Index: lib/CodeGen/AsmPrinter/DwarfDebug.cpp
===================================================================
--- lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -1313,6 +1313,20 @@
   }
 }
 
+static const DebugLoc &
+findNextDebugLoc(MachineBasicBlock::const_iterator MBBI,
+                 MachineBasicBlock::const_iterator MBBE) {
+  static DebugLoc NoLocation;
+  for ( ; MBBI != MBBE; ++MBBI) {
+    if (MBBI->isDebugInstr())
+      continue;
+    const DebugLoc &DL = MBBI->getDebugLoc();
+    if (DL)
+      return DL;
+  }
+  return NoLocation;
+}
+
 // Process beginning of an instruction.
 void DwarfDebug::beginInstruction(const MachineInstr *MI) {
   DebugHandlerBase::beginInstruction(MI);
@@ -1327,43 +1341,48 @@
   // any line record, as there is no correspondence with any user code.
   if (MI->isMetaInstruction() || MI->getFlag(MachineInstr::FrameSetup))
     return;
-  const DebugLoc &DL = MI->getDebugLoc();
+  const DebugLoc *DL = &MI->getDebugLoc();
   // When we emit a line-0 record, we don't update PrevInstLoc; so look at
   // the last line number actually emitted, to see if it was line 0.
   unsigned LastAsmLine =
       Asm->OutStreamer->getContext().getCurrentDwarfLoc().getLine();
 
-  if (DL == PrevInstLoc) {
+  if (*DL == PrevInstLoc) {
     // If we have an ongoing unspecified location, nothing to do here.
-    if (!DL)
+    if (!*DL)
       return;
     // We have an explicit location, same as the previous location.
     // But we might be coming back to it after a line 0 record.
-    if (LastAsmLine == 0 && DL.getLine() != 0) {
+    if (LastAsmLine == 0 && DL->getLine() != 0) {
       // Reinstate the source location but not marked as a statement.
-      const MDNode *Scope = DL.getScope();
-      recordSourceLine(DL.getLine(), DL.getCol(), Scope, /*Flags=*/0);
+      const MDNode *Scope = DL->getScope();
+      recordSourceLine(DL->getLine(), DL->getCol(), Scope, /*Flags=*/0);
     }
     return;
   }
 
-  if (!DL) {
+  if (!*DL) {
     // We have an unspecified location, which might want to be line 0.
     // If we have already emitted a line-0 record, don't repeat it.
     if (LastAsmLine == 0)
       return;
-    // If user said Don't Do That, don't do that.
-    if (UnknownLocations == Disable)
-      return;
-    // See if we have a reason to emit a line-0 record now.
-    // Reasons to emit a line-0 record include:
-    // - User asked for it (UnknownLocations).
-    // - Instruction has a label, so it's referenced from somewhere else,
-    //   possibly debug information; we want it to have a source location.
-    // - Instruction is at the top of a block; we don't want to inherit the
-    //   location from the physically previous (maybe unrelated) block.
-    if (UnknownLocations == Enable || PrevLabel ||
-        (PrevInstBB && PrevInstBB != MI->getParent())) {
+    // By default we emit nothing to avoid line table bloat. However at the
+    // beginning of a basic block or after a label it is undesirable to let
+    // the previous location unchanged. In these cases do a forward search for
+    // the next valid debug location.
+    if (UnknownLocations == Default) {
+      const MachineBasicBlock &MBB = *MI->getParent();
+      if (PrevLabel || PrevInstBB != &MBB) {
+        DL = &findNextDebugLoc(MI->getIterator(), MBB.end());
+      } else {
+        return;
+      }
+    }
+    if (!*DL) {
+      // If user said Don't Do That, don't do that.
+      if (UnknownLocations == Disable)
+        return;
+      // Emit a line-0 record now.
       // Preserve the file and column numbers, if we can, to save space in
       // the encoded line table.
       // Do not update PrevInstLoc, it remembers the last non-0 line.
@@ -1374,32 +1393,33 @@
         Column = PrevInstLoc.getCol();
       }
       recordSourceLine(/*Line=*/0, Column, Scope, /*Flags=*/0);
+      return;
     }
-    return;
   }
 
   // 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)
+  unsigned Line = DL->getLine();
+  if (PrevInstLoc && Line == 0 && LastAsmLine == 0)
     return;
   unsigned Flags = 0;
-  if (DL == PrologEndLoc) {
+  if (*DL == PrologEndLoc) {
     Flags |= DWARF2_FLAG_PROLOGUE_END | DWARF2_FLAG_IS_STMT;
     PrologEndLoc = DebugLoc();
   }
   // If the line changed, we call that a new statement; unless we went to
   // line 0 and came back, in which case it is not a new statement.
   unsigned OldLine = PrevInstLoc ? PrevInstLoc.getLine() : LastAsmLine;
-  if (DL.getLine() && DL.getLine() != OldLine)
+  if (Line && Line != OldLine)
     Flags |= DWARF2_FLAG_IS_STMT;
 
