[MC] Support .cfi_label

GNU assembler 2.26 introduced the .cfi_label directive. It does not
expand to any CFI instructions, but defines a label in
.eh_frame/.debug_frame, which can be used by runtime patching code to
locate the FDE. .cfi_label is not allowed for CIE's initial
instructions, and can therefore be used to force the next instruction to
be placed in a FDE instead of a CIE.

In glibc since 2018, sysdeps/riscv/start.S utilizes .cfi_label to force
DW_CFA_undefined to be placed in a FDE. arc/csky/loongarch ports have
copied this use.
```
.cfi_startproc
// DW_CFA_undefined is allowed for CIE's initial instructions.
// Without .cfi_label, gas would place DW_CFA_undefined in a CIE.
.cfi_label .Ldummy
.cfi_undefined ra
.cfi_endproc
```

No CFI instruction is associated with .cfi_label, so the `case
MCCFIInstruction::OpLabel:` code in BOLT is unreachable and onlt to make
-Wswitch happy.

Close llvm#97222

Pull Request: llvm#97922

Change-Id: Ic236d33bf52ac5631bd202c2896d23463c8e099b

bolt/lib/Core/BinaryFunction.cpp

@@ -2538,6 +2538,7 @@ struct CFISnapshot {
     case MCCFIInstruction::OpWindowSave:
     case MCCFIInstruction::OpNegateRAState:
     case MCCFIInstruction::OpLLVMDefAspaceCfa:
+    case MCCFIInstruction::OpLabel:
       llvm_unreachable("unsupported CFI opcode");
       break;
     case MCCFIInstruction::OpRememberState:
@@ -2675,6 +2676,7 @@ struct CFISnapshotDiff : public CFISnapshot {
     case MCCFIInstruction::OpWindowSave:
     case MCCFIInstruction::OpNegateRAState:
     case MCCFIInstruction::OpLLVMDefAspaceCfa:
+    case MCCFIInstruction::OpLabel:
       llvm_unreachable("unsupported CFI opcode");
       return false;
     case MCCFIInstruction::OpRememberState:
@@ -2823,6 +2825,7 @@ BinaryFunction::unwindCFIState(int32_t FromState, int32_t ToState,
     case MCCFIInstruction::OpWindowSave:
     case MCCFIInstruction::OpNegateRAState:
     case MCCFIInstruction::OpLLVMDefAspaceCfa:
+    case MCCFIInstruction::OpLabel:
       llvm_unreachable("unsupported CFI opcode");
       break;
     case MCCFIInstruction::OpGnuArgsSize:

llvm/include/llvm/MC/MCDwarf.h

@@ -501,6 +501,7 @@ class MCCFIInstruction {
     OpWindowSave,
     OpNegateRAState,
     OpGnuArgsSize,
+    OpLabel,
     OpLLVMRegisterPair,
     OpLLVMVectorRegisters,
     OpLLVMVectorOffset,
@@ -546,6 +547,7 @@ class MCCFIInstruction {
       unsigned Register;
       unsigned Register2;
     } RR;
+    MCSymbol *CfiLabel;
   } U;
   OpType Operation;
   SMLoc Loc;
@@ -586,6 +588,12 @@ class MCCFIInstruction {
     U.RI = {R, O};
   }
 
+  MCCFIInstruction(OpType Op, MCSymbol *L, MCSymbol *CfiLabel, SMLoc Loc)
+      : Label(L), Operation(Op), Loc(Loc) {
+    assert(Op == OpLabel);
+    U.CfiLabel = CfiLabel;
+  }
+
 public:
   /// .cfi_def_cfa defines a rule for computing CFA as: take address from
   /// Register and add Offset to it.
@@ -706,6 +714,11 @@ class MCCFIInstruction {
     return MCCFIInstruction(OpGnuArgsSize, L, 0, Size, Loc);
   }
 
