Index: llvm/lib/Transforms/Scalar/MergeICmps.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/MergeICmps.cpp
+++ llvm/lib/Transforms/Scalar/MergeICmps.cpp
@@ -330,7 +330,8 @@
 
 // Visit the given comparison block. If this is a comparison between two valid
 // BCE atoms, returns the comparison.
-std::optional<BCECmpBlock> visitCmpBlock(Value *const Val,
+std::optional<BCECmpBlock> visitCmpBlock(Value *const Baseline,
+                                         Value *const Val,
                                          BasicBlock *const Block,
                                          const BasicBlock *const PhiBlock,
                                          BaseIdentifier &BaseId) {
@@ -348,15 +349,20 @@
     // that this does not mean that this is the last incoming value, blocks
     // can be reordered).
     Cond = Val;
-    ExpectedPredicate = ICmpInst::ICMP_EQ;
+    const auto *const ConstBase = cast<ConstantInt>(Baseline);
+    ExpectedPredicate =
+        ConstBase->isOne() ? ICmpInst::ICMP_NE : ICmpInst::ICMP_EQ;
   } else {
+    // All the incoming values must be consistent.
+    if (Baseline != Val)
+      return std::nullopt;
     // In this case, we expect a constant incoming value (the comparison is
     // chained).
     const auto *const Const = cast<ConstantInt>(Val);
     LLVM_DEBUG(dbgs() << "const\n");
-    if (!Const->isZero())
+    if (!Const->isZero() && !Const->isOne())
       return std::nullopt;
-    LLVM_DEBUG(dbgs() << "false\n");
+    LLVM_DEBUG(dbgs() << *Const << "\n");
     assert(BranchI->getNumSuccessors() == 2 && "expecting a cond branch");
     BasicBlock *const FalseBlock = BranchI->getSuccessor(1);
     Cond = BranchI->getCondition();
@@ -475,9 +481,10 @@
   // Now look inside blocks to check for BCE comparisons.
   std::vector<BCECmpBlock> Comparisons;
   BaseIdentifier BaseId;
+  Value *const Baseline = Phi.getIncomingValueForBlock(Blocks[0]);
   for (BasicBlock *const Block : Blocks) {
     assert(Block && "invalid block");
-    std::optional<BCECmpBlock> Comparison = visitCmpBlock(
+    std::optional<BCECmpBlock> Comparison = visitCmpBlock(Baseline,
         Phi.getIncomingValueForBlock(Block), Block, Phi.getParent(), BaseId);
     if (!Comparison) {
       LLVM_DEBUG(dbgs() << "chain with invalid BCECmpBlock, no merge.\n");
@@ -637,6 +644,19 @@
     ToSplit->split(BB, AA);
   }
 
+  BasicBlock *LastBlock = nullptr;
+  for (unsigned I = 0; I < Phi.getNumIncomingValues(); ++I) {
+    if (isa<ConstantInt>(Phi.getIncomingValue(I)))
+      continue;
+    // The only non-constant incoming value is from last block.
+    LastBlock = Phi.getIncomingBlock(I);
+  }
+  auto *const BranchI = cast<BranchInst>(LastBlock->getTerminator());
+  assert(BranchI->isUnconditional() &&
+         "The last link in the chain of comparisons");
+  auto *CmpI = cast<ICmpInst>(Phi.getIncomingValueForBlock(LastBlock));
+  ICmpInst::Predicate Predicate = CmpI->getPredicate();
+
   if (Comparisons.size() == 1) {
     LLVM_DEBUG(dbgs() << "Only one comparison, updating branches\n");
     Value *const LhsLoad =
@@ -644,7 +664,10 @@
     Value *const RhsLoad =
         Builder.CreateLoad(FirstCmp.Rhs().LoadI->getType(), Rhs);
     // There are no blocks to merge, just do the comparison.
-    IsEqual = Builder.CreateICmpEQ(LhsLoad, RhsLoad);
+    if (Predicate == ICmpInst::ICMP_NE)
+      IsEqual = Builder.CreateICmpNE(LhsLoad, RhsLoad);
+    else
+      IsEqual = Builder.CreateICmpEQ(LhsLoad, RhsLoad);
   } else {
     const unsigned TotalSizeBits = std::accumulate(
         Comparisons.begin(), Comparisons.end(), 0u,
@@ -660,8 +683,12 @@
         Lhs, Rhs,
         ConstantInt::get(Builder.getIntNTy(SizeTBits), TotalSizeBits / 8),
         Builder, DL, &TLI);
-    IsEqual = Builder.CreateICmpEQ(
-        MemCmpCall, ConstantInt::get(Builder.getIntNTy(IntBits), 0));
+    if (Predicate == ICmpInst::ICMP_NE)
+      IsEqual = Builder.CreateICmpNE(
+          MemCmpCall, ConstantInt::get(Builder.getIntNTy(IntBits), 0));
+    else
+      IsEqual = Builder.CreateICmpEQ(
+          MemCmpCall, ConstantInt::get(Builder.getIntNTy(IntBits), 0));
   }
 
