Index: llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -5796,8 +5796,9 @@
         if (!SE.isSCEVable(Phi.getType()))
           continue;
         auto PhiSCEV = SE.getSCEV(&Phi);
-        if (Optional<APInt> Offset =
-                SE.computeConstantDifference(DbgValueSCEV, PhiSCEV))
+        Optional<APInt> Offset =
+                SE.computeConstantDifference(DbgValueSCEV, PhiSCEV);
+        if (Offset && Offset->getMinSignedBits() <= 64)
           EqSet.emplace_back(std::make_tuple(
               &Phi, Offset.getValue().getSExtValue(), DVI->getExpression()));
       }
Index: llvm/test/Transforms/LoopStrengthReduce/dbg-preserve-1.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopStrengthReduce/dbg-preserve-1.ll
@@ -0,0 +1,73 @@
+; RUN: opt < %s -loop-reduce -S | FileCheck %s
+;
+; Test that LSR avoids crashing on very large integer inputs. It should
+; discard the variable location by creating an undef dbg.value.
+; 
+; CHECK: call void @llvm.dbg.value(metadata i128 undef,
+
+source_filename = "<stdin>"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define dso_local void @qux() local_unnamed_addr !dbg !10 {
+entry:
+  br label %for.body, !dbg !17
+
+for.body:                                         ; preds = %for.body.for.body_crit_edge, %entry
+  %0 = phi i128 [ 0, %entry ], [ %.pre8, %for.body.for.body_crit_edge ], !dbg !19
+  %add6.i.i = add i128 %0, 18446744073709551615, !dbg !35
+  call void @llvm.dbg.value(metadata i128 %add6.i.i, metadata !25, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 128)), !dbg !36
+  br label %for.body.for.body_crit_edge
+
+for.body.for.body_crit_edge:                      ; preds = %for.body
+  %.pre8 = load i128, i128* undef, align 16, !dbg !19, !tbaa !37
+  br label %for.body, !dbg !17
+}
+
+; Function Attrs: nofree nosync nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!7, !8, !9}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !3, globals: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "/tmp/beans.c", directory: "/tmp")
+!2 = !{}
+!3 = !{!4}
+!4 = !DIDerivedType(tag: DW_TAG_typedef, name: "uint64_t", file: !5, line: 1, baseType: !6)
+!5 = !DIFile(filename: "/tmp/beans.c", directory: "")
+!6 = !DIBasicType(name: "long unsigned int", size: 64, encoding: DW_ATE_unsigned)
+!7 = !{i32 7, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 4}
+!10 = distinct !DISubprogram(name: "qux", scope: !5, file: !5, line: 27, type: !11, scopeLine: 27, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!11 = !DISubroutineType(types: !12)
+!12 = !{null, !13, !13}
+!13 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !14, size: 64)
+!14 = !DIDerivedType(tag: DW_TAG_typedef, name: "croix", file: !5, line: 2, baseType: !15)
+!15 = !DIDerivedType(tag: DW_TAG_typedef, name: "__uint128_t", file: !1, baseType: !16)
+!16 = !DIBasicType(name: "unsigned __int128", size: 128, encoding: DW_ATE_unsigned)
+!17 = !DILocation(line: 30, column: 3, scope: !18)
+!18 = distinct !DILexicalBlock(scope: !10, file: !5, line: 30, column: 3)
+!19 = !DILocation(line: 12, column: 25, scope: !20, inlinedAt: !30)
+!20 = distinct !DISubprogram(name: "bar", scope: !5, file: !5, line: 8, type: !21, scopeLine: 8, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !24)
+!21 = !DISubroutineType(types: !22)
+!22 = !{null, !23, !13}
+!23 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !4, size: 64)
+!24 = !{!25}
+!25 = !DILocalVariable(name: "tmp", scope: !20, file: !5, line: 9, type: !26)
+!26 = !DIDerivedType(tag: DW_TAG_typedef, name: "xyzzy", file: !5, line: 3, baseType: !27)
+!27 = !DICompositeType(tag: DW_TAG_array_type, baseType: !14, size: 512, elements: !28)
+!28 = !{!29}
+!29 = !DISubrange(count: 4)
+!30 = distinct !DILocation(line: 23, column: 3, scope: !31, inlinedAt: !32)
+!31 = distinct !DISubprogram(name: "baz", scope: !5, file: !5, line: 21, type: !11, scopeLine: 21, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!32 = distinct !DILocation(line: 31, column: 5, scope: !33)
+!33 = distinct !DILexicalBlock(scope: !34, file: !5, line: 30, column: 31)
+!34 = distinct !DILexicalBlock(scope: !18, file: !5, line: 30, column: 3)
+!35 = !DILocation(line: 12, column: 23, scope: !20, inlinedAt: !30)
+!36 = !DILocation(line: 0, scope: !20, inlinedAt: !30)
+!37 = !{!38, !38, i64 0}
+!38 = !{!"__int128", !39, i64 0}
+!39 = !{!"omnipotent char", !40, i64 0}
+!40 = !{!"Simple C/C++ TBAA"}