+  static MCCFIInstruction createLabel(MCSymbol *L, MCSymbol *CfiLabel,
+                                      SMLoc Loc) {
+    return MCCFIInstruction(OpLabel, L, CfiLabel, Loc);
+  }
+
   /// .cfi_llvm_register_pair Previous value of Register is saved in R1:R2.
   static MCCFIInstruction
   createLLVMRegisterPair(MCSymbol *L, unsigned Register, unsigned R1,
@@ -783,6 +796,11 @@ class MCCFIInstruction {
     return U.RI.Offset;
   }
 
+  MCSymbol *getCfiLabel() const {
+    assert(Operation == OpLabel);
+    return U.CfiLabel;
+  }
+
   StringRef getValues() const {
     assert(Operation == OpEscape);
     return StringRef(&Values[0], Values.size());

llvm/include/llvm/MC/MCStreamer.h

@@ -1031,6 +1031,7 @@ class MCStreamer {
                                        int64_t MaskRegister,
                                        int64_t MaskRegisterSizeInBits,
                                        int64_t Offset, SMLoc Loc = {});
+  virtual void emitCFILabelDirective(SMLoc Loc, StringRef Name);
 
   virtual void emitWinCFIStartProc(const MCSymbol *Symbol, SMLoc Loc = SMLoc());
   virtual void emitWinCFIEndProc(SMLoc Loc = SMLoc());

llvm/lib/CodeGen/CFIInstrInserter.cpp

@@ -251,6 +251,7 @@ void CFIInstrInserter::calculateOutgoingCFAInfo(MBBCFAInfo &MBBInfo) {
       case MCCFIInstruction::OpLLVMRegisterPair:
       case MCCFIInstruction::OpLLVMVectorRegisters:
       case MCCFIInstruction::OpLLVMVectorOffset:
+      case MCCFIInstruction::OpLabel:
         break;
       }
       if (CSRReg || CSROffset) {

llvm/lib/MC/MCAsmStreamer.cpp

@@ -365,6 +365,7 @@ class MCAsmStreamer final : public MCStreamer {
   void emitCFILLVMVectorOffset(int64_t Register, int64_t RegisterSize,
                                int64_t MaskRegister, int64_t MaskRegisterSize,
                                int64_t Offset, SMLoc Loc) override;
+  void emitCFILabelDirective(SMLoc Loc, StringRef Name) override;
 
   void emitWinCFIStartProc(const MCSymbol *Symbol, SMLoc Loc) override;
   void emitWinCFIEndProc(SMLoc Loc) override;
@@ -2178,6 +2179,12 @@ void MCAsmStreamer::emitCFIReturnColumn(int64_t Register) {
   EmitEOL();
 }
 
+void MCAsmStreamer::emitCFILabelDirective(SMLoc Loc, StringRef Name) {
+  MCStreamer::emitCFILabelDirective(Loc, Name);
+  OS << "\t.cfi_label " << Name;
+  EmitEOL();
+}
+
 void MCAsmStreamer::emitCFIBKeyFrame() {
   MCStreamer::emitCFIBKeyFrame();
   OS << "\t.cfi_b_key_frame";

llvm/lib/MC/MCDwarf.cpp

@@ -1512,6 +1512,10 @@ void FrameEmitterImpl::emitCFIInstruction(const MCCFIInstruction &Instr) {
     Streamer.emitBytes(Instr.getValues());
     return;
 
+  case MCCFIInstruction::OpLabel:
+    Streamer.emitLabel(Instr.getCfiLabel(), Instr.getLoc());
+    return;
+
   case MCCFIInstruction::OpLLVMRegisterPair: {
     // CFI for a register spilled to a pair of SGPRs is implemented as an
     // expression(E) rule where E is a composite location description with

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -523,6 +523,7 @@ class AsmParser : public MCAsmParser {
     DK_CFI_UNDEFINED,
     DK_CFI_REGISTER,
     DK_CFI_WINDOW_SAVE,
+    DK_CFI_LABEL,
     DK_CFI_B_KEY_FRAME,
     DK_MACROS_ON,
     DK_MACROS_OFF,
@@ -628,6 +629,7 @@ class AsmParser : public MCAsmParser {
   bool parseDirectiveCFILLVMRegisterPair(SMLoc DirectiveLoc);
   bool parseDirectiveCFILLVMVectorRegisters(SMLoc DirectiveLoc);
   bool parseDirectiveCFILLVMVectorOffset(SMLoc DirectiveLoc);
+  bool parseDirectiveCFILabel(SMLoc DirectiveLoc);
 
