diff --git a/include/llvm/MC/MCAsmInfo.h b/include/llvm/MC/MCAsmInfo.h
index d53318c6631..0e6c4e7c75e 100644
--- a/include/llvm/MC/MCAsmInfo.h
+++ b/include/llvm/MC/MCAsmInfo.h
@@ -197,6 +197,13 @@ namespace llvm {
     /// HasSetDirective - True if the assembler supports the .set directive.
     bool HasSetDirective;                    // Defaults to true.
 
+    /// NeedsSetToChangeDiffSize - True if the assembler requires that we do
+    /// Lc = a - b
+    /// .long Lc
+    /// instead of doing
+    /// .long a - b
+    bool NeedsSetToChangeDiffSize;           // Defaults to false.
+
     /// HasLCOMMDirective - This is true if the target supports the .lcomm
     /// directive.
     bool HasLCOMMDirective;                  // Defaults to false.
@@ -400,6 +407,7 @@ namespace llvm {
       return ExternDirective;
     }
     bool hasSetDirective() const { return HasSetDirective; }
+    bool needsSetToChangeDiffSize() const { return NeedsSetToChangeDiffSize; }
     bool hasLCOMMDirective() const { return HasLCOMMDirective; }
     bool hasDotTypeDotSizeDirective() const {return HasDotTypeDotSizeDirective;}
     bool getCOMMDirectiveAlignmentIsInBytes() const {
diff --git a/include/llvm/MC/MCObjectStreamer.h b/include/llvm/MC/MCObjectStreamer.h
index a3f532d8c00..d130ba4bcda 100644
--- a/include/llvm/MC/MCObjectStreamer.h
+++ b/include/llvm/MC/MCObjectStreamer.h
@@ -60,8 +60,7 @@ public:
   /// @{
 
   virtual void EmitLabel(MCSymbol *Symbol);
-  virtual void EmitValue(const MCExpr *Value, unsigned Size,unsigned AddrSpace,
-                         bool UseSet = false);
+  virtual void EmitValue(const MCExpr *Value, unsigned Size,unsigned AddrSpace);
   virtual void EmitULEB128Value(const MCExpr *Value, unsigned AddrSpace = 0);
   virtual void EmitSLEB128Value(const MCExpr *Value, unsigned AddrSpace = 0);
   virtual void EmitWeakReference(MCSymbol *Alias, const MCSymbol *Symbol);
diff --git a/include/llvm/MC/MCStreamer.h b/include/llvm/MC/MCStreamer.h
index 16646c96d3e..d89ee9ae126 100644
--- a/include/llvm/MC/MCStreamer.h
+++ b/include/llvm/MC/MCStreamer.h
@@ -242,7 +242,7 @@ namespace llvm {
     /// @param Size - The size of the integer (in bytes) to emit. This must
     /// match a native machine width.
     virtual void EmitValue(const MCExpr *Value, unsigned Size,
-                           unsigned AddrSpace = 0, bool UseSet = false) = 0;
+                           unsigned AddrSpace = 0) = 0;
 
     /// EmitIntValue - Special case of EmitValue that avoids the client having
     /// to pass in a MCExpr for constant integers.
diff --git a/lib/MC/MCAsmInfo.cpp b/lib/MC/MCAsmInfo.cpp
index 9cf4019e312..1bc63d03fba 100644
--- a/lib/MC/MCAsmInfo.cpp
+++ b/lib/MC/MCAsmInfo.cpp
@@ -54,6 +54,7 @@ MCAsmInfo::MCAsmInfo() {
   GPRel32Directive = 0;
   GlobalDirective = "\t.globl\t";
   HasSetDirective = true;
+  NeedsSetToChangeDiffSize = false;
   HasLCOMMDirective = false;
   COMMDirectiveAlignmentIsInBytes = true;
   HasDotTypeDotSizeDirective = true;
diff --git a/lib/MC/MCAsmStreamer.cpp b/lib/MC/MCAsmStreamer.cpp
index 40a234ea85f..ab807edd9ea 100644
--- a/lib/MC/MCAsmStreamer.cpp
+++ b/lib/MC/MCAsmStreamer.cpp
@@ -44,6 +44,8 @@ class MCAsmStreamer : public MCStreamer {
   unsigned IsVerboseAsm : 1;
   unsigned ShowInst : 1;
 
