[System Model] [TTI] Update cache and prefetch TTI interfaces

[greened]

Summary

Rework the TTI cache and software prefetching APIs to prepare for the
introduction of a general system model. Changes include:

• Marking existing interfaces const and/or override as appropriate
• Adding comments
• Adding BasicTTIImpl interfaces that delegate to a subtarget implementation
• Adding a default "no information" subtarget implementation

Only a handful of targets use these interfaces currently: AArch64,
Hexagon, PPC and SystemZ. AArch64 already has a custom subtarget
implementation, so its custom TTI implementation is migrated to use
the new facilities in BasicTTIImpl to invoke its custom subtarget
implementation. The custom TTI implementations continue to exist for
the other targets with this change. They are not moved over to
subtarget-based implementations.

The end goal is to have the default subtarget implementation defer to
the system model defined by the target. With this change, the default
subtarget implementation essentially returns "no information" for
these interfaces. None of the existing users of TTI will hit that
implementation because they define their own custom TTI
implementations and won't use the BasicTTIImpl implementations.

Once system models are in place for the targets that use these
interfaces, their custom TTI implementations can be removed.

[greened]

This is the subset of D58736 covering changes to TTI and related classes. It does not introduce any new functionality, only reorganizes things a bit to move implementations into subtargets in preparation for defining system models for targets.

[greened]

Ping.

[hfinkel]

I'd prefer we remove the redundant comments in TTIImpl.h (see below), but otherwise, LGTM.

llvm/include/llvm/Analysis/TargetTransformInfoImpl.h

393

I don't see any value in repeating these interface comments here. They're already present on the declarations in TargetTransformInfo.h, and they'll get out of sync here.

[greened]

llvm/include/llvm/Analysis/TargetTransformInfoImpl.h

393

Ok. I wasn't sure what the policy is. Some implementations have them, others don't. I agree that less duplication is better.

[greened]

Updated to latest master and removed comments on implementations

[greened]

I know this now says "ready to land" but is one review really sufficient?

[rengolin]

As far as I can see this is an NFC refactoring with obvious benefits to readability and extensibility. LGTM too. :)

[hfinkel]

In D63614#1571616, @greened wrote:

I know this now says "ready to land" but is one review really sufficient?

This is a refactoring of existing functionality (and, FWIW, functionality originally contributed by me, later generalized by @anemet, and others). If you're looking for particular feedback from others, please call that out explicitly. The patch has been online for a couple of weeks, and there's been time for feedback. In short, yes, you should feel free to proceed.

Also, it is useful to note that all code is subject to post-commit code review, even for patches that have received pre-commit review. You may yet get feedback after you commit the change.

[beanz]

This patch causes lots of warning spews because it adds virtual methods to a class that doesn't have a virtual destructor.

[greened]

In D63614#1702176, @beanz wrote:

This patch causes lots of warning spews because it adds virtual methods to a class that doesn't have a virtual destructor.

Vitaly fixed it before I could get to it. Thanks for the heads-up!

[ychen]

llvm/trunk/include/llvm/CodeGen/BasicTTIImpl.h

509 ↗

(On Diff #209023)

BasicTTIImplBase is a CRTP base. The virtual here seems odd. Why not just move the SystemZ & Hexagon implementation to be under the MCSubtarget hierarchy and get rid of the virtual here?

llvm/include/llvm/Analysis/TargetTransformInfo.h

public:
   };
 
   /// \return The size of the cache level in bytes, if available.
   llvm::Optional<unsigned> getCacheSize(CacheLevel Level) const;
 
   /// \return The associativity of the cache level, if available.
   llvm::Optional<unsigned> getCacheAssociativity(CacheLevel Level) const;
 
-  /// \return How much before a load we should place the prefetch instruction.
-  /// This is currently measured in number of instructions.
+  /// \return How much before a load we should place the prefetch
+  /// instruction.  This is currently measured in number of
+  /// instructions.
   unsigned getPrefetchDistance() const;
 
-  /// \return Some HW prefetchers can handle accesses up to a certain constant
-  /// stride.  This is the minimum stride in bytes where it makes sense to start
-  /// adding SW prefetches.  The default is 1, i.e. prefetch with any stride.
+  /// \return Some HW prefetchers can handle accesses up to a certain
+  /// constant stride.  This is the minimum stride in bytes where it
+  /// makes sense to start adding SW prefetches.  The default is 1,
+  /// i.e. prefetch with any stride.
   unsigned getMinPrefetchStride() const;
 