   // macro directives
   bool parseDirectivePurgeMacro(SMLoc DirectiveLoc);
@@ -2236,6 +2238,8 @@ bool AsmParser::parseStatement(ParseStatementInfo &Info,
       return parseDirectiveCFIRegister(IDLoc);
     case DK_CFI_WINDOW_SAVE:
       return parseDirectiveCFIWindowSave(IDLoc);
+    case DK_CFI_LABEL:
+      return parseDirectiveCFILabel(IDLoc);
     case DK_MACROS_ON:
     case DK_MACROS_OFF:
       return parseDirectiveMacrosOnOff(IDVal);
@@ -4545,6 +4549,19 @@ bool AsmParser::parseDirectiveCFILLVMVectorOffset(SMLoc DirectiveLoc) {
   return false;
 }
 
+/// parseDirectiveCFILabel
+/// ::= .cfi_label label
+bool AsmParser::parseDirectiveCFILabel(SMLoc Loc) {
+  StringRef Name;
+  Loc = Lexer.getLoc();
+  if (parseIdentifier(Name))
+    return TokError("expected identifier");
+  if (parseEOL())
+    return true;
+  getStreamer().emitCFILabelDirective(Loc, Name);
+  return false;
+}
+
 /// parseDirectiveAltmacro
 /// ::= .altmacro
 /// ::= .noaltmacro
@@ -5620,6 +5637,7 @@ void AsmParser::initializeDirectiveKindMap() {
   DirectiveKindMap[".cfi_undefined"] = DK_CFI_UNDEFINED;
   DirectiveKindMap[".cfi_register"] = DK_CFI_REGISTER;
   DirectiveKindMap[".cfi_window_save"] = DK_CFI_WINDOW_SAVE;
+  DirectiveKindMap[".cfi_label"] = DK_CFI_LABEL;
   DirectiveKindMap[".cfi_b_key_frame"] = DK_CFI_B_KEY_FRAME;
   DirectiveKindMap[".cfi_mte_tagged_frame"] = DK_CFI_MTE_TAGGED_FRAME;
   DirectiveKindMap[".macros_on"] = DK_MACROS_ON;

llvm/lib/MC/MCStreamer.cpp

@@ -728,6 +728,13 @@ void MCStreamer::emitCFIReturnColumn(int64_t Register) {
   CurFrame->RAReg = Register;
 }
 
+void MCStreamer::emitCFILabelDirective(SMLoc Loc, StringRef Name) {
+  MCSymbol *Label = emitCFILabel();
+  MCSymbol *Sym = getContext().getOrCreateSymbol(Name);
+  if (MCDwarfFrameInfo *F = getCurrentDwarfFrameInfo())
+    F->Instructions.push_back(MCCFIInstruction::createLabel(Label, Sym, Loc));
+}
+
 WinEH::FrameInfo *MCStreamer::EnsureValidWinFrameInfo(SMLoc Loc) {
   const MCAsmInfo *MAI = Context.getAsmInfo();
   if (!MAI->usesWindowsCFI()) {

llvm/test/MC/ELF/cfi-label.s

@@ -1,4 +1,3 @@
-# XFAIL: *
 # RUN: llvm-mc -triple x86_64 %s | FileCheck %s --check-prefix=ASM
 # RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t
 # RUN: llvm-readelf -sX %t | FileCheck %s --check-prefix=SYMTAB

revert_patches.txt

@@ -100,7 +100,3 @@ Revert: breaking MIOpen test 2
 f8b1ca4992a2 [MCParser] .altmacro: Support argument expansion not preceded by
 812f9e81d2f7 [MCParser] .altmacro: ignore & after a token
 ---
-revert: needs downstream merging from Emma
-9abb574f9a68 [MC] Make MCCFIInstruction smaller    
-2718654c542c [MC] Support .cfi_label
----