[LoongArch] Initial implementation for enableMemCmpExpansion hook (llvm#166526)

After overriding `TargetTransformInfo::enableMemCmpExpansion` in this
commit, `MergeICmps` and `ExpandMemCmp` passes will be enabled on
LoongArch.

Author: zhaoqi5

llvm/lib/Target/LoongArch/LoongArchTargetTransformInfo.cpp

@@ -111,4 +111,25 @@ bool LoongArchTTIImpl::shouldExpandReduction(const IntrinsicInst *II) const {
   }
 }
 
-// TODO: Implement more hooks to provide TTI machinery for LoongArch.
+LoongArchTTIImpl::TTI::MemCmpExpansionOptions
+LoongArchTTIImpl::enableMemCmpExpansion(bool OptSize, bool IsZeroCmp) const {
+  TTI::MemCmpExpansionOptions Options;
+
+  if (!ST->hasUAL())
+    return Options;
+
+  Options.MaxNumLoads = TLI->getMaxExpandSizeMemcmp(OptSize);
+  Options.NumLoadsPerBlock = Options.MaxNumLoads;
+  Options.AllowOverlappingLoads = true;
+
+  // TODO: Support for vectors.
+  if (ST->is64Bit()) {
+    Options.LoadSizes = {8, 4, 2, 1};
+    Options.AllowedTailExpansions = {3, 5, 6};
+  } else {
+    Options.LoadSizes = {4, 2, 1};
+    Options.AllowedTailExpansions = {3};
+  }
+
+  return Options;
+}

llvm/lib/Target/LoongArch/LoongArchTargetTransformInfo.h

@@ -55,7 +55,8 @@ class LoongArchTTIImpl : public BasicTTIImplBase<LoongArchTTIImpl> {
 
   bool shouldExpandReduction(const IntrinsicInst *II) const override;
 
-  // TODO: Implement more hooks to provide TTI machinery for LoongArch.
+  TTI::MemCmpExpansionOptions
+  enableMemCmpExpansion(bool OptSize, bool IsZeroCmp) const override;
 };
 
 } // end namespace llvm