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 //===--------------------- ResourceManager.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
 //
 //===----------------------------------------------------------------------===//
 /// \file
 ///
 /// The classes here represent processor resource units and their management
 /// strategy.  These classes are managed by the Scheduler.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_MCA_HARDWAREUNITS_RESOURCEMANAGER_H
 #define LLVM_MCA_HARDWAREUNITS_RESOURCEMANAGER_H
 
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/MC/MCSchedule.h"
 #include "llvm/MCA/Instruction.h"
 #include "llvm/MCA/Support.h"
 
 namespace llvm {
 namespace mca {
 
 /// Used to notify the internal state of a processor resource.
 ///
 /// A processor resource is available if it is not reserved, and there are
 /// available slots in the buffer.  A processor resource is unavailable if it
 /// is either reserved, or the associated buffer is full. A processor resource
 /// with a buffer size of -1 is always available if it is not reserved.
 ///
 /// Values of type ResourceStateEvent are returned by method
 /// ResourceManager::canBeDispatched()
 ///
 /// The naming convention for resource state events is:
 ///  * Event names start with prefix RS_
 ///  * Prefix RS_ is followed by a string describing the actual resource state.
 enum ResourceStateEvent {
   RS_BUFFER_AVAILABLE,
   RS_BUFFER_UNAVAILABLE,
   RS_RESERVED
 };
 
 /// Resource allocation strategy used by hardware scheduler resources.
 class ResourceStrategy {
   ResourceStrategy(const ResourceStrategy &) = delete;
   ResourceStrategy &operator=(const ResourceStrategy &) = delete;
 
 public:
   ResourceStrategy() = default;
   virtual ~ResourceStrategy();
 
   /// Selects a processor resource unit from a ReadyMask.
   virtual uint64_t select(uint64_t ReadyMask) = 0;
 
   /// Called by the ResourceManager when a processor resource group, or a
   /// processor resource with multiple units has become unavailable.
   ///
   /// The default strategy uses this information to bias its selection logic.
   virtual void used(uint64_t ResourceMask) {}
 };
 
 /// Default resource allocation strategy used by processor resource groups and
 /// processor resources with multiple units.
 class DefaultResourceStrategy final : public ResourceStrategy {
   /// A Mask of resource unit identifiers.
   ///
   /// There is one bit set for every available resource unit.
   /// It defaults to the value of field ResourceSizeMask in ResourceState.
   const uint64_t ResourceUnitMask;
 
   /// A simple round-robin selector for processor resource units.
   /// Each bit of this mask identifies a sub resource within a group.
   ///
   /// As an example, lets assume that this is a default policy for a
   /// processor resource group composed by the following three units:
   ///   ResourceA -- 0b001
   ///   ResourceB -- 0b010
   ///   ResourceC -- 0b100
   ///
   /// Field NextInSequenceMask is used to select the next unit from the set of
   /// resource units. It defaults to the value of field `ResourceUnitMasks` (in
   /// this example, it defaults to mask '0b111').
   ///
   /// The round-robin selector would firstly select 'ResourceC', then
   /// 'ResourceB', and eventually 'ResourceA'.  When a resource R is used, the
   /// corresponding bit in NextInSequenceMask is cleared.  For example, if
   /// 'ResourceC' is selected, then the new value of NextInSequenceMask becomes
   /// 0xb011.
   ///
   /// When NextInSequenceMask becomes zero, it is automatically reset to the
   /// default value (i.e. ResourceUnitMask).
   uint64_t NextInSequenceMask;
 
   /// This field is used to track resource units that are used (i.e. selected)
   /// by other groups other than the one associated with this strategy object.
   ///
   /// In LLVM processor resource groups are allowed to partially (or fully)
   /// overlap. That means, a same unit may be visible to multiple groups.
   /// This field keeps track of uses that have originated from outside of
   /// this group. The idea is to bias the selection strategy, so that resources
   /// that haven't been used by other groups get prioritized.
   ///
   /// The end goal is to (try to) keep the resource distribution as much uniform
   /// as possible. By construction, this mask only tracks one-level of resource
   /// usage. Therefore, this strategy is expected to be less accurate when same
   /// units are used multiple times by other groups within a single round of
   /// select.
   ///
   /// Note: an LRU selector would have a better accuracy at the cost of being
   /// slightly more expensive (mostly in terms of runtime cost). Methods
   /// 'select' and 'used', are always in the hot execution path of llvm-mca.
   /// Therefore, a slow implementation of 'select' would have a negative impact
   /// on the overall performance of the tool.
   uint64_t RemovedFromNextInSequence;
 
 public:
   DefaultResourceStrategy(uint64_t UnitMask)
       : ResourceUnitMask(UnitMask), NextInSequenceMask(UnitMask),
         RemovedFromNextInSequence(0) {}
   virtual ~DefaultResourceStrategy() = default;
 
   uint64_t select(uint64_t ReadyMask) override;
   void used(uint64_t Mask) override;
 };
 
