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+//==------ llvm/CodeGen/LoopTraversal.h - Loop Traversal -*- C++ -*---------==//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+/// \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), therfore 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 exmple 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 proccessed 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() {}
+
+ /// \brief 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