-  /// \return The maximum number of iterations to prefetch ahead.  If the
-  /// required number of iterations is more than this number, no prefetching is
-  /// performed.
+  /// \return The maximum number of iterations to prefetch ahead.  If
+  /// the required number of iterations is more than this number, no
+  /// prefetching is performed.
   unsigned getMaxPrefetchIterationsAhead() const;
 
   /// \return The maximum interleave factor that any transform should try to
   /// perform for this target. This number depends on the level of parallelism
   /// and the number of execution units in the CPU.
   unsigned getMaxInterleaveFactor(unsigned VF) const;
 
   /// Collect properties of V used in cost analysis, e.g. OP_PowerOf2.
virtual int getIntImmCost(Intrinsic::ID IID, unsigned Idx, const APInt &Imm,
                             Type *Ty) = 0;
   virtual unsigned getNumberOfRegisters(bool Vector) = 0;
   virtual unsigned getRegisterBitWidth(bool Vector) const = 0;
   virtual unsigned getMinVectorRegisterBitWidth() = 0;
   virtual bool shouldMaximizeVectorBandwidth(bool OptSize) const = 0;
   virtual unsigned getMinimumVF(unsigned ElemWidth) const = 0;
   virtual bool shouldConsiderAddressTypePromotion(
       const Instruction &I, bool &AllowPromotionWithoutCommonHeader) = 0;
-  virtual unsigned getCacheLineSize() = 0;
-  virtual llvm::Optional<unsigned> getCacheSize(CacheLevel Level) = 0;
-  virtual llvm::Optional<unsigned> getCacheAssociativity(CacheLevel Level) = 0;
-  virtual unsigned getPrefetchDistance() = 0;
-  virtual unsigned getMinPrefetchStride() = 0;
-  virtual unsigned getMaxPrefetchIterationsAhead() = 0;
+  virtual unsigned getCacheLineSize() const = 0;
+  virtual llvm::Optional<unsigned> getCacheSize(CacheLevel Level) const = 0;
+  virtual llvm::Optional<unsigned> getCacheAssociativity(CacheLevel Level) const = 0;
+
+  /// \return How much before a load we should place the prefetch
+  /// instruction.  This is currently measured in number of
+  /// instructions.
+  virtual unsigned getPrefetchDistance() const = 0;
+
+  /// \return Some HW prefetchers can handle accesses up to a certain
+  /// constant stride.  This is the minimum stride in bytes where it
+  /// makes sense to start adding SW prefetches.  The default is 1,
+  /// i.e. prefetch with any stride.
+  virtual unsigned getMinPrefetchStride() const = 0;
+
+  /// \return The maximum number of iterations to prefetch ahead.  If
+  /// the required number of iterations is more than this number, no
+  /// prefetching is performed.
+  virtual unsigned getMaxPrefetchIterationsAhead() const = 0;
+
   virtual unsigned getMaxInterleaveFactor(unsigned VF) = 0;
   virtual unsigned
   getArithmeticInstrCost(unsigned Opcode, Type *Ty, OperandValueKind Opd1Info,
                          OperandValueKind Opd2Info,
                          OperandValueProperties Opd1PropInfo,
                          OperandValueProperties Opd2PropInfo,
                          ArrayRef<const Value *> Args) = 0;
   virtual int getShuffleCost(ShuffleKind Kind, Type *Tp, int Index,
public:
   unsigned getMinimumVF(unsigned ElemWidth) const override {
     return Impl.getMinimumVF(ElemWidth);
   }
   bool shouldConsiderAddressTypePromotion(
       const Instruction &I, bool &AllowPromotionWithoutCommonHeader) override {
     return Impl.shouldConsiderAddressTypePromotion(
         I, AllowPromotionWithoutCommonHeader);
   }
-  unsigned getCacheLineSize() override {
+  unsigned getCacheLineSize() const override {
     return Impl.getCacheLineSize();
   }
-  llvm::Optional<unsigned> getCacheSize(CacheLevel Level) override {
+  llvm::Optional<unsigned> getCacheSize(CacheLevel Level) const override {
     return Impl.getCacheSize(Level);
   }
-  llvm::Optional<unsigned> getCacheAssociativity(CacheLevel Level) override {
+  llvm::Optional<unsigned> getCacheAssociativity(CacheLevel Level) const override {
     return Impl.getCacheAssociativity(Level);
   }
-  unsigned getPrefetchDistance() override { return Impl.getPrefetchDistance(); }
-  unsigned getMinPrefetchStride() override {
+
+  /// Return the preferred prefetch distance in terms of instructions.
+  ///
+  unsigned getPrefetchDistance() const override {
+    return Impl.getPrefetchDistance();
+  }
+
+  /// Return the minimum stride necessary to trigger software
+  /// prefetching.
+  ///
+  unsigned getMinPrefetchStride() const override {
     return Impl.getMinPrefetchStride();
   }
-  unsigned getMaxPrefetchIterationsAhead() override {
+
+  /// Return the maximum prefetch distance in terms of loop
+  /// iterations.
+  ///
+  unsigned getMaxPrefetchIterationsAhead() const override {
     return Impl.getMaxPrefetchIterationsAhead();
   }
+
   unsigned getMaxInterleaveFactor(unsigned VF) override {
     return Impl.getMaxInterleaveFactor(VF);
   }
   unsigned getEstimatedNumberOfCaseClusters(const SwitchInst &SI,
                                             unsigned &JTSize) override {
     return Impl.getEstimatedNumberOfCaseClusters(SI, JTSize);
   }
   unsigned

