[MC] Teach the MachO object writer about N_FUNC_COLD

N_FUNC_COLD is a new MachO symbol attribute. It's a hint to the linker
to order a symbol towards the end of its section, to improve locality.

Example:

```
void a1() {}
__attribute__((cold)) void a2() {}
void a3() {}
int main() {
  a1();
  a2();
  a3();
  return 0;
}
```

A linker that supports N_FUNC_COLD will order _a2 to the end of the text
section. From `nm -njU` output, we see:

```
_a1
_a3
_main
_a2
```

Differential Revision: https://reviews.llvm.org/D57190

llvm-svn: 352227

llvm/include/llvm/BinaryFormat/MachO.h

@@ -334,6 +334,7 @@ enum {
   N_WEAK_DEF = 0x0080u,
   N_SYMBOL_RESOLVER = 0x0100u,
   N_ALT_ENTRY = 0x0200u,
+  N_COLD_FUNC = 0x0400u,
   // For undefined symbols coming from libraries, see GET_LIBRARY_ORDINAL()
   // as these are in the top 8 bits.
   SELF_LIBRARY_ORDINAL = 0x0,

llvm/include/llvm/MC/MCDirectives.h

@@ -19,6 +19,7 @@ enum MCSymbolAttr {
   MCSA_Invalid = 0,    ///< Not a valid directive.
 
   // Various directives in alphabetical order.
+  MCSA_Cold,                ///< .cold (MachO)
   MCSA_ELF_TypeFunction,    ///< .type _foo, STT_FUNC  # aka @function
   MCSA_ELF_TypeIndFunction, ///< .type _foo, STT_GNU_IFUNC
   MCSA_ELF_TypeObject,      ///< .type _foo, STT_OBJECT  # aka @object

llvm/include/llvm/MC/MCSymbolMachO.h

@@ -34,6 +34,7 @@ class MCSymbolMachO : public MCSymbol {
     SF_WeakDefinition                       = 0x0080,
     SF_SymbolResolver                       = 0x0100,
     SF_AltEntry                             = 0x0200,
+    SF_Cold                                 = 0x0400,
 
     // Common alignment
     SF_CommonAlignmentMask                  = 0xF0FF,
@@ -97,6 +98,10 @@ public:
     return getFlags() & SF_AltEntry;
   }
 
+  void setCold() const { modifyFlags(SF_Cold, SF_Cold); }
+
+  bool isCold() const { return getFlags() & SF_Cold; }
+
   void setDesc(unsigned Value) const {
     assert(Value == (Value & SF_DescFlagsMask) &&
            "Invalid .desc value!");

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -663,6 +663,9 @@ void AsmPrinter::EmitFunctionHeader() {
   if (MAI->hasDotTypeDotSizeDirective())
     OutStreamer->EmitSymbolAttribute(CurrentFnSym, MCSA_ELF_TypeFunction);
 
+  if (F.hasFnAttribute(Attribute::Cold))
+    OutStreamer->EmitSymbolAttribute(CurrentFnSym, MCSA_Cold);
+
   if (isVerbose()) {
     F.printAsOperand(OutStreamer->GetCommentOS(),
                    /*PrintType=*/false, F.getParent());

llvm/lib/MC/MCAsmStreamer.cpp

@@ -656,6 +656,9 @@ bool MCAsmStreamer::EmitSymbolAttribute(MCSymbol *Symbol,
       // .weak_reference
   case MCSA_WeakReference:  OS << MAI->getWeakRefDirective(); break;
   case MCSA_WeakDefAutoPrivate: OS << "\t.weak_def_can_be_hidden\t"; break;
+  case MCSA_Cold:
+    // Assemblers currently do not support a .cold directive.
+    return false;
   }
 
