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

[via llvm-commits]

https://github.com/zhaoqi5 created https://github.com/llvm/llvm-project/pull/166526

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

>From 79e394bf93bddfd7cb2a383c31811035c1f03cf8 Mon Sep 17 00:00:00 2001
From: Qi Zhao <zhaoqi01 at loongson.cn>
Date: Wed, 5 Nov 2025 18:01:05 +0800
Subject: [PATCH] [LoongArch] Initial implementation for
 `enableMemCmpExpansion` hook

After overriding `TargetTransformInfo::enableMemCmpExpansion`
in this commit, `MergeICmps` and `ExpandMemCmp` passes will be
enabled on LoongArch.
---
 .../LoongArchTargetTransformInfo.cpp          | 21 ++++++++++++++++++-
 .../LoongArch/LoongArchTargetTransformInfo.h  |  3 ++-
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Target/LoongArch/LoongArchTargetTransformInfo.cpp b/llvm/lib/Target/LoongArch/LoongArchTargetTransformInfo.cpp
index f548a8dd0532b..1eaac7348ce24 100644
--- a/llvm/lib/Target/LoongArch/LoongArchTargetTransformInfo.cpp
+++ b/llvm/lib/Target/LoongArch/LoongArchTargetTransformInfo.cpp
@@ -111,4 +111,23 @@ 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;
+
+  // TODO: Set same as the default value of MaxLoadsPerMemcmp or
+  // MaxLoadsPerMemcmpOptSize. May need more consideration?
+  Options.MaxNumLoads = TLI->getMaxExpandSizeMemcmp(OptSize);
+  Options.NumLoadsPerBlock = Options.MaxNumLoads;
+  Options.AllowOverlappingLoads = true;
+
+  // TODO: Support for vectors.
+  if (ST->is64Bit())
+    Options.LoadSizes.push_back(8);
+  Options.LoadSizes.append({4, 2, 1});
+
+  return Options;
+}
diff --git a/llvm/lib/Target/LoongArch/LoongArchTargetTransformInfo.h b/llvm/lib/Target/LoongArch/LoongArchTargetTransformInfo.h
index e3f16c7804994..9b479f9dc0dc5 100644
--- a/llvm/lib/Target/LoongArch/LoongArchTargetTransformInfo.h
+++ b/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