# This patch allows ISPC code to be debugged in Visual Studio 2012 and above
# Requires LLVM 3.5
Index: include/llvm/Support/COFF.h
===================================================================
--- include/llvm/Support/COFF.h	(revision 215535)
+++ include/llvm/Support/COFF.h	(working copy)
@@ -642,9 +642,14 @@
 
   enum CodeViewLineTableIdentifiers {
     DEBUG_SECTION_MAGIC           = 0x4,
+    DEBUG_SYMBOL_SUBSECTION       = 0xF1,
     DEBUG_LINE_TABLE_SUBSECTION   = 0xF2,
     DEBUG_STRING_TABLE_SUBSECTION = 0xF3,
-    DEBUG_INDEX_SUBSECTION        = 0xF4
+    DEBUG_INDEX_SUBSECTION        = 0xF4,
+
+    // Symbol subsections are split into subsubsections.
+    DEBUG_SYMBOL_PROC_START_SUBSUBSECTION = 0x1147,
+    DEBUG_SYMBOL_PROC_END_SUBSUBSECTION   = 0x114F
   };
 
   inline bool isReservedSectionNumber(int N) {
Index: lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp
===================================================================
--- lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp	(revision 215535)
+++ lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp	(working copy)
@@ -117,14 +117,15 @@
 }
 
 static void EmitLabelDiff(MCStreamer &Streamer,
-                          const MCSymbol *From, const MCSymbol *To) {
+                          const MCSymbol *From, const MCSymbol *To,
+                          unsigned int Size = 4) {
   MCSymbolRefExpr::VariantKind Variant = MCSymbolRefExpr::VK_None;
   MCContext &Context = Streamer.getContext();
   const MCExpr *FromRef = MCSymbolRefExpr::Create(From, Variant, Context),
                *ToRef   = MCSymbolRefExpr::Create(To, Variant, Context);
   const MCExpr *AddrDelta =
       MCBinaryExpr::Create(MCBinaryExpr::Sub, ToRef, FromRef, Context);
-  Streamer.EmitValue(AddrDelta, 4);
+  Streamer.EmitValue(AddrDelta, Size);
 }
 
 void WinCodeViewLineTables::emitDebugInfoForFunction(const Function *GV) {
@@ -132,12 +133,50 @@
   // which holds the PC to file:line table.
   const MCSymbol *Fn = Asm->getSymbol(GV);
   assert(Fn);
+  const StringRef FuncName = Fn->getName();
 
   const FunctionInfo &FI = FnDebugInfo[GV];
   if (FI.Instrs.empty())
     return;
   assert(FI.End && "Don't know where the function ends?");
 
+  // Emit a symbol subsection, required by VS2012+ to find function boundaries.
+  MCSymbol *SymbolsBegin = Asm->MMI->getContext().CreateTempSymbol(),
+           *SymbolsEnd = Asm->MMI->getContext().CreateTempSymbol();
+  Asm->OutStreamer.AddComment("Symbol subsection for " + Twine(FuncName));
+  Asm->EmitInt32(COFF::DEBUG_SYMBOL_SUBSECTION);
+  EmitLabelDiff(Asm->OutStreamer, SymbolsBegin, SymbolsEnd);
+  Asm->OutStreamer.EmitLabel(SymbolsBegin);
+  {
+    MCSymbol *ProcSegmentBegin = Asm->MMI->getContext().CreateTempSymbol(),
+             *ProcSegmentEnd = Asm->MMI->getContext().CreateTempSymbol();
+    EmitLabelDiff(Asm->OutStreamer, ProcSegmentBegin, ProcSegmentEnd, 2);
+    Asm->OutStreamer.EmitLabel(ProcSegmentBegin);
+
+    Asm->EmitInt16(COFF::DEBUG_SYMBOL_PROC_START_SUBSUBSECTION);
+    // Some bytes of this segment don't seem to be required for basic debugging,
+    // so just fill them with zeroes.
+    Asm->OutStreamer.EmitFill(12, 0);
+    // This is the important bit that tells the debugger where the function
+    // code is located:
+    EmitLabelDiff(Asm->OutStreamer, Fn, FI.End);
+    Asm->OutStreamer.EmitFill(12, 0);
+    Asm->OutStreamer.EmitCOFFSecRel32(Fn);
+    Asm->OutStreamer.EmitCOFFSectionIndex(Fn);
+    Asm->EmitInt8(0);
+    // Emit the function name as a null-terminated string.
+    Asm->OutStreamer.EmitBytes(FuncName);
+    Asm->EmitInt8(0);
+    Asm->OutStreamer.EmitLabel(ProcSegmentEnd);
+
+    // We're done with this function.
+    Asm->EmitInt16(0x0002);
+    Asm->EmitInt16(COFF::DEBUG_SYMBOL_PROC_END_SUBSUBSECTION);
+  }
+  Asm->OutStreamer.EmitLabel(SymbolsEnd);
+  // Every subsection must be aligned to a 4-byte boundary.
+  Asm->OutStreamer.EmitFill((-FuncName.size()) % 4, 0);
+
   // PCs/Instructions are grouped into segments sharing the same filename.
   // Pre-calculate the lengths (in instructions) of these segments and store
   // them in a map for convenience.  Each index in the map is the sequential
@@ -154,18 +193,18 @@
   }
   FilenameSegmentLengths[LastSegmentEnd] = FI.Instrs.size() - LastSegmentEnd;
 
