Index: llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
+++ llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
@@ -592,8 +592,10 @@
   /// function that describe the same variable. If the resulting 
   /// list has only one entry that is valid for entire variable's
   /// scope return true.
-  bool buildLocationList(SmallVectorImpl<DebugLocEntry> &DebugLoc,
-                         const DbgValueHistoryMap::Entries &Entries);
+  bool buildLocationList(
+      SmallVectorImpl<DebugLocEntry> &DebugLoc,
+      const DbgValueHistoryMap::Entries &Entries,
+      SmallPtrSetImpl<const MachineBasicBlock *> &VeryLargeBlocks);
 
   /// Collect variable information from the side table maintained by MF.
   void collectVariableInfoFromMFTable(DwarfCompileUnit &TheCU,
Index: llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -167,6 +167,11 @@
                                             "Abstract subprograms")),
                       cl::init(DefaultLinkageNames));
 
+static cl::opt<unsigned> LocationAnalysisSizeLimit(
+    "singlevarlocation-input-bb-limit",
+    cl::desc("Maximum block size to analyze for single-location variables"),
+    cl::init(30000), cl::Hidden);
+
 static const char *const DWARFGroupName = "dwarf";
 static const char *const DWARFGroupDescription = "DWARF Emission";
 static const char *const DbgTimerName = "writer";
@@ -1637,8 +1642,10 @@
 // [1-3)    [(reg0, fragment 0, 32), (reg1, fragment 32, 32)]
 // [3-4)    [(reg1, fragment 32, 32), (123, fragment 64, 32)]
 // [4-)     [(@g, fragment 0, 96)]
-bool DwarfDebug::buildLocationList(SmallVectorImpl<DebugLocEntry> &DebugLoc,
-                                   const DbgValueHistoryMap::Entries &Entries) {
+bool DwarfDebug::buildLocationList(
+    SmallVectorImpl<DebugLocEntry> &DebugLoc,
+    const DbgValueHistoryMap::Entries &Entries,
+    SmallPtrSetImpl<const MachineBasicBlock *> &VeryLargeBlocks) {
   using OpenRange =
       std::pair<DbgValueHistoryMap::EntryIndex, DbgValueLoc>;
   SmallVector<OpenRange, 4> OpenRanges;
@@ -1734,8 +1741,14 @@
       DebugLoc.pop_back();
   }
 
-  return DebugLoc.size() == 1 && isSafeForSingleLocation &&
-         validThroughout(LScopes, StartDebugMI, EndMI);
+  // If there's a single entry, safe for a single location, and not part of
+  // an over-sized basic block, then ask validThroughout whether this
+  // location can be represented as a single variable location.
+  if (DebugLoc.size() != 1 || !isSafeForSingleLocation)
+    return false;
+  if (VeryLargeBlocks.count(StartDebugMI->getParent()))
+    return false;
+  return validThroughout(LScopes, StartDebugMI, EndMI);
 }
 
 DbgEntity *DwarfDebug::createConcreteEntity(DwarfCompileUnit &TheCU,
@@ -1767,6 +1780,13 @@
   // Grab the variable info that was squirreled away in the MMI side-table.
   collectVariableInfoFromMFTable(TheCU, Processed);
 
+  // Identify blocks that are unreasonably sized, so that we can later
+  // skip lexical scope analysis over them.
+  SmallPtrSet<const MachineBasicBlock *, 8> VeryLargeBlocks;
+  for (const auto &MBB : *CurFn)
+    if (MBB.size() > LocationAnalysisSizeLimit)
+      VeryLargeBlocks.insert(&MBB);
+
   for (const auto &I : DbgValues) {
     InlinedEntity IV = I.first;
     if (Processed.count(IV))
@@ -1803,7 +1823,8 @@
     if (HistSize == 1 || SingleValueWithClobber) {
       const auto *End =
           SingleValueWithClobber ? HistoryMapEntries[1].getInstr() : nullptr;
-      if (validThroughout(LScopes, MInsn, End)) {
+      if (VeryLargeBlocks.count(MInsn->getParent()) == 0 &&
+          validThroughout(LScopes, MInsn, End)) {
         RegVar->initializeDbgValue(MInsn);
         continue;
       }
@@ -1818,7 +1839,8 @@
 
     // Build the location list for this variable.
     SmallVector<DebugLocEntry, 8> Entries;
-    bool isValidSingleLocation = buildLocationList(Entries, HistoryMapEntries);
+    bool isValidSingleLocation =
+        buildLocationList(Entries, HistoryMapEntries, VeryLargeBlocks);
 
     // Check whether buildLocationList managed to merge all locations to one
     // that is valid throughout the variable's scope. If so, produce single
Index: llvm/test/DebugInfo/MIR/X86/singlelocation-cutoffs.mir
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/MIR/X86/singlelocation-cutoffs.mir
@@ -0,0 +1,65 @@
+# Test cutoffs for single-location variable analysis.
+# Disable validThroughout if the input size exceeds the specified limit
+
+# RUN: llc %s -o - -start-after=livedebugvalues -mtriple=x86_64-unknown-unknown \
+# RUN:   --singlevarlocation-input-bb-limit=0 -filetype=obj\
+# RUN:   | llvm-dwarfdump -v -\
+# RUN:   | FileCheck %s -check-prefix=LIMITED
+
+# RUN: llc %s -o - -start-after=livedebugvalues -mtriple=x86_64-unknown-unknown \
+# RUN: --singlevarlocation-input-bb-limit=20 -filetype=obj | llvm-dwarfdump -v -\
+# RUN:   | FileCheck %s -check-prefix=UNLIMITED
+
+# LIMITED: DW_AT_location [DW_FORM_sec_offset]
+
+# UNLIMITED: DW_AT_location [DW_FORM_exprloc]
+
+--- |
+  target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+  declare i32 @use(i32)
+
+  define i32 @foo(i32 %x) !dbg !6 {
+  entry:
+    ret i32 1, !dbg !15
+  }
+
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.debugify = !{!3, !4}
+  !llvm.module.flags = !{!5}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+  !1 = !DIFile(filename: "/tmp/t.ll", directory: "/")
+  !2 = !{}
+  !3 = !{i32 4}
+  !4 = !{i32 2}
+  !5 = !{i32 2, !"Debug Info Version", i32 3}
+  !6 = distinct !DISubprogram(name: "foo", linkageName: "foo", scope: null, file: !1, line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !8)
+  !7 = !DISubroutineType(types: !2)
+  !8 = !{!9, !11}
+  !9 = !DILocalVariable(name: "1", scope: !6, file: !1, line: 1, type: !10)
+  !10 = !DIBasicType(name: "ty32", size: 32, encoding: DW_ATE_unsigned)
+  !11 = !DILocalVariable(name: "2", scope: !6, file: !1, line: 3, type: !10)
+  !12 = !DILocation(line: 1, column: 1, scope: !6)
+  !13 = !DILocation(line: 2, column: 1, scope: !6)
+  !14 = !DILocation(line: 3, column: 1, scope: !6)
+  !15 = !DILocation(line: 4, column: 1, scope: !6)
+
+...
+---
+name:            foo
+liveins:
+  - { reg: '$edi', virtual-reg: '' }
+stack:
+  - { id: 0, name: '', type: spill-slot, offset: -12, size: 4, alignment: 4,
+      stack-id: default, callee-saved-register: '', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+body:             |
+  bb.0.entry:
+    liveins: $edi
+    DBG_VALUE renamable $edi, $noreg, !11, !DIExpression(), debug-location !14
+    RETQ debug-location !14
+
+...