[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 #97222

Pull Request: https://github.com/llvm/llvm-project/pull/97922

bolt/lib/Core/BinaryFunction.cpp

@@ -2538,6 +2538,7 @@ private:
     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

@@ -500,7 +500,8 @@ public:
     OpRegister,
     OpWindowSave,
     OpNegateRAState,
-    OpGnuArgsSize
+    OpGnuArgsSize,
+    OpLabel,
   };
 
 private:
@@ -519,6 +520,7 @@ private:
       unsigned Register;
       unsigned Register2;
     } RR;
+    MCSymbol *CfiLabel;
   } U;
   OpType Operation;
   SMLoc Loc;
@@ -544,6 +546,12 @@ private:
     U.RIA = {R, O, AS};
   }
 
+  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.
@@ -664,6 +672,11 @@ public:
     return MCCFIInstruction(OpGnuArgsSize, L, 0, Size, Loc);
   }
 
+  static MCCFIInstruction createLabel(MCSymbol *L, MCSymbol *CfiLabel,
+                                      SMLoc Loc) {
+    return MCCFIInstruction(OpLabel, L, CfiLabel, Loc);
+  }
+
   OpType getOperation() const { return Operation; }
   MCSymbol *getLabel() const { return Label; }
 
@@ -698,6 +711,11 @@ public:
     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

@@ -1019,6 +1019,7 @@ public:
                                SMLoc Loc = {});
   virtual void emitCFIWindowSave(SMLoc Loc = {});
   virtual void emitCFINegateRAState(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

@@ -248,6 +248,7 @@ void CFIInstrInserter::calculateOutgoingCFAInfo(MBBCFAInfo &MBBInfo) {
       case MCCFIInstruction::OpWindowSave:
       case MCCFIInstruction::OpNegateRAState:
       case MCCFIInstruction::OpGnuArgsSize:
+      case MCCFIInstruction::OpLabel:
         break;
       }
       if (CSRReg || CSROffset) {

llvm/lib/MC/MCAsmStreamer.cpp

@@ -356,6 +356,7 @@ public:
   void emitCFIWindowSave(SMLoc Loc) override;
   void emitCFINegateRAState(SMLoc Loc) override;
   void emitCFIReturnColumn(int64_t Register) override;
+  void emitCFILabelDirective(SMLoc Loc, StringRef Name) override;
 
   void emitWinCFIStartProc(const MCSymbol *Symbol, SMLoc Loc) override;
   void emitWinCFIEndProc(SMLoc Loc) override;
@@ -2126,6 +2127,12 @@ 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

@@ -1465,6 +1465,9 @@ void FrameEmitterImpl::emitCFIInstruction(const MCCFIInstruction &Instr) {
   case MCCFIInstruction::OpEscape:
     Streamer.emitBytes(Instr.getValues());
     return;
+  case MCCFIInstruction::OpLabel:
+    Streamer.emitLabel(Instr.getCfiLabel(), Instr.getLoc());
+    return;
   }
   llvm_unreachable("Unhandled case in switch");
 }

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -520,6 +520,7 @@ private:
     DK_CFI_UNDEFINED,
     DK_CFI_REGISTER,
     DK_CFI_WINDOW_SAVE,
+    DK_CFI_LABEL,
     DK_CFI_B_KEY_FRAME,
     DK_MACROS_ON,
     DK_MACROS_OFF,
@@ -622,6 +623,7 @@ private:
   bool parseDirectiveCFIReturnColumn(SMLoc DirectiveLoc);
   bool parseDirectiveCFISignalFrame(SMLoc DirectiveLoc);
   bool parseDirectiveCFIUndefined(SMLoc DirectiveLoc);
+  bool parseDirectiveCFILabel(SMLoc DirectiveLoc);
 
   // macro directives
   bool parseDirectivePurgeMacro(SMLoc DirectiveLoc);
@@ -2224,6 +2226,8 @@ 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);
@@ -4488,6 +4492,19 @@ bool AsmParser::parseDirectiveCFIUndefined(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
@@ -5560,6 +5577,7 @@ 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

@@ -689,6 +689,13 @@ 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

@@ -0,0 +1,61 @@
+# 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
+# RUN: llvm-dwarfdump --eh-frame %t | FileCheck %s
+
+# RUN: not llvm-mc -filetype=obj -triple=x86_64 --defsym ERR=1 %s -o /dev/null 2>&1 | \
+# RUN:   FileCheck %s --check-prefix=ERR --implicit-check-not=error:
+
+# ASM:      nop
+# ASM-NEXT: .cfi_label cfi1
+# ASM-NEXT: .cfi_escape 0x00
+# ASM:      .globl cfi2
+# ASM-NEXT: .cfi_label cfi2
+# ASM:      nop
+# ASM-NEXT: .cfi_label .Lcfi3
+
+# SYMTAB:      000000000000002b     0 NOTYPE  LOCAL  DEFAULT     3 (.eh_frame) cfi1
+# SYMTAB:      000000000000002d     0 NOTYPE  GLOBAL DEFAULT     3 (.eh_frame) cfi2
+# SYMTAB-NOT:  {{.}}
+
+# CHECK:       DW_CFA_remember_state:
+# CHECK-NEXT:  DW_CFA_advance_loc: 1 to 0x1
+# CHECK-NEXT:  DW_CFA_nop:
+# CHECK-NEXT:  DW_CFA_advance_loc: 1 to 0x2
+# CHECK-NEXT:  DW_CFA_nop:
+# CHECK-NEXT:  DW_CFA_nop:
+# CHECK-NEXT:  DW_CFA_advance_loc: 1 to 0x3
+# CHECK-NEXT:  DW_CFA_nop:
+# CHECK-NEXT:  DW_CFA_nop:
+# CHECK-NEXT:  DW_CFA_nop:
+# CHECK-NEXT:  DW_CFA_restore_state:
+
+.globl foo
+foo:
+.cfi_startproc
+.cfi_remember_state
+nop
+.cfi_label cfi1
+.cfi_escape 0
+nop
+.globl cfi2
+.cfi_label cfi2
+.cfi_escape 0, 0
+nop
+.cfi_label .Lcfi3
+.cfi_escape 0, 0, 0
+.cfi_restore_state
+ret
+
+# ERR: [[#@LINE+10]]:1: error: this directive must appear between .cfi_startproc and .cfi_endproc directives
+.ifdef ERR
+# ERR: [[#@LINE+1]]:12: error: symbol 'foo' is already defined
+.cfi_label foo
+# ERR: [[#@LINE+1]]:12: error: symbol '.Lcfi3' is already defined
+.cfi_label .Lcfi3
+.endif
+.cfi_endproc
+
+.ifdef ERR
+.cfi_label after_endproc
+.endif