[RISCV] Add initial support of memcmp expansion

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -2491,3 +2491,18 @@ bool RISCVTTIImpl::isProfitableToSinkOperands(
   }
   return true;
 }
+
+RISCVTTIImpl::TTI::MemCmpExpansionOptions
+RISCVTTIImpl::enableMemCmpExpansion(bool OptSize, bool IsZeroCmp) const {
+  TTI::MemCmpExpansionOptions Options;
+  // FIXME: Vector haven't been tested.
+  Options.AllowOverlappingLoads =
+      (ST->enableUnalignedScalarMem() || ST->enableUnalignedVectorMem());
+  Options.MaxNumLoads = TLI->getMaxExpandSizeMemcmp(OptSize);
+  Options.NumLoadsPerBlock = Options.MaxNumLoads;
+  if (ST->is64Bit())
+    Options.LoadSizes = {8, 4, 2, 1};
+  else
+    Options.LoadSizes = {4, 2, 1};
+  return Options;
+}

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h

@@ -427,6 +427,9 @@ class RISCVTTIImpl : public BasicTTIImplBase<RISCVTTIImpl> {
 
   bool isProfitableToSinkOperands(Instruction *I,
                                   SmallVectorImpl<Use *> &Ops) const;
+
+  TTI::MemCmpExpansionOptions enableMemCmpExpansion(bool OptSize,
+                                                    bool IsZeroCmp) const;
 };
 
 } // end namespace llvm