Index: lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp
===================================================================
--- lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp
+++ lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp
@@ -166,6 +166,8 @@
   // Identify the function this subsection is for.
   Asm->OutStreamer.EmitCOFFSecRel32(Fn);
   Asm->OutStreamer.EmitCOFFSectionIndex(Fn);
+  // Padding after 16-bit section index
+  Asm->EmitInt16(0);
 
   // Length of the function's code, in bytes.
   EmitLabelDiff(Asm->OutStreamer, Fn, FI.End);
Index: lib/MC/WinCOFFStreamer.cpp
===================================================================
--- lib/MC/WinCOFFStreamer.cpp
+++ lib/MC/WinCOFFStreamer.cpp
@@ -162,7 +162,8 @@
   const MCSymbolRefExpr *SRE = MCSymbolRefExpr::Create(Symbol, getContext());
   MCFixup Fixup = MCFixup::Create(DF->getContents().size(), SRE, FK_SecRel_2);
   DF->getFixups().push_back(Fixup);
-  DF->getContents().resize(DF->getContents().size() + 4, 0);
+  // +2, not +4: unlike 32-bit SECREL, SECTION is 16-bit
+  DF->getContents().resize(DF->getContents().size() + 2, 0);
 }
 
 void MCWinCOFFStreamer::EmitCOFFSecRel32(MCSymbol const *Symbol) {
Index: test/DebugInfo/COFF/asm.ll
===================================================================
--- test/DebugInfo/COFF/asm.ll
+++ test/DebugInfo/COFF/asm.ll
@@ -28,6 +28,7 @@
 ; X86-NEXT: [[F2_START]]:
 ; X86-NEXT: .secrel32 _f
 ; X86-NEXT: .secidx _f
+; X86-NEXT: .short 0
 ; X86-NEXT: .long [[END_OF_F]]-_f
 ; X86-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
 ; X86-NEXT: .long   0
@@ -97,6 +98,7 @@
 ; X64-NEXT: [[F2_START]]:
 ; X64-NEXT: .secrel32 f
 ; X64-NEXT: .secidx f
+; X64-NEXT: .short 0
 ; X64-NEXT: .long [[END_OF_F]]-f
 ; X64-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
 ; X64-NEXT: .long   0
Index: test/DebugInfo/COFF/multifile.ll
===================================================================
--- test/DebugInfo/COFF/multifile.ll
+++ test/DebugInfo/COFF/multifile.ll
@@ -35,6 +35,7 @@
 ; X86-NEXT: [[F2_START]]:
 ; X86-NEXT: .secrel32 _f
 ; X86-NEXT: .secidx _f
+; X86-NEXT: .short 0
 ; X86-NEXT: .long [[END_OF_F]]-_f
 ; Segment for file 'D:\\one.c' begins
 ; X86-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
@@ -129,6 +130,7 @@
 ; X64-NEXT: [[F2_START]]:
 ; X64-NEXT: .secrel32 f
 ; X64-NEXT: .secidx f
+; X64-NEXT: .short 0
 ; X64-NEXT: .long [[END_OF_F]]-f
 ; Segment for file 'D:\\input.c' begins
 ; X64-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
Index: test/DebugInfo/COFF/multifunction.ll
===================================================================
--- test/DebugInfo/COFF/multifunction.ll
+++ test/DebugInfo/COFF/multifunction.ll
@@ -58,6 +58,7 @@
 ; X86-NEXT: [[F2_START]]:
 ; X86-NEXT: .secrel32       _x
 ; X86-NEXT: .secidx _x
+; X86-NEXT: .short 0
 ; X86-NEXT: .long [[END_OF_X]]-_x
 ; X86-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
 ; X86-NEXT: .long   0
@@ -75,6 +76,7 @@
 ; X86-NEXT: [[F2_START]]:
 ; X86-NEXT: .secrel32       _y
 ; X86-NEXT: .secidx _y
+; X86-NEXT: .short 0
 ; X86-NEXT: .long [[END_OF_Y]]-_y
 ; X86-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
 ; X86-NEXT: .long   0
@@ -92,6 +94,7 @@
 ; X86-NEXT: [[F2_START]]:
 ; X86-NEXT: .secrel32 _f
 ; X86-NEXT: .secidx _f
+; X86-NEXT: .short 0
 ; X86-NEXT: .long [[END_OF_F]]-_f
 ; X86-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
 ; X86-NEXT: .long   0
@@ -208,6 +211,7 @@
 ; X64-NEXT: [[F2_START]]:
 ; X64-NEXT: .secrel32 x
 ; X64-NEXT: .secidx x
+; X64-NEXT: .short 0
 ; X64-NEXT: .long [[END_OF_X]]-x
 ; X64-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
 ; X64-NEXT: .long   0
@@ -227,6 +231,7 @@
 ; X64-NEXT: [[F2_START]]:
 ; X64-NEXT: .secrel32 y
 ; X64-NEXT: .secidx y
+; X64-NEXT: .short 0
 ; X64-NEXT: .long [[END_OF_Y]]-y
 ; X64-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
 ; X64-NEXT: .long   0
@@ -246,6 +251,7 @@
 ; X64-NEXT: [[F2_START]]:
 ; X64-NEXT: .secrel32 f
 ; X64-NEXT: .secidx f
+; X64-NEXT: .short 0
 ; X64-NEXT: .long [[END_OF_F]]-f
 ; X64-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
 ; X64-NEXT: .long   0
Index: test/DebugInfo/COFF/simple.ll
===================================================================
--- test/DebugInfo/COFF/simple.ll
+++ test/DebugInfo/COFF/simple.ll
@@ -26,6 +26,7 @@
 ; X86-NEXT: [[F2_START]]:
 ; X86-NEXT: .secrel32 _f
 ; X86-NEXT: .secidx _f
+; X86-NEXT: .short 0
 ; X86-NEXT: .long [[END_OF_F]]-_f
 ; X86-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
 ; X86-NEXT: .long   0
@@ -88,6 +89,7 @@
 ; X64-NEXT: [[F2_START]]:
 ; X64-NEXT: .secrel32 f
 ; X64-NEXT: .secidx f
+; X64-NEXT: .short 0
 ; X64-NEXT: .long [[END_OF_F]]-f
 ; X64-NEXT: [[FILE_SEGMENT_START:[^:]*]]:
 ; X64-NEXT: .long   0
Index: test/MC/COFF/secidx.s
===================================================================
--- test/MC/COFF/secidx.s
+++ test/MC/COFF/secidx.s
@@ -12,5 +12,5 @@
 
 // CHECK:       Relocations [
 // CHECK-NEXT:    0x0 IMAGE_REL_I386_SECTION .text
-// CHECK-NEXT:    0x4 IMAGE_REL_I386_SECTION spam
+// CHECK-NEXT:    0x2 IMAGE_REL_I386_SECTION spam
 // CHECK-NEXT:  ]