+  bool needsSet(const MCExpr *Value);
+
 public:
   MCAsmStreamer(MCContext &Context, formatted_raw_ostream &os,
                 bool isLittleEndian, bool isVerboseAsm,
@@ -150,8 +152,7 @@ public:
 
   virtual void EmitBytes(StringRef Data, unsigned AddrSpace);
 
-  virtual void EmitValue(const MCExpr *Value, unsigned Size,unsigned AddrSpace,
-                         bool UseSet = false);
+  virtual void EmitValue(const MCExpr *Value, unsigned Size,unsigned AddrSpace);
   virtual void EmitIntValue(uint64_t Value, unsigned Size,
                             unsigned AddrSpace = 0);
 
@@ -511,8 +512,34 @@ void MCAsmStreamer::EmitIntValue(uint64_t Value, unsigned Size,
   EmitValue(MCConstantExpr::Create(Value, getContext()), Size, AddrSpace);
 }
 
+static bool hasSymbolDifference(const MCExpr *Value) {
+  switch (Value->getKind()) {
+  case MCExpr::Target: llvm_unreachable("Can't handle target exprs yet!");
+  case MCExpr::Constant:
+  case MCExpr::SymbolRef:
+    return false;
+  case MCExpr::Unary:
+    return hasSymbolDifference(cast<MCUnaryExpr>(Value)->getSubExpr());
+  case MCExpr::Binary: {
+    const MCBinaryExpr *BE = cast<MCBinaryExpr>(Value);
+    if (BE->getOpcode() == MCBinaryExpr::Sub &&
+	BE->getLHS()->getKind() == MCExpr::SymbolRef &&
+	BE->getRHS()->getKind() == MCExpr::SymbolRef)
+      return true;
+    return hasSymbolDifference(BE->getLHS()) ||
+      hasSymbolDifference(BE->getRHS());
+  }
+  }
+  llvm_unreachable("Switch covers all cases");
+}
+
+bool MCAsmStreamer::needsSet(const MCExpr *Value) {
+  return getContext().getAsmInfo().needsSetToChangeDiffSize() &&
+    hasSymbolDifference(Value);
+}
+
 void MCAsmStreamer::EmitValue(const MCExpr *Value, unsigned Size,
-                              unsigned AddrSpace, bool UseSet) {
+                              unsigned AddrSpace) {
   assert(CurSection && "Cannot emit contents before setting section!");
   const char *Directive = 0;
   switch (Size) {
@@ -538,7 +565,7 @@ void MCAsmStreamer::EmitValue(const MCExpr *Value, unsigned Size,
   }
 
   assert(Directive && "Invalid size for machine code value!");
-  if (UseSet && MAI.hasSetDirective()) {
+  if (needsSet(Value)) {
     MCSymbol *SetLabel = getContext().CreateTempSymbol();
     EmitAssignment(SetLabel, Value);
     OS << Directive << *SetLabel;
diff --git a/lib/MC/MCDwarf.cpp b/lib/MC/MCDwarf.cpp
index a33b0c596ba..1b1d5097042 100644
--- a/lib/MC/MCDwarf.cpp
+++ b/lib/MC/MCDwarf.cpp
@@ -213,8 +213,15 @@ void MCDwarfFileTable::Emit(MCStreamer *MCOS,
 
