diff --git a/llvm/include/llvm/Transforms/Utils/LoopUtils.h b/llvm/include/llvm/Transforms/Utils/LoopUtils.h
--- a/llvm/include/llvm/Transforms/Utils/LoopUtils.h
+++ b/llvm/include/llvm/Transforms/Utils/LoopUtils.h
@@ -73,7 +73,8 @@
 ///
 /// Returns true if any modifications are made.
 bool formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
-                              DominatorTree &DT, LoopInfo &LI);
+                              DominatorTree &DT, LoopInfo &LI,
+                              ScalarEvolution *SE);
 
 /// Put loop into LCSSA form.
 ///
diff --git a/llvm/lib/Transforms/Utils/LCSSA.cpp b/llvm/lib/Transforms/Utils/LCSSA.cpp
--- a/llvm/lib/Transforms/Utils/LCSSA.cpp
+++ b/llvm/lib/Transforms/Utils/LCSSA.cpp
@@ -74,7 +74,8 @@
 /// that are outside the current loop.  If so, insert LCSSA PHI nodes and
 /// rewrite the uses.
 bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
-                                    DominatorTree &DT, LoopInfo &LI) {
+                                    DominatorTree &DT, LoopInfo &LI,
+                                    ScalarEvolution *SE) {
   SmallVector<Use *, 16> UsesToRewrite;
   SmallSetVector<PHINode *, 16> PHIsToRemove;
   PredIteratorCache PredCache;
@@ -134,6 +135,11 @@
     SSAUpdater SSAUpdate(&InsertedPHIs);
     SSAUpdate.Initialize(I->getType(), I->getName());
 
+    // Force re-computation of I, as some users now need to use the new PHI
+    // node.
+    if (SE)
+      SE->forgetValue(I);
+
     // Insert the LCSSA phi's into all of the exit blocks dominated by the
     // value, and add them to the Phi's map.
     for (BasicBlock *ExitBB : ExitBlocks) {
@@ -368,7 +374,7 @@
       Worklist.push_back(&I);
     }
   }
-  Changed = formLCSSAForInstructions(Worklist, DT, *LI);
+  Changed = formLCSSAForInstructions(Worklist, DT, *LI, SE);
 
   // If we modified the code, remove any caches about the loop from SCEV to
   // avoid dangling entries.
diff --git a/llvm/test/Transforms/LoopUnroll/unroll-preserve-scev-lcssa.ll b/llvm/test/Transforms/LoopUnroll/unroll-preserve-scev-lcssa.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/LoopUnroll/unroll-preserve-scev-lcssa.ll
@@ -0,0 +1,71 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -loop-reduce -loop-unroll -unroll-runtime -verify-scev -verify-loop-lcssa < %s -S | FileCheck %s
+
+; Test case for PR43458. We check that we do properly invalidate LCSSA phi
+; users in SCEV. Run -loop-reduce first, so SCEV is already populated.
+
+define void @spam() {
+; CHECK-LABEL: @spam(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    br label [[BB3:%.*]]
+; CHECK:       bb3:
+; CHECK-NEXT:    br i1 false, label [[BB9:%.*]], label [[BB5_PREHEADER:%.*]]
+; CHECK:       bb5.preheader:
+; CHECK-NEXT:    br label [[BB5:%.*]]
+; CHECK:       bb5:
+; CHECK-NEXT:    br label [[BB9]]
+; CHECK:       bb9:
+; CHECK-NEXT:    [[TMP10:%.*]] = phi i64 [ 0, [[BB3]] ], [ 5, [[BB5]] ]
+; CHECK-NEXT:    [[TMP11:%.*]] = trunc i64 [[TMP10]] to i32
+; CHECK-NEXT:    [[TMP21:%.*]] = sext i32 [[TMP11]] to i64
+; CHECK-NEXT:    [[TMP22:%.*]] = icmp slt i64 0, [[TMP21]]
+; CHECK-NEXT:    br i1 [[TMP22]], label [[BB24_PREHEADER:%.*]], label [[BB29:%.*]]
+; CHECK:       bb24.preheader:
+; CHECK-NEXT:    br label [[BB24:%.*]]
+; CHECK:       bb24:
+; CHECK-NEXT:    [[TMP25:%.*]] = phi i64 [ [[TMP26:%.*]], [[BB24]] ], [ 0, [[BB24_PREHEADER]] ]
+; CHECK-NEXT:    [[TMP26]] = add nuw nsw i64 [[TMP25]], 1
+; CHECK-NEXT:    [[TMP27:%.*]] = icmp slt i64 [[TMP26]], [[TMP21]]
+; CHECK-NEXT:    br i1 [[TMP27]], label [[BB24]], label [[BB29_LOOPEXIT:%.*]]
+; CHECK:       bb29.loopexit:
+; CHECK-NEXT:    br label [[BB29]]
+; CHECK:       bb29:
+; CHECK-NEXT:    ret void
+;
+bb:
+  br label %bb3
+
+bb3:                                              ; preds = %bb9, %bb
+  %pv1 = phi i64 [ 0, %bb ], [ %iv1, %bb9]
+  %c1 = icmp eq i64 %pv1, 5
+  br i1 %c1, label %bb9, label %bb5
+
+bb5:                                              ; preds = %bb5, %bb3
+  %pv = phi i64 [ 0, %bb3], [ %tmp6, %bb5]
+  %tmp6 = add nsw i64 %pv, 1
+  %cond = icmp eq i64 %tmp6, 5
+  br i1 %cond, label %bb8, label %bb5
+
+bb8:                                              ; preds = %bb5
+  br label %bb9
+
+bb9:                                              ; preds = %bb8, %bb3
+  %tmp10 = phi i64 [ 0, %bb3 ], [ %tmp6, %bb8 ]
+  %tmp11 = trunc i64 %tmp10 to i32
+  %iv1 = add nsw i64 %pv1, 1
+  br i1 false, label %bb3, label %bb20
+
+bb20:                                             ; preds = %bb9
+  %tmp21 = sext i32 %tmp11 to i64
+  %tmp22 = icmp slt i64 0, %tmp21
+  br i1 %tmp22, label %bb24, label %bb29
+
+bb24:                                             ; preds = %bb24, %bb20
+  %tmp25 = phi i64 [ %tmp26, %bb24 ], [ 0, %bb20 ]
+  %tmp26 = add nuw nsw i64 %tmp25, 1
+  %tmp27 = icmp slt i64 %tmp26, %tmp21
+  br i1 %tmp27, label %bb24, label %bb29
+
+bb29:                                             ; preds = %bb24, %bb20
+  ret void
+}