Index: lib/CodeGen/AsmPrinter/DwarfDebug.cpp
===================================================================
--- lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -1489,7 +1489,7 @@
   // We have an explicit location, different from the previous location.
   // Don't repeat a line-0 record, but otherwise emit the new location.
   // (The new location might be an explicit line 0, which we do emit.)
-  if (PrevInstLoc && DL.getLine() == 0 && LastAsmLine == 0)
+  if (DL.getLine() == 0 && LastAsmLine == 0)
     return;
   unsigned Flags = 0;
   if (DL == PrologEndLoc) {
Index: test/DebugInfo/MIR/X86/debug-loc-0.mir
===================================================================
--- /dev/null
+++ test/DebugInfo/MIR/X86/debug-loc-0.mir
@@ -0,0 +1,116 @@
+# Check that subsequent .loc 0 0 directives are collapsed.
+
+# RUN: llc -O2 -filetype=asm -mtriple=x86_64-apple-macosx10.9.0 -o - %s | FileCheck %s
+
+# CHECK: Ltmp0:
+# CHECK: .loc 1 0 0 prologue_end
+# CHECK-NOT: .loc 1 0 0
+# CHECK: .loc 1 37 1
+
+--- |
+  ; ModuleID = '<stdin>'
+  source_filename = "<stdin>"
+  target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+  target triple = "x86_64-apple-macosx10.9.0"
+
+  %swift.opaque = type opaque
+  %swift.metadata_response = type { %swift.type*, i64 }
+  %swift.type = type { i64 }
+
+  define hidden swiftcc void @"$S4main1fyyF"() !dbg !5 {
+  entry:
+    %s1.addr = alloca i8*, align 8
+    call void @llvm.dbg.declare(metadata i8** %s1.addr, metadata !11, metadata !DIExpression(DW_OP_deref)), !dbg !16
+    %0 = bitcast i8** %s1.addr to %swift.opaque**
+    store %swift.opaque* null, %swift.opaque** %0, align 8
+    %1 = call swiftcc %swift.metadata_response @"$S16resilient_struct4SizeVMa"(i64 0) #2, !dbg !17
+    %2 = extractvalue %swift.metadata_response %1, 0, !dbg !17
+    %3 = bitcast %swift.type* %2 to i8***, !dbg !17
+    %4 = getelementptr inbounds i8**, i8*** %3, i64 -1, !dbg !17
+    ret void, !dbg !18
+  }
+
+  ; Function Attrs: nounwind readnone speculatable
+  declare void @llvm.dbg.declare(metadata, metadata, metadata) #0
+
+  declare swiftcc %swift.metadata_response @"$S16resilient_struct4SizeVMa"(i64)
+
+  ; Function Attrs: nounwind
+  declare void @llvm.stackprotector(i8*, i8**) #1
+
+  attributes #0 = { nounwind readnone speculatable }
+  attributes #1 = { nounwind }
+  attributes #2 = { nounwind readnone }
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!3, !4}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_Swift, file: !1, producer: "Swift version 4.2.1-dev effective-3.4.1 (LLVM 4ba03d9389, Clang 78aa734eee, Swift f4134ebef6)", isOptimized: false, flags: "-target x86_64-apple-macosx10.9 -module-cache-path /Users/jonas/swift/build/Ninja-ReleaseAssert/swift-macosx-x86_64/./swift-test-results/x86_64-apple-macosx10.9/clang-module-cache -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk -swift-version 3 -g -I /Users/jonas/swift/build/Ninja-ReleaseAssert/swift-macosx-x86_64/test-macosx-x86_64/DebugInfo/Output/struct_resilience.swift.tmp -emit-ir -enable-resilience /Users/jonas/swift/swift/test/DebugInfo/struct_resilience.swift -o /tmp/41.ll -resource-dir /Users/jonas/swift/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift", runtimeVersion: 4, emissionKind: FullDebug, enums: !2, imports: !2)
+  !1 = !DIFile(filename: "/Users/jonas/swift/swift/test/DebugInfo/struct_resilience.swift", directory: "/Users/jonas/swift")
+  !2 = !{}
+  !3 = !{i32 2, !"Dwarf Version", i32 4}
+  !4 = !{i32 2, !"Debug Info Version", i32 3}
+  !5 = distinct !DISubprogram(name: "f", linkageName: "$S4main1fyyF", scope: !7, file: !6, line: 23, type: !8, scopeLine: 23, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+  !6 = !DIFile(filename: "struct_resilience.swift", directory: "/Users/jonas/swift/swift/test/DebugInfo")
+  !7 = !DIModule(scope: null, name: "main", includePath: "/Users/jonas/swift/swift/test/DebugInfo", isysroot: "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk")
+  !8 = !DISubroutineType(types: !9)
+  !9 = !{!10}
+  !10 = !DICompositeType(tag: DW_TAG_structure_type, name: "$SytD", file: !6, elements: !2, runtimeLang: DW_LANG_Swift, identifier: "$SytD")
+  !11 = !DILocalVariable(name: "s1", scope: !12, file: !6, line: 24, type: !13)
+  !12 = distinct !DILexicalBlock(scope: !5, file: !6, line: 23, column: 10)
+  !13 = !DICompositeType(tag: DW_TAG_structure_type, name: "Size", scope: !15, file: !14, size: 512, elements: !2, runtimeLang: DW_LANG_Swift, identifier: "$S16resilient_struct4SizeVD")
+  !14 = !DIFile(filename: "resilient_struct.swiftmodule", directory: "/Users/jonas/swift/build/Ninja-ReleaseAssert/swift-macosx-x86_64/test-macosx-x86_64/DebugInfo/Output/struct_resilience.swift.tmp")
+  !15 = !DIModule(scope: null, name: "resilient_struct", includePath: "/Users/jonas/swift/build/Ninja-ReleaseAssert/swift-macosx-x86_64/test-macosx-x86_64/DebugInfo/Output/struct_resilience.swift.tmp/resilient_struct.swiftmodule", isysroot: "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk")
+  !16 = !DILocation(line: 24, column: 7, scope: !12)
+  !17 = !DILocation(line: 0, scope: !12)
+  !18 = !DILocation(line: 37, column: 1, scope: !12)
+
+...
+---
+name:            '$S4main1fyyF'
+alignment:       4
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+registers:       []
+liveins:         []
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    8
+  adjustsStack:    false
+  hasCalls:        true
+  stackProtector:  ''
+  maxCallFrameSize: 4294967295
+  cvBytesOfCalleeSavedRegisters: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:      []
+stack:
+  - { id: 0, name: s1.addr, type: default, offset: 0, size: 8, alignment: 8,
+      stack-id: 0, callee-saved-register: '', callee-saved-restored: true,
+      debug-info-variable: '!11', debug-info-expression: '!DIExpression(DW_OP_deref)',
+      debug-info-location: '!16' }
+constants:       []
+body:             |
+  bb.0.entry:
+    MOV64mi32 %stack.0.s1.addr, 1, $noreg, 0, $noreg, 0 :: (store 8 into %ir.0)
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !17
+    dead $edi = MOV32r0 implicit-def dead $eflags, implicit-def $rdi, debug-location !17
+    CALL64pcrel32 @"$S16resilient_struct4SizeVMa", csr_64, implicit $rsp, implicit $ssp, implicit killed $rdi, implicit-def $rsp, implicit-def $ssp, implicit-def dead $rax, implicit-def dead $rdx, debug-location !17
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !17
+    RET 0, debug-location !18
+
+...