 /// A processor resource descriptor.
 ///
 /// There is an instance of this class for every processor resource defined by
 /// the machine scheduling model.
 /// Objects of class ResourceState dynamically track the usage of processor
 /// resource units.
 class ResourceState {
   /// An index to the MCProcResourceDesc entry in the processor model.
   const unsigned ProcResourceDescIndex;
   /// A resource mask. This is generated by the tool with the help of
   /// function `mca::computeProcResourceMasks' (see Support.h).
   ///
   /// Field ResourceMask only has one bit set if this resource state describes a
   /// processor resource unit (i.e. this is not a group). That means, we can
   /// quickly check if a resource is a group by simply counting the number of
   /// bits that are set in the mask.
   ///
   /// The most significant bit of a mask (MSB) uniquely identifies a resource.
   /// Remaining bits are used to describe the composition of a group (Group).
   ///
   /// Example (little endian):
   ///            Resource |  Mask      |  MSB       |  Group
   ///            ---------+------------+------------+------------
   ///            A        |  0b000001  |  0b000001  |  0b000000
   ///                     |            |            |
   ///            B        |  0b000010  |  0b000010  |  0b000000
   ///                     |            |            |
   ///            C        |  0b010000  |  0b010000  |  0b000000
   ///                     |            |            |
   ///            D        |  0b110010  |  0b100000  |  0b010010
   ///
   /// In this example, resources A, B and C are processor resource units.
   /// Only resource D is a group resource, and it contains resources B and C.
   /// That is because MSB(B) and MSB(C) are both contained within Group(D).
   const uint64_t ResourceMask;
 
   /// A ProcResource can have multiple units.
   ///
   /// For processor resource groups this field is a mask of contained resource
   /// units. It is obtained from ResourceMask by clearing the highest set bit.
   /// The number of resource units in a group can be simply computed as the
   /// population count of this field.
   ///
   /// For normal (i.e. non-group) resources, the number of bits set in this mask
   /// is equivalent to the number of units declared by the processor model (see
   /// field 'NumUnits' in 'ProcResourceUnits').
   uint64_t ResourceSizeMask;
 
   /// A mask of ready units.
   uint64_t ReadyMask;
 
   /// Buffered resources will have this field set to a positive number different
   /// than zero. A buffered resource behaves like a reservation station
   /// implementing its own buffer for out-of-order execution.
   ///
   /// A BufferSize of 1 is used by scheduler resources that force in-order
   /// execution.
   ///
   /// A BufferSize of 0 is used to model in-order issue/dispatch resources.
   /// Since in-order issue/dispatch resources don't implement buffers, dispatch
   /// events coincide with issue events.
   /// Also, no other instruction ca be dispatched/issue while this resource is
   /// in use. Only when all the "resource cycles" are consumed (after the issue
   /// event), a new instruction ca be dispatched.
   const int BufferSize;
 
   /// Available slots in the buffer (zero, if this is not a buffered resource).
   unsigned AvailableSlots;
 
   /// This field is set if this resource is currently reserved.
   ///
   /// Resources can be reserved for a number of cycles.
   /// Instructions can still be dispatched to reserved resources. However,
   /// istructions dispatched to a reserved resource cannot be issued to the
   /// underlying units (i.e. pipelines) until the resource is released.
   bool Unavailable;
 
   const bool IsAGroup;
 
   /// Checks for the availability of unit 'SubResMask' in the group.
   bool isSubResourceReady(uint64_t SubResMask) const {
     return ReadyMask & SubResMask;
   }
 
 public:
   ResourceState(const MCProcResourceDesc &Desc, unsigned Index, uint64_t Mask);
 
   unsigned getProcResourceID() const { return ProcResourceDescIndex; }
   uint64_t getResourceMask() const { return ResourceMask; }
   uint64_t getReadyMask() const { return ReadyMask; }
   int getBufferSize() const { return BufferSize; }
 
   bool isBuffered() const { return BufferSize > 0; }
   bool isInOrder() const { return BufferSize == 1; }
 
   /// Returns true if this is an in-order dispatch/issue resource.
   bool isADispatchHazard() const { return BufferSize == 0; }
   bool isReserved() const { return Unavailable; }
 
   void setReserved() { Unavailable = true; }
   void clearReserved() { Unavailable = false; }
 