llvm/include/llvm/Analysis/TargetTransformInfoImpl.h

public:
 
   bool
   shouldConsiderAddressTypePromotion(const Instruction &I,
                                      bool &AllowPromotionWithoutCommonHeader) {
     AllowPromotionWithoutCommonHeader = false;
     return false;
   }
 
-  unsigned getCacheLineSize() { return 0; }
+  unsigned getCacheLineSize() const { return 0; }
 
-  llvm::Optional<unsigned> getCacheSize(TargetTransformInfo::CacheLevel Level) {
+  llvm::Optional<unsigned> getCacheSize(TargetTransformInfo::CacheLevel Level) const {
     switch (Level) {
     case TargetTransformInfo::CacheLevel::L1D:
       LLVM_FALLTHROUGH;
     case TargetTransformInfo::CacheLevel::L2D:
       return llvm::Optional<unsigned>();
     }
 
     llvm_unreachable("Unknown TargetTransformInfo::CacheLevel");
   }
 
   llvm::Optional<unsigned> getCacheAssociativity(
-    TargetTransformInfo::CacheLevel Level) {
+    TargetTransformInfo::CacheLevel Level) const {
     switch (Level) {
     case TargetTransformInfo::CacheLevel::L1D:
       LLVM_FALLTHROUGH;
     case TargetTransformInfo::CacheLevel::L2D:
       return llvm::Optional<unsigned>();
     }
 
     llvm_unreachable("Unknown TargetTransformInfo::CacheLevel");
   }
 
-  unsigned getPrefetchDistance() { return 0; }
+  unsigned getPrefetchDistance() const { return 0; }
 
-  unsigned getMinPrefetchStride() { return 1; }
+  unsigned getMinPrefetchStride() const { return 1; }
 
-  unsigned getMaxPrefetchIterationsAhead() { return UINT_MAX; }
+  unsigned getMaxPrefetchIterationsAhead() const { return UINT_MAX; }
 
   unsigned getMaxInterleaveFactor(unsigned VF) { return 1; }
 
   unsigned getArithmeticInstrCost(unsigned Opcode, Type *Ty,
                                   TTI::OperandValueKind Opd1Info,
                                   TTI::OperandValueKind Opd2Info,
                                   TTI::OperandValueProperties Opd1PropInfo,
                                   TTI::OperandValueProperties Opd2PropInfo,

llvm/include/llvm/CodeGen/BasicTTIImpl.h

public:
 
   int getInstructionLatency(const Instruction *I) {
     if (isa<LoadInst>(I))
       return getST()->getSchedModel().DefaultLoadLatency;
 