   BasicBlock *const PhiBB = Phi.getParent();
Index: llvm/test/Transforms/MergeICmps/X86/pr59740.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/MergeICmps/X86/pr59740.ll
@@ -0,0 +1,48 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=mergeicmps -verify-dom-info -S -mtriple=x86_64-unknown-unknown | FileCheck %s
+
+%struct.S = type { i8, i8, i8, i8 }
+
+define noundef i1 @_Z2neR1SS0_(ptr nocapture readonly align 1 dereferenceable(4) %s0, ptr nocapture readonly align 1 dereferenceable(4) %s1) {
+; CHECK-LABEL: @_Z2neR1SS0_(
+; CHECK-NEXT:  "bb0+bb1+bb2+bb3":
+; CHECK-NEXT:    [[MEMCMP:%.*]] = call i32 @memcmp(ptr [[S0:%.*]], ptr [[S1:%.*]], i64 4)
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp ne i32 [[MEMCMP]], 0
+; CHECK-NEXT:    br label [[BB4:%.*]]
+; CHECK:       bb4:
+; CHECK-NEXT:    ret i1 [[TMP0]]
+;
+bb0:
+  %v0 = load i8, ptr %s0, align 1
+  %v1 = load i8, ptr %s1, align 1
+  %cmp0 = icmp eq i8 %v0, %v1
+  br i1 %cmp0, label %bb1, label %bb4
+
+bb1:                                              ; preds = %bb0
+  %s2 = getelementptr inbounds %struct.S, ptr %s0, i64 0, i32 1
+  %v2 = load i8, ptr %s2, align 1
+  %s3 = getelementptr inbounds %struct.S, ptr %s1, i64 0, i32 1
+  %v3 = load i8, ptr %s3, align 1
+  %cmp1 = icmp eq i8 %v2, %v3
+  br i1 %cmp1, label %bb2, label %bb4
+
+bb2:                                             ; preds = %bb1
+  %s4 = getelementptr inbounds %struct.S, ptr %s0, i64 0, i32 2
+  %v4 = load i8, ptr %s4, align 1
+  %s5 = getelementptr inbounds %struct.S, ptr %s1, i64 0, i32 2
+  %v5 = load i8, ptr %s5, align 1
+  %cmp2 = icmp eq i8 %v4, %v5
+  br i1 %cmp2, label %bb3, label %bb4
+
+bb3:                                               ; preds = %bb2
+  %s6 = getelementptr inbounds %struct.S, ptr %s0, i64 0, i32 3
+  %v6 = load i8, ptr %s6, align 1
+  %s7 = getelementptr inbounds %struct.S, ptr %s1, i64 0, i32 3
+  %v7 = load i8, ptr %s7, align 1
+  %cmp3 = icmp ne i8 %v6, %v7
+  br label %bb4
+
+bb4:                                               ; preds = %bb0, %bb1, %bb2, %bb3
+  %cmp = phi i1 [ true, %bb0 ], [ true, %bb1 ], [ true, %bb2 ], [ %cmp3, %bb3 ]
+  ret i1 %cmp
+}