   /// Returs true if this resource is not reserved, and if there are at least
   /// `NumUnits` available units.
   bool isReady(unsigned NumUnits = 1) const;
 
   uint64_t getNumReadyUnits() const { return llvm::popcount(ReadyMask); }
 
   bool isAResourceGroup() const { return IsAGroup; }
 
   bool containsResource(uint64_t ID) const { return ResourceMask & ID; }
 
   void markSubResourceAsUsed(uint64_t ID) {
     assert(isSubResourceReady(ID));
     ReadyMask ^= ID;
   }
 
   void releaseSubResource(uint64_t ID) {
     assert(!isSubResourceReady(ID));
     ReadyMask ^= ID;
   }
 
   unsigned getNumUnits() const {
     return isAResourceGroup() ? 1U : llvm::popcount(ResourceSizeMask);
   }
 
   /// Checks if there is an available slot in the resource buffer.
   ///
   /// Returns RS_BUFFER_AVAILABLE if this is not a buffered resource, or if
   /// there is a slot available.
   ///
   /// Returns RS_RESERVED if this buffered resource is a dispatch hazard, and it
   /// is reserved.
   ///
   /// Returns RS_BUFFER_UNAVAILABLE if there are no available slots.
   ResourceStateEvent isBufferAvailable() const;
 
   /// Reserve a buffer slot.
   ///
   /// Returns true if the buffer is not full.
   /// It always returns true if BufferSize is set to zero.
   bool reserveBuffer() {
     if (BufferSize <= 0)
       return true;
 
     --AvailableSlots;
     assert(AvailableSlots <= static_cast<unsigned>(BufferSize));
     return AvailableSlots;
   }
 
   /// Releases a slot in the buffer.
   void releaseBuffer() {
     // Ignore dispatch hazards or invalid buffer sizes.
     if (BufferSize <= 0)
       return;
 
     ++AvailableSlots;
     assert(AvailableSlots <= static_cast<unsigned>(BufferSize));
   }
 
 #ifndef NDEBUG
   void dump() const;
 #endif
 };
 
 /// A resource unit identifier.
 ///
 /// This is used to identify a specific processor resource unit using a pair
 /// of indices where the 'first' index is a processor resource mask, and the
 /// 'second' index is an index for a "sub-resource" (i.e. unit).
 typedef std::pair<uint64_t, uint64_t> ResourceRef;
 
 // First: a MCProcResourceDesc index identifying a buffered resource.
 // Second: max number of buffer entries used in this resource.
 typedef std::pair<unsigned, unsigned> BufferUsageEntry;
 
 /// A resource manager for processor resource units and groups.
 ///
 /// This class owns all the ResourceState objects, and it is responsible for
 /// acting on requests from a Scheduler by updating the internal state of
 /// ResourceState objects.
 /// This class doesn't know about instruction itineraries and functional units.
 /// In future, it can be extended to support itineraries too through the same
 /// public interface.
 class ResourceManager {
   // Set of resources available on the subtarget.
   //
   // There is an instance of ResourceState for every resource declared by the
   // target scheduling model.
   //
   // Elements of this vector are ordered by resource kind. In particular,
   // resource units take precedence over resource groups.
   //
   // The index of a processor resource in this vector depends on the value of
   // its mask (see the description of field ResourceState::ResourceMask).  In
   // particular, it is computed as the position of the most significant bit set
   // (MSB) in the mask plus one (since we want to ignore the invalid resource
   // descriptor at index zero).
   //
   // Example (little endian):
   //
   //             Resource | Mask    |  MSB    | Index
   //             ---------+---------+---------+-------
   //                 A    | 0b00001 | 0b00001 |   1
   //                      |         |         |
   //                 B    | 0b00100 | 0b00100 |   3
   //                      |         |         |
   //                 C    | 0b10010 | 0b10000 |   5
   //
   //
   // The same index is also used to address elements within vector `Strategies`
   // and vector `Resource2Groups`.
   std::vector<std::unique_ptr<ResourceState>> Resources;
   std::vector<std::unique_ptr<ResourceStrategy>> Strategies;
 
   // Used to quickly identify groups that own a particular resource unit.
   std::vector<uint64_t> Resource2Groups;
 
   // A table that maps processor resource IDs to processor resource masks.
   SmallVector<uint64_t, 8> ProcResID2Mask;
 
   // A table that maps resource indices to actual processor resource IDs in the
   // scheduling model.
   SmallVector<unsigned, 8> ResIndex2ProcResID;
 
   // Keeps track of which resources are busy, and how many cycles are left
   // before those become usable again.
   SmallDenseMap<ResourceRef, unsigned> BusyResources;
 
   // Set of processor resource units available on the target.
   uint64_t ProcResUnitMask;
 
   // Set of processor resource units that are available during this cycle.
   uint64_t AvailableProcResUnits;
 
   // Set of processor resources that are currently reserved.
   uint64_t ReservedResourceGroups;
 