     return BaseT::getInstructionLatency(I);
   }
 
+  virtual Optional<unsigned>
+  getCacheSize(TargetTransformInfo::CacheLevel Level) const {
+    return Optional<unsigned>(
+      getST()->getCacheSize(static_cast<unsigned>(Level)));
+  }
+
+  virtual Optional<unsigned>
+  getCacheAssociativity(TargetTransformInfo::CacheLevel Level) const {
+    return Optional<unsigned>(
+      getST()->getCacheAssociativity(static_cast<unsigned>(Level)));
+  }
+
+  virtual unsigned getCacheLineSize() const {
+    return getST()->getCacheLineSize();
+  }
+
+  virtual unsigned getPrefetchDistance() const {
+    return getST()->getPrefetchDistance();
+  }
+
+  virtual unsigned getMinPrefetchStride() const {
+    return getST()->getMinPrefetchStride();
+  }
+
+  virtual unsigned getMaxPrefetchIterationsAhead() const {
+    return getST()->getMaxPrefetchIterationsAhead();
+  }
+
   /// @}
 
   /// \name Vector TTI Implementations
   /// @{
 
   unsigned getNumberOfRegisters(bool Vector) { return Vector ? 0 : 1; }
 
   unsigned getRegisterBitWidth(bool Vector) const { return 32; }

llvm/include/llvm/MC/MCSubtargetInfo.h

resolveVariantSchedClass(unsigned SchedClass, const MCInst *MI,
     return 0;
   }
 
   /// Check whether the CPU string is valid.
   bool isCPUStringValid(StringRef CPU) const {
     auto Found = std::lower_bound(ProcDesc.begin(), ProcDesc.end(), CPU);
     return Found != ProcDesc.end() && StringRef(Found->Key) == CPU;
   }
+
+  /// Return the cache size in bytes for the given level of cache.
+  /// Level is zero-based, so a value of zero means the first level of
+  /// cache.
+  ///
+  virtual Optional<unsigned> getCacheSize(unsigned Level) const;
+
+  /// Return the cache associatvity for the given level of cache.
+  /// Level is zero-based, so a value of zero means the first level of
+  /// cache.
+  ///
+  virtual Optional<unsigned> getCacheAssociativity(unsigned Level) const;
+
+  /// Return the target cache line size in bytes at a given level.
+  ///
+  virtual Optional<unsigned> getCacheLineSize(unsigned Level) const;
+
+  /// Return the target cache line size in bytes.  By default, return
+  /// the line size for the bottom-most level of cache.  This provides
+  /// a more convenient interface for the common case where all cache
+  /// levels have the same line size.  Return zero if there is no
+  /// cache model.
+  ///
+  virtual unsigned getCacheLineSize() const {
+    Optional<unsigned> Size = getCacheLineSize(0);
+    if (Size)
+      return *Size;
+
+    return 0;
+  }
+
+  /// Return the preferred prefetch distance in terms of instructions.
+  ///
+  virtual unsigned getPrefetchDistance() const;
+
+  /// Return the maximum prefetch distance in terms of loop
+  /// iterations.
+  ///
+  virtual unsigned getMaxPrefetchIterationsAhead() const;
+
+  /// Return the minimum stride necessary to trigger software
+  /// prefetching.
+  ///
+  virtual unsigned getMinPrefetchStride() const;
 };
 
 } // end namespace llvm
 
 #endif // LLVM_MC_MCSUBTARGETINFO_H

llvm/lib/MC/MCSubtargetInfo.cpp

MCSubtargetInfo::getInstrItineraryForCPU(StringRef CPU) const {
   const MCSchedModel &SchedModel = getSchedModelForCPU(CPU);
   return InstrItineraryData(SchedModel, Stages, OperandCycles, ForwardingPaths);
 }
 