   // The first 4 bytes is the total length of the information for this
   // compilation unit (not including these 4 bytes for the length).
-  MCOS->EmitValue(MakeStartMinusEndExpr(MCOS, LineStartSym, LineEndSym, 4),
-                  4, 0, true /*UseSet*/);
+  // FIXME: We create the dummy TotalLength variable because LineEndSym points
+  // to the end of the section and the darwin assembler doesn't consider that
+  // difference an assembly time constant. It might be better for this to be
+  // proected by a flag.
+  MCSymbol *TotalLength = MCOS->getContext().CreateTempSymbol();
+  MCOS->EmitAssignment(TotalLength,
+		       MakeStartMinusEndExpr(MCOS, LineStartSym, LineEndSym,
+					     4));
+  MCOS->EmitSymbolValue(TotalLength, 4, 0);
 
   // Next 2 bytes is the Version, which is Dwarf 2.
   MCOS->EmitIntValue(2, 2);
@@ -228,7 +235,7 @@ void MCDwarfFileTable::Emit(MCStreamer *MCOS,
   // length of the prologue.
   MCOS->EmitValue(MakeStartMinusEndExpr(MCOS, LineStartSym, ProEndSym,
                                         (4 + 2 + 4)),
-                  4, 0, true /*UseSet*/);
+                  4, 0);
 
   // Parameters of the state machine, are next.
   MCOS->EmitIntValue(DWARF2_LINE_MIN_INSN_LENGTH, 1);
diff --git a/lib/MC/MCLoggingStreamer.cpp b/lib/MC/MCLoggingStreamer.cpp
index 6cbef0b0250..134554b8006 100644
--- a/lib/MC/MCLoggingStreamer.cpp
+++ b/lib/MC/MCLoggingStreamer.cpp
@@ -154,8 +154,7 @@ public:
     return Child->EmitBytes(Data, AddrSpace);
   }
 
-  virtual void EmitValue(const MCExpr *Value, unsigned Size,unsigned AddrSpace,
-                         bool UseSet = false){
+  virtual void EmitValue(const MCExpr *Value, unsigned Size,unsigned AddrSpace){
     LogCall("EmitValue");
     return Child->EmitValue(Value, Size, AddrSpace);
   }
diff --git a/lib/MC/MCNullStreamer.cpp b/lib/MC/MCNullStreamer.cpp
index 86ab628e602..c8f9c636174 100644
--- a/lib/MC/MCNullStreamer.cpp
+++ b/lib/MC/MCNullStreamer.cpp
@@ -69,7 +69,7 @@ namespace {
     virtual void EmitBytes(StringRef Data, unsigned AddrSpace) {}
 
     virtual void EmitValue(const MCExpr *Value, unsigned Size,
-                           unsigned AddrSpace, bool UseSet = false) {}
+                           unsigned AddrSpace) {}
     virtual void EmitULEB128Value(const MCExpr *Value,
                                   unsigned AddrSpace = 0) {}
     virtual void EmitSLEB128Value(const MCExpr *Value,
diff --git a/lib/MC/MCObjectStreamer.cpp b/lib/MC/MCObjectStreamer.cpp
index 1538a589966..ad8fb495840 100644
--- a/lib/MC/MCObjectStreamer.cpp
+++ b/lib/MC/MCObjectStreamer.cpp
@@ -77,7 +77,7 @@ const MCExpr *MCObjectStreamer::AddValueSymbols(const MCExpr *Value) {
 }
 
 void MCObjectStreamer::EmitValue(const MCExpr *Value, unsigned Size,
-                                 unsigned AddrSpace, bool UseSet) {
+                                 unsigned AddrSpace) {
   assert(AddrSpace == 0 && "Address space must be 0!");
   MCDataFragment *DF = getOrCreateDataFragment();
 
diff --git a/lib/Target/PTX/PTXMCAsmStreamer.cpp b/lib/Target/PTX/PTXMCAsmStreamer.cpp
index 8043baf8cfc..4e517ad76a2 100644
--- a/lib/Target/PTX/PTXMCAsmStreamer.cpp
+++ b/lib/Target/PTX/PTXMCAsmStreamer.cpp
@@ -147,8 +147,7 @@ public:
 
   virtual void EmitBytes(StringRef Data, unsigned AddrSpace);
 