   // Set of unavailable scheduler buffer resources. This is used internally to
   // speedup `canBeDispatched()` queries.
   uint64_t AvailableBuffers;
 
   // Set of dispatch hazard buffer resources that are currently unavailable.
   uint64_t ReservedBuffers;
 
   // Returns the actual resource unit that will be used.
   ResourceRef selectPipe(uint64_t ResourceID);
 
   void use(const ResourceRef &RR);
   void release(const ResourceRef &RR);
 
   unsigned getNumUnits(uint64_t ResourceID) const;
 
   // Overrides the selection strategy for the processor resource with the given
   // mask.
   void setCustomStrategyImpl(std::unique_ptr<ResourceStrategy> S,
                              uint64_t ResourceMask);
 
 public:
   ResourceManager(const MCSchedModel &SM);
   virtual ~ResourceManager() = default;
 
   // Overrides the selection strategy for the resource at index ResourceID in
   // the MCProcResourceDesc table.
   void setCustomStrategy(std::unique_ptr<ResourceStrategy> S,
                          unsigned ResourceID) {
     assert(ResourceID < ProcResID2Mask.size() &&
            "Invalid resource index in input!");
     return setCustomStrategyImpl(std::move(S), ProcResID2Mask[ResourceID]);
   }
 
   // Returns RS_BUFFER_AVAILABLE if buffered resources are not reserved, and if
   // there are enough available slots in the buffers.
   ResourceStateEvent canBeDispatched(uint64_t ConsumedBuffers) const;
 
   // Return the processor resource identifier associated to this Mask.
   unsigned resolveResourceMask(uint64_t Mask) const;
 
   // Acquires a slot from every buffered resource in mask `ConsumedBuffers`.
   // Units that are dispatch hazards (i.e. BufferSize=0) are marked as reserved.
   void reserveBuffers(uint64_t ConsumedBuffers);
 
   // Releases a slot from every buffered resource in mask `ConsumedBuffers`.
   // ConsumedBuffers is a bitmask of previously acquired buffers (using method
   // `reserveBuffers`). Units that are dispatch hazards (i.e. BufferSize=0) are
   // not automatically unreserved by this method.
   void releaseBuffers(uint64_t ConsumedBuffers);
 
   // Reserve a processor resource. A reserved resource is not available for
   // instruction issue until it is released.
   void reserveResource(uint64_t ResourceID);
 
   // Release a previously reserved processor resource.
   void releaseResource(uint64_t ResourceID);
 
   // Returns a zero mask if resources requested by Desc are all available during
   // this cycle. It returns a non-zero mask value only if there are unavailable
   // processor resources; each bit set in the mask represents a busy processor
   // resource unit or a reserved processor resource group.
   uint64_t checkAvailability(const InstrDesc &Desc) const;
 
   uint64_t getProcResUnitMask() const { return ProcResUnitMask; }
   uint64_t getAvailableProcResUnits() const { return AvailableProcResUnits; }
 
   using ResourceWithCycles = std::pair<ResourceRef, ReleaseAtCycles>;
 
   void issueInstruction(const InstrDesc &Desc,
                         SmallVectorImpl<ResourceWithCycles> &Pipes) {
     if (Desc.HasPartiallyOverlappingGroups)
       return issueInstructionImpl(Desc, Pipes);
 
     return fastIssueInstruction(Desc, Pipes);
   }
 
   // Selects pipeline resources consumed by an instruction.
   // This method works under the assumption that used group resources don't
   // partially overlap. The logic is guaranteed to find a valid resource unit
   // schedule, no matter in which order individual uses are processed. For that
   // reason, the vector of resource uses is simply (and quickly) processed in
   // sequence. The resulting schedule is eventually stored into vector `Pipes`.
   void fastIssueInstruction(const InstrDesc &Desc,
                             SmallVectorImpl<ResourceWithCycles> &Pipes);
 
   // Selects pipeline resources consumed by an instruction.
   // This method works under the assumption that used resource groups may
   // partially overlap. This complicates the selection process, because the
   // order in which uses are processed matters. The logic internally prioritizes
   // groups which are more constrained than others.
   void issueInstructionImpl(const InstrDesc &Desc,
                             SmallVectorImpl<ResourceWithCycles> &Pipes);
 
   void cycleEvent(SmallVectorImpl<ResourceRef> &ResourcesFreed);
 
 #ifndef NDEBUG
   void dump() const {
     for (const std::unique_ptr<ResourceState> &Resource : Resources)
       Resource->dump();
   }
 #endif
 };
 } // namespace mca
 } // namespace llvm
 
 #endif // LLVM_MCA_HARDWAREUNITS_RESOURCEMANAGER_H

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