-  const MDNode *Scope = DL.getScope();
-  recordSourceLine(DL.getLine(), DL.getCol(), Scope, Flags);
+  const MDNode *Scope = DL->getScope();
+  recordSourceLine(Line, DL->getCol(), Scope, Flags);
 
   // If we're not at line 0, remember this location.
-  if (DL.getLine())
-    PrevInstLoc = DL;
+  if (Line)
+    PrevInstLoc = *DL;
 }
 
 static DebugLoc findPrologueEndLoc(const MachineFunction *MF) {
Index: test/DebugInfo/AArch64/line-header.ll
===================================================================
--- test/DebugInfo/AArch64/line-header.ll
+++ test/DebugInfo/AArch64/line-header.ll
@@ -3,4 +3,4 @@
 
 ; check line table length is correctly calculated for both big and little endian
 CHECK-LABEL: .debug_line contents:
-CHECK: total_length: 0x0000003f
+CHECK: total_length: 0x0000003c
Index: test/DebugInfo/ARM/single-constant-use-preserves-dbgloc.ll
===================================================================
--- test/DebugInfo/ARM/single-constant-use-preserves-dbgloc.ll
+++ test/DebugInfo/ARM/single-constant-use-preserves-dbgloc.ll
@@ -31,11 +31,10 @@
 
 if.end:                                           ; preds = %entry
 ; Materialize the constant.
-; CHECK:      .loc    1 0
+; CHECK:      .loc    1 7 5
 ; CHECK-NEXT: mvn     r0, #0
 
 ; The backend performs the store to %retval first, for some reason.
-; CHECK-NEXT: .loc    1 7 5
 ; CHECK-NEXT: str     r0, [sp, #4]
   store i32 -1, i32* %x, align 4, !dbg !19
 
Index: test/DebugInfo/Mips/delay-slot.ll
===================================================================
--- test/DebugInfo/Mips/delay-slot.ll
+++ test/DebugInfo/Mips/delay-slot.ll
@@ -16,7 +16,7 @@
 ; CHECK: 0x0000000000000004      2      0      1   0             0  is_stmt prologue_end
 ; CHECK: 0x0000000000000024      3      0      1   0             0  is_stmt
 ; CHECK: 0x0000000000000034      4      0      1   0             0  is_stmt
-; CHECK: 0x0000000000000048      5      0      1   0             0  is_stmt
+; CHECK: 0x0000000000000044      5      0      1   0             0  is_stmt
 ; CHECK: 0x0000000000000058      5      0      1   0             0  is_stmt end_sequence
 
 
Index: test/DebugInfo/NVPTX/debug-info.ll
===================================================================
--- test/DebugInfo/NVPTX/debug-info.ll
+++ test/DebugInfo/NVPTX/debug-info.ll
@@ -36,6 +36,7 @@
 ; CHECK: setp.ge.s32     %p{{.+}}, %r{{.+}}, %r{{.+}};
 ; CHECK: .loc [[DEBUG_INFO_CU]] 7 7
 ; CHECK: @%p{{.+}} bra   [[BB:.+]];
+; CHECK: .loc [[DEBUG_INFO_CU]] 8 13
 ; CHECK: ld.param.f32    %f{{.+}}, [{{.+}}];
 ; CHECK: ld.param.u64    %rd{{.+}}, [{{.+}}];
 ; CHECK: cvta.to.global.u64      %rd{{.+}}, %rd{{.+}};
@@ -43,7 +44,6 @@
 ; CHECK: cvta.to.global.u64      %rd{{.+}}, %rd{{.+}};
 ; CHECK: mul.wide.u32    %rd{{.+}}, %r{{.+}}, 4;
 ; CHECK: add.s64         %rd{{.+}}, %rd{{.+}}, %rd{{.+}};
-; CHECK: .loc [[DEBUG_INFO_CU]] 8 13
 ; CHECK: ld.global.f32   %f{{.+}}, [%rd{{.+}}];
 ; CHECK: add.s64         %rd{{.+}}, %rd{{.+}}, %rd{{.+}};
 ; CHECK: .loc [[DEBUG_INFO_CU]] 8 19
Index: test/DebugInfo/X86/dwarf-no-source-loc.ll
===================================================================
--- test/DebugInfo/X86/dwarf-no-source-loc.ll
+++ test/DebugInfo/X86/dwarf-no-source-loc.ll
@@ -40,15 +40,14 @@
   ret void, !dbg !14
 }
 