-  // Emit the control code of the subsection followed by the payload size.
-  Asm->OutStreamer.AddComment(
-      "Linetable subsection for " + Twine(Fn->getName()));
+  // Emit a linetable subsection, requred to do PC-to-file:line lookup.
+  Asm->OutStreamer.AddComment("Linetable subsection for " + Twine(FuncName));
   Asm->EmitInt32(COFF::DEBUG_LINE_TABLE_SUBSECTION);
-  MCSymbol *SubsectionBegin = Asm->MMI->getContext().CreateTempSymbol(),
-           *SubsectionEnd = Asm->MMI->getContext().CreateTempSymbol();
-  EmitLabelDiff(Asm->OutStreamer, SubsectionBegin, SubsectionEnd);
-  Asm->OutStreamer.EmitLabel(SubsectionBegin);
+  MCSymbol *LineTableBegin = Asm->MMI->getContext().CreateTempSymbol(),
+           *LineTableEnd = Asm->MMI->getContext().CreateTempSymbol();
+  EmitLabelDiff(Asm->OutStreamer, LineTableBegin, LineTableEnd);
+  Asm->OutStreamer.EmitLabel(LineTableBegin);
 
   // Identify the function this subsection is for.
   Asm->OutStreamer.EmitCOFFSecRel32(Fn);
   Asm->OutStreamer.EmitCOFFSectionIndex(Fn);
+  Asm->EmitInt16(0);
 
   // Length of the function's code, in bytes.
   EmitLabelDiff(Asm->OutStreamer, Fn, FI.End);
@@ -209,7 +248,7 @@
 
   if (FileSegmentEnd)
     Asm->OutStreamer.EmitLabel(FileSegmentEnd);
-  Asm->OutStreamer.EmitLabel(SubsectionEnd);
+  Asm->OutStreamer.EmitLabel(LineTableEnd);
 }
 
 void WinCodeViewLineTables::endModule() {
@@ -254,8 +293,7 @@
     Asm->OutStreamer.EmitBytes(FileNameRegistry.Filenames[I]);
     Asm->EmitInt8(0);
   }
-
-  // No more subsections. Fill with zeros to align the end of the section by 4.
+  // Every subsection must be aligned to a 4-byte boundary.
   Asm->OutStreamer.EmitFill((-FileNameRegistry.LastOffset) % 4, 0);
 
   clear();
Index: lib/MC/WinCOFFStreamer.cpp
===================================================================
--- lib/MC/WinCOFFStreamer.cpp	(revision 215535)
+++ lib/MC/WinCOFFStreamer.cpp	(working copy)
@@ -163,7 +163,7 @@
   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);
+  DF->getContents().resize(DF->getContents().size() + 2, 0);
 }
 
 void MCWinCOFFStreamer::EmitCOFFSecRel32(MCSymbol const *Symbol) {