ReachingDefAnalysis and ExecutionDomainFix.
+/// It identifies basic blocks that are part of loops and should to be visited
+/// twice and returns efficient traversal order for all the blocks.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_LOOPTRAVERSAL_H
+#define LLVM_CODEGEN_LOOPTRAVERSAL_H
+
+#include "llvm/ADT/SmallVector.h"
+
+namespace llvm {
+
+class MachineBasicBlock;
+class MachineFunction;
+
+/// This class provides the basic blocks traversal order used by passes like
+/// ReachingDefAnalysis and ExecutionDomainFix.
+/// It identifies basic blocks that are part of loops and should to be visited
+/// twice and returns efficient traversal order for all the blocks.
+///
+/// We want to visit every instruction in every basic block in order to update
+/// it's execution domain or collect clearance information. However, for the
+/// clearance calculation, we need to know clearances from all predecessors
+/// (including any backedges), therefore we need to visit some blocks twice.
+/// As an example, consider the following loop.
+///
+///
+///    PH -> A -> B (xmm<Undef> -> xmm<Def>) -> C -> D -> EXIT
+///          ^                                  |
+///          +----------------------------------+
+///
+/// The iteration order this pass will return is as follows:
+/// Optimized: PH A B C A' B' C' D
+///
+/// The basic block order is constructed as follows:
+/// Once we finish processing some block, we update the counters in MBBInfos
+/// and re-process any successors that are now 'done'.
+/// We call a block that is ready for its final round of processing `done`
+/// (isBlockDone), e.g. when all predecessor information is known.
+///
+/// Note that a naive traversal order would be to do two complete passes over
+/// all basic blocks/instructions, the first for recording clearances, the
+/// second for updating clearance based on backedges.
+/// However, for functions without backedges, or functions with a lot of
+/// straight-line code, and a small loop, that would be a lot of unnecessary
+/// work (since only the BBs that are part of the loop require two passes).
+///
+/// E.g., the naive iteration order for the above example is as follows:
+/// Naive: PH A B C D A' B' C' D'
+///
+/// In the optimized approach we avoid processing D twice, because we
+/// can entirely process the predecessors before getting to D.
+class LoopTraversal {
+private:
+  struct MBBInfo {
+    /// Whether we have gotten to this block in primary processing yet.
+    bool PrimaryCompleted = false;
+
+    /// The number of predecessors for which primary processing has completed
+    unsigned IncomingProcessed = 0;
+
+    /// The value of `IncomingProcessed` at the start of primary processing
+    unsigned PrimaryIncoming = 0;
+
+    /// The number of predecessors for which all processing steps are done.
+    unsigned IncomingCompleted = 0;
+
+    MBBInfo() = default;
+  };
+  using MBBInfoMap = SmallVector<MBBInfo, 4>;
+  /// Helps keep track if we processed this block and all its predecessors.
+  MBBInfoMap MBBInfos;
+
+public:
+  struct TraversedMBBInfo {
+    /// The basic block.
+    MachineBasicBlock *MBB = nullptr;
+
+    /// True if this is the first time we process the basic block.
+    bool PrimaryPass = true;
+
+    /// True if the block that is ready for its final round of processing.
+    bool IsDone = true;
+
+    TraversedMBBInfo(MachineBasicBlock *BB = nullptr, bool Primary = true,
+                     bool Done = true)
+        : MBB(BB), PrimaryPass(Primary), IsDone(Done) {}
+  };
+  LoopTraversal() = default;
+
+  /// Identifies basic blocks that are part of loops and should to be
+  ///  visited twice and returns efficient traversal order for all the blocks.
+  typedef SmallVector<TraversedMBBInfo, 4> TraversalOrder;
+  TraversalOrder traverse(MachineFunction &MF);
+
+private:
+  /// Returens true if the block is ready for its final round of processing.
+  bool isBlockDone(MachineBasicBlock *MBB);
+};
+
+} // namespace llvm
+
+#endif // LLVM_CODEGEN_LOOPTRAVERSAL_H