 void MCSubtargetInfo::initInstrItins(InstrItineraryData &InstrItins) const {
   InstrItins = InstrItineraryData(getSchedModel(), Stages, OperandCycles,
                                   ForwardingPaths);
 }
+
+Optional<unsigned> MCSubtargetInfo::getCacheSize(unsigned Level) const {
+  return Optional<unsigned>();
+}
+
+Optional<unsigned>
+MCSubtargetInfo::getCacheAssociativity(unsigned Level) const {
+  return Optional<unsigned>();
+}
+
+Optional<unsigned> MCSubtargetInfo::getCacheLineSize(unsigned Level) const {
+  return Optional<unsigned>();
+}
+
+unsigned MCSubtargetInfo::getPrefetchDistance() const {
+  return 0;
+}
+
+unsigned MCSubtargetInfo::getMaxPrefetchIterationsAhead() const {
+  return 0;
+}
+
+unsigned MCSubtargetInfo::getMinPrefetchStride() const {
+  return 0;
+}

llvm/lib/Target/AArch64/AArch64Subtarget.h

public:
   bool useEL3ForTP() const { return UseEL3ForTP; }
 
   bool useRSqrt() const { return UseRSqrt; }
   bool force32BitJumpTables() const { return Force32BitJumpTables; }
   unsigned getMaxInterleaveFactor() const { return MaxInterleaveFactor; }
   unsigned getVectorInsertExtractBaseCost() const {
     return VectorInsertExtractBaseCost;
   }
-  unsigned getCacheLineSize() const { return CacheLineSize; }
-  unsigned getPrefetchDistance() const { return PrefetchDistance; }
-  unsigned getMinPrefetchStride() const { return MinPrefetchStride; }
-  unsigned getMaxPrefetchIterationsAhead() const {
+  unsigned getCacheLineSize() const override { return CacheLineSize; }
+  unsigned getPrefetchDistance() const override { return PrefetchDistance; }
+  unsigned getMinPrefetchStride() const override { return MinPrefetchStride; }
+  unsigned getMaxPrefetchIterationsAhead() const override {
     return MaxPrefetchIterationsAhead;
   }
   unsigned getPrefFunctionAlignment() const { return PrefFunctionAlignment; }
   unsigned getPrefLoopAlignment() const { return PrefLoopAlignment; }
 
   unsigned getMaximumJumpTableSize() const { return MaxJumpTableSize; }
 
   unsigned getWideningBaseCost() const { return WideningBaseCost; }

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h

int getInterleavedMemoryOpCost(unsigned Opcode, Type *VecTy, unsigned Factor,
                                  unsigned AddressSpace,
                                  bool UseMaskForCond = false,
                                  bool UseMaskForGaps = false);
 
   bool
   shouldConsiderAddressTypePromotion(const Instruction &I,
                                      bool &AllowPromotionWithoutCommonHeader);
 
-  unsigned getCacheLineSize();
-
-  unsigned getPrefetchDistance();
-
-  unsigned getMinPrefetchStride();
-
-  unsigned getMaxPrefetchIterationsAhead();
-
   bool shouldExpandReduction(const IntrinsicInst *II) const {
     return false;
   }
 
   unsigned getGISelRematGlobalCost() const {
     return 2;
   }
 

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

if (const GetElementPtrInst *GEPInst = dyn_cast<GetElementPtrInst>(U)) {
         AllowPromotionWithoutCommonHeader = true;
         break;
       }
     }
   }
   return Considerable;
 }
 
-unsigned AArch64TTIImpl::getCacheLineSize() {
-  return ST->getCacheLineSize();
-}
-
-unsigned AArch64TTIImpl::getPrefetchDistance() {
-  return ST->getPrefetchDistance();
-}
-
-unsigned AArch64TTIImpl::getMinPrefetchStride() {
-  return ST->getMinPrefetchStride();
-}
-
-unsigned AArch64TTIImpl::getMaxPrefetchIterationsAhead() {
-  return ST->getMaxPrefetchIterationsAhead();
-}
-
 bool AArch64TTIImpl::useReductionIntrinsic(unsigned Opcode, Type *Ty,
                                            TTI::ReductionFlags Flags) const {
   assert(isa<VectorType>(Ty) && "Expected Ty to be a vector type");
   unsigned ScalarBits = Ty->getScalarSizeInBits();
   switch (Opcode) {
   case Instruction::FAdd:
   case Instruction::FMul:
   case Instruction::And:

llvm/lib/Target/Hexagon/HexagonTargetTransformInfo.h

public:
   // The Hexagon target can unroll loops with run-time trip counts.
   void getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
                                TTI::UnrollingPreferences &UP);
 
