Index: llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -6216,6 +6216,13 @@
         DVIRec.DVI->setExpression(DVIRec.Expr);
       }
 
+      // SCEVUnknowns may have null Value pointers if the corresponding
+      // instruction was deleted, these are not safe to pass to
+      // the SCEV utilities.
+      if (auto *U = dyn_cast<SCEVUnknown>(DVIRec.SCEV))
+        if (U->getValue() == nullptr)
+          continue;
+
       LLVM_DEBUG(dbgs() << "scev-salvage: value to recover SCEV: "
                         << *DVIRec.SCEV << '\n');
 
Index: llvm/test/Transforms/LoopStrengthReduce/deleted-debug-phi.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopStrengthReduce/deleted-debug-phi.ll
@@ -0,0 +1,106 @@
+; The phi in while.cond179 is dead and is removed by LoopStrengthReduce.
+; When the SCEVs are salvaged to update the debug MD, the SCEV for the phi
+; should not be analyzed, as it is a SCEVUnknown with a null Value.
+; This is exposed by the debug printout which will crash when dumping that SCEV.
+
+; REQUIRES: asserts
+; RUN: opt -debugify -loop-reduce -debug -S < %s | FileCheck %s
+source_filename = "deleted-debug-phi.ll"
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
+target triple = "x86_64-apple-darwin10.0.0"
+
+; Function Attrs: nounwind
+define void @nopreheader(i8* %end) #0 {
+
+; CHECK-LABEL: @nopreheader(
+; CHECK:       while.cond179:
+; CHECK-NEXT:    call void @llvm.dbg.value(metadata i8* undef, metadata [[META9:![0-9]+]], metadata !DIExpression()), !dbg [[DBG16:![0-9]+]]
+
+; CHECK:       while.cond238:
+; CHECK-NEXT:    [[TMP0:%.*]] = phi i64 {{.*}} !dbg [[DBG25:![0-9]+]]
+; CHECK-NEXT:    call void @llvm.dbg.value(metadata i64 [[TMP0]], metadata [[META11:![0-9]+]], metadata !DIExpression()), !dbg [[DBG25]]
+; CHECK-NEXT:    [[TMPX:%.*]] = add i64 undef, [[TMP0]], !dbg [[DBG26:![0-9]+]]
+; CHECK-NEXT:    call void @llvm.dbg.value(metadata i64 [[TMPX]], metadata [[META12:![0-9]+]], metadata !DIExpression()), !dbg [[DBG26]]
+
+; CHECK:       land.rhs243:
+; CHECK-NEXT:    [[INDVAR_NEXT15:%.*]] = add i64 [[TMP0]], 1, !dbg [[DBG28:![0-9]+]]
+; CHECK-NEXT:    call void @llvm.dbg.value(metadata i64 [[INDVAR_NEXT15]], metadata [[META13:![0-9]+]], metadata !DIExpression()), !dbg [[DBG28]]
+
+entry:
+  br label %while.cond179
+
+while.cond179:                                    ; preds = %if.end348, %entry
+  %s.1 = phi i8* [ %incdec.ptr356, %if.end348 ], [ undef, %entry ]
+  indirectbr i8* undef, [label %land.rhs184, label %while.end453]
+
+land.rhs184:                                      ; preds = %while.cond179
+  indirectbr i8* undef, [label %while.end453, label %while.cond197]
+
+while.cond197:                                    ; preds = %land.rhs202, %land.rhs184
+  indirectbr i8* undef, [label %land.rhs202, label %while.end215]
+
+land.rhs202:                                      ; preds = %while.cond197
+  indirectbr i8* undef, [label %while.end215, label %while.cond197]
+
+while.end215:                                     ; preds = %land.rhs202, %while.cond197
+  indirectbr i8* undef, [label %PREMATURE, label %if.end221]
+
+if.end221:                                        ; preds = %while.end215
+  indirectbr i8* undef, [label %while.cond238.preheader, label %lor.lhs.false227]
+
+lor.lhs.false227:                                 ; preds = %if.end221
+  indirectbr i8* undef, [label %while.cond238.preheader, label %if.else]
+
+while.cond238.preheader:                          ; preds = %lor.lhs.false227, %if.end221
+  indirectbr i8* undef, [label %while.cond238]
+
+while.cond238:                                    ; preds = %land.rhs243, %while.cond238.preheader
+  %0 = phi i64 [ %indvar.next15, %land.rhs243 ], [ 0, %while.cond238.preheader ]
+  %tmpx = add i64 undef, %0
+  indirectbr i8* undef, [label %land.rhs243, label %while.end256]
+
+land.rhs243:                                      ; preds = %while.cond238
+  %indvar.next15 = add i64 %0, 1
+  indirectbr i8* undef, [label %while.end256, label %while.cond238]
+
+while.end256:                                     ; preds = %land.rhs243, %while.cond238
+  unreachable
+
+if.else:                                          ; preds = %lor.lhs.false227
+  indirectbr i8* undef, [label %if.then297, label %if.else386]
+
+if.then297:                                       ; preds = %if.else
+  indirectbr i8* undef, [label %PREMATURE, label %if.end307]
+
+if.end307:                                        ; preds = %if.then297
+  indirectbr i8* undef, [label %if.end314, label %FAIL]
+
+if.end314:                                        ; preds = %if.end307
+  indirectbr i8* undef, [label %PREMATURE, label %if.end348]
+
+if.end348:                                        ; preds = %if.end314
+  %incdec.ptr356 = getelementptr inbounds i8, i8* undef, i64 2
+  indirectbr i8* undef, [label %while.cond179]
+
+if.else386:                                       ; preds = %if.else
+  unreachable
+
+while.end453:                                     ; preds = %land.rhs184, %while.cond179
+  unreachable
+
+FAIL:                                             ; preds = %if.end307
+  unreachable
+
+PREMATURE:                                        ; preds = %if.end314, %if.then297, %while.end215
+  unreachable
+}
+
+; Function Attrs: nofree nosync nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+attributes #0 = { nounwind }
+attributes #1 = { nofree nosync nounwind readnone speculatable willreturn }
+
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 2, !"Debug Info Version", i32 3}