[RISCV] Add initial support of memcmp expansion

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -2508,3 +2508,22 @@ bool RISCVTTIImpl::isProfitableToSinkOperands(
   }
   return true;
 }
+
+RISCVTTIImpl::TTI::MemCmpExpansionOptions
+RISCVTTIImpl::enableMemCmpExpansion(bool OptSize, bool IsZeroCmp) const {
+  TTI::MemCmpExpansionOptions Options;
+  // TODO: Enable expansion when unaligned access is not supported after we fix
+  // issues in ExpandMemcmp.
+  if (!(ST->enableUnalignedScalarMem() &&
+        (ST->hasStdExtZbb() || ST->hasStdExtZbkb() || IsZeroCmp)))
+    return Options;
+
+  Options.AllowOverlappingLoads = true;
+  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

@@ -434,6 +434,9 @@ class RISCVTTIImpl : public BasicTTIImplBase<RISCVTTIImpl> {
 
   bool isProfitableToSinkOperands(Instruction *I,
                                   SmallVectorImpl<Use *> &Ops) const;
+
+  TTI::MemCmpExpansionOptions enableMemCmpExpansion(bool OptSize,
+                                                    bool IsZeroCmp) const;
 };
 
 } // end namespace llvm