Index: llvm/lib/Transforms/Scalar/LoopBoundSplit.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopBoundSplit.cpp
+++ llvm/lib/Transforms/Scalar/LoopBoundSplit.cpp
@@ -265,6 +265,17 @@
         SplitCandidateCond.BoundSCEV->getType())
       continue;
 
+    // Check SplitCandidateCond's condition with its AddRec start value. The
+    // split cond should be true with the start value because the first
+    // iteration is always executed in pre-loop and we assmue the split
+    // condition is always true in pre-loop.
+    const SCEV *SplitAddRecStartSCEV =
+        dyn_cast<SCEVAddRecExpr>(SplitCandidateCond.AddRecSCEV)->getStart();
+    if (SE.isKnownPredicate(
+            ICmpInst::getInversePredicate(SplitCandidateCond.Pred),
+            SplitAddRecStartSCEV, SplitCandidateCond.BoundSCEV))
+      continue;
+
     SplitCandidateCond.BI = BI;
     return BI;
   }
Index: llvm/test/Transforms/LoopBoundSplit/bug51766.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopBoundSplit/bug51766.ll
@@ -0,0 +1,55 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt  -passes=loop-bound-split -S < %s | FileCheck %s
+
+; LoopBoundSplit pass should fail to split this test's loop because the split
+; condition is false in first iteration of the loop.
+
+define i16 @main(i16 %qqq) {
+; CHECK-LABEL: @main(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[T0:%.*]] = phi i16 [ 0, [[ENTRY:%.*]] ], [ [[T8:%.*]], [[CONT19:%.*]] ]
+; CHECK-NEXT:    [[T1:%.*]] = shl nuw nsw i16 [[T0]], 1
+; CHECK-NEXT:    [[T2:%.*]] = add i16 [[T1]], [[QQQ:%.*]]
+; CHECK-NEXT:    [[DOTNOT9:%.*]] = icmp ult i16 [[T2]], [[QQQ]]
+; CHECK-NEXT:    br i1 [[DOTNOT9]], label [[HANDLER_POINTER_OVERFLOW:%.*]], label [[CONT15_CRITEDGE:%.*]]
+; CHECK:       handler.pointer_overflow:
+; CHECK-NEXT:    call void @__ubsan_handle_pointer_overflow()
+; CHECK-NEXT:    br label [[CONT19]]
+; CHECK:       cont15.critedge:
+; CHECK-NEXT:    br label [[CONT19]]
+; CHECK:       cont19:
+; CHECK-NEXT:    [[T8]] = add nuw nsw i16 [[T0]], 1
+; CHECK-NEXT:    [[EXITCOND_NOT:%.*]] = icmp eq i16 [[T8]], 3
+; CHECK-NEXT:    br i1 [[EXITCOND_NOT]], label [[FOR_COND_CLEANUP:%.*]], label [[FOR_BODY]]
+; CHECK:       for.cond.cleanup:
+; CHECK-NEXT:    ret i16 0
+;
+entry:
+  br label %for.body
+
+for.body:
+  %t0 = phi i16 [ 0, %entry ], [ %t8, %cont19 ]
+  %t1 = shl nuw nsw i16 %t0, 1
+  %t2 = add i16 %t1, %qqq
+  %.not9 = icmp ult i16 %t2, %qqq
+  br i1 %.not9, label %handler.pointer_overflow, label %cont15.critedge
+
+handler.pointer_overflow:
+  call void @__ubsan_handle_pointer_overflow()
+  br label %cont19
+
+cont15.critedge:
+  br label %cont19
+
+cont19:
+  %t8 = add nuw nsw i16 %t0, 1
+  %exitcond.not = icmp eq i16 %t8, 3
+  br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:
+  ret i16 0
+}
+
+declare dso_local void @__ubsan_handle_pointer_overflow()