   Symbol->print(OS, MAI);

llvm/lib/MC/MCELFStreamer.cpp

@@ -201,6 +201,7 @@ bool MCELFStreamer::EmitSymbolAttribute(MCSymbol *S, MCSymbolAttr Attribute) {
   // In the future it might be worth trying to make these operations more well
   // defined.
   switch (Attribute) {
+  case MCSA_Cold:
   case MCSA_LazyReference:
   case MCSA_Reference:
   case MCSA_SymbolResolver:

llvm/lib/MC/MCMachOStreamer.cpp

@@ -386,6 +386,10 @@ bool MCMachOStreamer::EmitSymbolAttribute(MCSymbol *Sym,
     Symbol->setWeakDefinition();
     Symbol->setWeakReference();
     break;
+
+  case MCSA_Cold:
+    Symbol->setCold();
+    break;
   }
 
   return true;

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -426,6 +426,7 @@ private:
     DK_WEAK_DEFINITION,
     DK_WEAK_REFERENCE,
     DK_WEAK_DEF_CAN_BE_HIDDEN,
+    DK_COLD,
     DK_COMM,
     DK_COMMON,
     DK_LCOMM,
@@ -1983,6 +1984,8 @@ bool AsmParser::parseStatement(ParseStatementInfo &Info,
       return parseDirectiveSymbolAttribute(MCSA_WeakReference);
     case DK_WEAK_DEF_CAN_BE_HIDDEN:
       return parseDirectiveSymbolAttribute(MCSA_WeakDefAutoPrivate);
+    case DK_COLD:
+      return parseDirectiveSymbolAttribute(MCSA_Cold);
     case DK_COMM:
     case DK_COMMON:
       return parseDirectiveComm(/*IsLocal=*/false);
@@ -5223,6 +5226,7 @@ void AsmParser::initializeDirectiveKindMap() {
   DirectiveKindMap[".weak_definition"] = DK_WEAK_DEFINITION;
   DirectiveKindMap[".weak_reference"] = DK_WEAK_REFERENCE;
   DirectiveKindMap[".weak_def_can_be_hidden"] = DK_WEAK_DEF_CAN_BE_HIDDEN;
+  DirectiveKindMap[".cold"] = DK_COLD;
   DirectiveKindMap[".comm"] = DK_COMM;
   DirectiveKindMap[".common"] = DK_COMMON;
   DirectiveKindMap[".lcomm"] = DK_LCOMM;

llvm/test/MC/AArch64/cold.s

@@ -0,0 +1,13 @@
+// RUN: llvm-mc -triple aarch64-apple-ios -o - -filetype=obj %s | \
+// RUN:   llvm-readobj -symbols | FileCheck %s -check-prefix=READOBJ
+
+// READOBJ-LABEL: Name: cold_func
+// READOBJ-NEXT: Type: Section
+// READOBJ-NEXT: Section: __text
+// READOBJ-NEXT: RefType: UndefinedNonLazy (0x0)
+// READOBJ-NEXT: Flags [ (0x400)
+
+  .text
+  .cold cold_func
+cold_func:
+  ret

llvm/test/tools/llvm-nm/AArch64/Inputs/cold-func.ll

@@ -0,0 +1,3 @@
+define void @cold_func() cold {
+  ret void
+}

llvm/test/tools/llvm-nm/AArch64/macho-cold.test

@@ -0,0 +1,4 @@
+RUN: llc -O0 -mtriple=aarch64-apple-ios %p/Inputs/cold-func.ll -filetype=obj -o %t.aarch64.o
+RUN: llvm-nm -m %t.aarch64.o | FileCheck %s
+
+CHECK: [cold] _cold_func

llvm/tools/llvm-nm/llvm-nm.cpp

@@ -554,6 +554,11 @@ static void darwinPrintSymbol(SymbolicFile &Obj, SymbolListT::iterator I,
       (NDesc & MachO::N_ALT_ENTRY) == MachO::N_ALT_ENTRY)
     outs() << "[alt entry] ";
 
+  if (Filetype == MachO::MH_OBJECT &&
+      ((NType & MachO::N_TYPE) != MachO::N_UNDF) &&
+      (NDesc & MachO::N_COLD_FUNC) == MachO::N_COLD_FUNC)
+    outs() << "[cold] ";
+
   if ((NDesc & MachO::N_ARM_THUMB_DEF) == MachO::N_ARM_THUMB_DEF)
     outs() << "[Thumb] ";