   /// Bias LSR towards creating post-increment opportunities.
   bool shouldFavorPostInc() const;
 
   // L1 cache prefetch.
-  unsigned getPrefetchDistance() const;
-  unsigned getCacheLineSize() const;
+  unsigned getPrefetchDistance() const override;
+  unsigned getCacheLineSize() const override;
 
   /// @}
 
   /// \name Vector TTI Implementations
   /// @{
 
   unsigned getNumberOfRegisters(bool vector) const;
   unsigned getMaxInterleaveFactor(unsigned VF);

llvm/lib/Target/PowerPC/PPCTargetTransformInfo.h

public:
   /// @{
   bool useColdCCForColdCall(Function &F);
   bool enableAggressiveInterleaving(bool LoopHasReductions);
   TTI::MemCmpExpansionOptions enableMemCmpExpansion(bool OptSize,
                                                     bool IsZeroCmp) const;
   bool enableInterleavedAccessVectorization();
   unsigned getNumberOfRegisters(bool Vector);
   unsigned getRegisterBitWidth(bool Vector) const;
-  unsigned getCacheLineSize();
-  unsigned getPrefetchDistance();
+  unsigned getCacheLineSize() const override;
+  unsigned getPrefetchDistance() const override;
   unsigned getMaxInterleaveFactor(unsigned VF);
   int vectorCostAdjustment(int Cost, unsigned Opcode, Type *Ty1, Type *Ty2);
   int getArithmeticInstrCost(
       unsigned Opcode, Type *Ty,
       TTI::OperandValueKind Opd1Info = TTI::OK_AnyValue,
       TTI::OperandValueKind Opd2Info = TTI::OK_AnyValue,
       TTI::OperandValueProperties Opd1PropInfo = TTI::OP_None,
       TTI::OperandValueProperties Opd2PropInfo = TTI::OP_None,

llvm/lib/Target/PowerPC/PPCTargetTransformInfo.cpp

unsigned PPCTTIImpl::getRegisterBitWidth(bool Vector) const {
   }
 
   if (ST->isPPC64())
     return 64;
   return 32;
 
 }
 
-unsigned PPCTTIImpl::getCacheLineSize() {
+unsigned PPCTTIImpl::getCacheLineSize() const {
   // Check first if the user specified a custom line size.
   if (CacheLineSize.getNumOccurrences() > 0)
     return CacheLineSize;
 
   // On P7, P8 or P9 we have a cache line size of 128.
   unsigned Directive = ST->getDarwinDirective();
   if (Directive == PPC::DIR_PWR7 || Directive == PPC::DIR_PWR8 ||
       Directive == PPC::DIR_PWR9)
     return 128;
 
   // On other processors return a default of 64 bytes.
   return 64;
 }
 
-unsigned PPCTTIImpl::getPrefetchDistance() {
+unsigned PPCTTIImpl::getPrefetchDistance() const {
   // This seems like a reasonable default for the BG/Q (this pass is enabled, by
   // default, only on the BG/Q).
   return 300;
 }
 
 unsigned PPCTTIImpl::getMaxInterleaveFactor(unsigned VF) {
   unsigned Directive = ST->getDarwinDirective();
   // The 440 has no SIMD support, but floating-point instructions

llvm/lib/Target/SystemZ/SystemZTargetTransformInfo.h

public:
   /// @}
 
   /// \name Vector TTI Implementations
   /// @{
 
   unsigned getNumberOfRegisters(bool Vector);
   unsigned getRegisterBitWidth(bool Vector) const;
 
-  unsigned getCacheLineSize() { return 256; }
-  unsigned getPrefetchDistance() { return 2000; }
-  unsigned getMinPrefetchStride() { return 2048; }
+  unsigned getCacheLineSize() const override { return 256; }
+  unsigned getPrefetchDistance() const override { return 2000; }
+  unsigned getMinPrefetchStride() const override { return 2048; }
 
   bool hasDivRemOp(Type *DataType, bool IsSigned);
   bool prefersVectorizedAddressing() { return false; }
   bool LSRWithInstrQueries() { return true; }
   bool supportsEfficientVectorElementLoadStore() { return true; }
   bool enableInterleavedAccessVectorization() { return true; }
 
   int getArithmeticInstrCost(