-; CHECK:      .loc 1 7 7
+; CHECK:      .loc 1 7 7 prologue_end
 ; CHECK-NOT:  .loc
-; CHECK:      .loc 1 0 7 is_stmt 0
+; CHECK:      # %bb.1
+; CHECK-NEXT: .file 2 "/tests/include.h"
+; CHECK-NEXT: .loc 2 20 5
 ; CHECK-NOT:  .loc
-; CHECK:      .loc 2 20 5 is_stmt 1
 ; CHECK:      .LBB0_2:
-; CHECK-NEXT: .loc 2 0 5 is_stmt 0
-; CHECK-NOT:  .loc
-; CHECK:      .loc 1 10 3 is_stmt 1
+; CHECK:      .loc 1 10 3
 ;
 ; DISABLE-NOT: .loc 1 0
 
Index: test/DebugInfo/X86/dwarf-no-source-loc.mir
===================================================================
--- /dev/null
+++ test/DebugInfo/X86/dwarf-no-source-loc.mir
@@ -0,0 +1,74 @@
+# RUN: llc -o - %s -start-before=patchable-function -use-unknown-locations=Default | FileCheck %s --check-prefixes=CHECK,DEFAULT
+# RUN: llc -o - %s -start-before=patchable-function -use-unknown-locations=Enable | FileCheck %s --check-prefixes=CHECK,ENABLE
+# RUN: llc -o - %s -start-before=patchable-function -use-unknown-locations=Disable | FileCheck %s --check-prefixes=CHECK,DISABLE
+--- |
+  target triple = "x86_64--"
+  
+  !0 = !DIFile(filename: "dwarf-no-source-loc.mir", directory: "/")
+  !1 = distinct !DICompileUnit(file: !0, language: DW_LANG_C, emissionKind: LineTablesOnly)
+  !2 = distinct !DISubprogram(name: "func", unit: !1)
+  !3 = !DILocation(line: 17, scope: !2)
+  !4 = !DILocation(line: 42, scope: !2)
+
+  !llvm.dbg.cu = !{!1}
+  !llvm.module.flags = !{!10, !11}
+  !10 = !{i32 2, !"Dwarf Version", i32 4}
+  !11 = !{i32 2, !"Debug Info Version", i32 3}
+  
+  define void @func() !dbg !2 {
+    unreachable
+  }
+...
+---
+name: func
+body: |
+  bb.0:
+    NOOP
+    NOOP
+    $eax = MOV32ri 1, debug-location !3
+    ; CHECK-LABEL: bb.0
+    ; CHECK: nop
+    ; CHECK: nop
+    ; CHECK: .loc 1 17 0 prologue_end
+    ; CHECK: movl $1, %eax
+
+  bb.1:
+    NOOP
+    $ebx = MOV32ri 2, debug-location !4
+    ; CHECK-LABEL: bb.1
+    ; DEFAULT: .loc 1 42 0
+    ; ENABLE: .loc 1 0
+    ; DISABLE-NOT: .loc 1 0
+    ; CHECK: nop
+    ; ENABLE: .loc 1 42 0
+    ; CHECK: movl $2, %ebx
+
+  bb.2:
+    NOOP
+    ; CHECK-LABEL: bb.2
+    ; DEFAULT: .loc 1 0 0 is_stmt 0
+    ; ENABLE: .loc 1 0 0 is_stmt 0
+    ; DISABLE-NOT: .loc 1 0
+    ; CHECK: nop
+
+  bb.3:
+    NOOP
+    $ecx = MOV32ri 3, debug-location !3
+    ; CHECK-LABEL: bb.3
+    ; CHECK: nop
+    ; DEFAULT: .loc 1 17 0 is_stmt 1
+    ; ENABLE: .loc 1 17 0 is_stmt 1
+    ; DISABLE-NOT: .loc 1 0
+    ; CHECK: movl $3, %ecx
+
+  bb.4:
+    NOOP
+    $edx = MOV32ri 4, debug-location !4
+    ; CHECK: bb.4
+    ; DEFAULT: .loc 1 42 0
+    ; ENABLE: .loc 1 0 0 is_stmt 0
+    ; DISABLE-NOT: .loc 1 0
+    ; CHECK: nop
+    ; ENABLE: .loc 1 42 0 is_stmt 1
+    ; CHECK: movl $4, %edx
+...