-  virtual void EmitValue(const MCExpr *Value, unsigned Size,unsigned AddrSpace,
-                         bool UseSet = false);
+  virtual void EmitValue(const MCExpr *Value, unsigned Size,unsigned AddrSpace);
   virtual void EmitULEB128Value(const MCExpr *Value, unsigned AddrSpace = 0);
   virtual void EmitSLEB128Value(const MCExpr *Value, unsigned AddrSpace = 0);
   virtual void EmitGPRel32Value(const MCExpr *Value);
@@ -361,7 +360,7 @@ void PTXMCAsmStreamer::EmitBytes(StringRef Data, unsigned AddrSpace) {
 }
 
 void PTXMCAsmStreamer::EmitValue(const MCExpr *Value, unsigned Size,
-                                 unsigned AddrSpace, bool UseSet) {
+                                 unsigned AddrSpace) {
   assert(CurSection && "Cannot emit contents before setting section!");
   const char *Directive = 0;
   switch (Size) {
diff --git a/lib/Target/PowerPC/PPCMCAsmInfo.cpp b/lib/Target/PowerPC/PPCMCAsmInfo.cpp
index 3644c79d041..89c8cb664c7 100644
--- a/lib/Target/PowerPC/PPCMCAsmInfo.cpp
+++ b/lib/Target/PowerPC/PPCMCAsmInfo.cpp
@@ -21,6 +21,10 @@ PPCMCAsmInfoDarwin::PPCMCAsmInfoDarwin(bool is64Bit) {
 
   if (!is64Bit)
     Data64bitsDirective = 0;      // We can't emit a 64-bit unit in PPC32 mode.
+
+  if (is64Bit)
+    NeedsSetToChangeDiffSize = true;
+
   AssemblerDialect = 1;           // New-Style mnemonics.
   SupportsDebugInformation= true; // Debug information.
 }
diff --git a/lib/Target/X86/X86MCAsmInfo.cpp b/lib/Target/X86/X86MCAsmInfo.cpp
index f45fdf5a3fb..1ac2d7e6c6d 100644
--- a/lib/Target/X86/X86MCAsmInfo.cpp
+++ b/lib/Target/X86/X86MCAsmInfo.cpp
@@ -56,6 +56,10 @@ X86MCAsmInfoDarwin::X86MCAsmInfoDarwin(const Triple &Triple) {
   if (!is64Bit)
     Data64bitsDirective = 0;       // we can't emit a 64-bit unit
 
+  // FIXME: Darwin 10 doesn't need this.
+  if (is64Bit)
+    NeedsSetToChangeDiffSize = true;
+
   // Use ## as a comment string so that .s files generated by llvm can go
   // through the GCC preprocessor without causing an error.  This is needed
   // because "clang foo.s" runs the C preprocessor, which is usually reserved
diff --git a/test/MC/MachO/empty-dwarf-lines.s b/test/MC/MachO/empty-dwarf-lines.s
index 29ef218fddc..8d06d7363da 100644
--- a/test/MC/MachO/empty-dwarf-lines.s
+++ b/test/MC/MachO/empty-dwarf-lines.s
@@ -17,7 +17,7 @@ _c:
 // CHECK-NEXT:  ('offset', 452)
 // CHECK-NEXT:  ('alignment', 0)
 // CHECK-NEXT:  ('reloc_offset', 496)
-// CHECK-NEXT:  ('num_reloc', 4)
+// CHECK-NEXT:  ('num_reloc', 2)
 // CHECK-NEXT:  ('flags', 0x2000000)
 // CHECK-NEXT:  ('reserved1', 0)
 // CHECK-NEXT:  ('reserved2', 0)