[MC] Add .loc_label instruction

Author: Alex B

llvm/include/llvm/MC/MCDwarf.h

@@ -194,11 +194,17 @@ class MCDwarfLineEntry : public MCDwarfLoc {
 
 public:
   // Constructor to create an MCDwarfLineEntry given a symbol and the dwarf loc.
-  MCDwarfLineEntry(MCSymbol *label, const MCDwarfLoc loc)
-      : MCDwarfLoc(loc), Label(label) {}
+  MCDwarfLineEntry(MCSymbol *label, const MCDwarfLoc loc,
+                   MCSymbol *lineStreamLabel = nullptr)
+      : MCDwarfLoc(loc), Label(label), LineStreamLabel(lineStreamLabel) {}
 
   MCSymbol *getLabel() const { return Label; }
 
+  // This is the label that is to be emitted into the line stream. If this is
+  // non-null and we need to emit a label, also make sure to restart the current
+  // line sequence.
+  MCSymbol *LineStreamLabel;
+
   // This indicates the line entry is synthesized for an end entry.
   bool IsEndEntry = false;
 

llvm/include/llvm/MC/MCStreamer.h

@@ -912,6 +912,9 @@ class MCStreamer {
                                      unsigned Isa, unsigned Discriminator,
                                      StringRef FileName);
 
+  /// This implements the '.loc_label Name' directive.
+  virtual void emitDwarfLocLabelDirective(SMLoc Loc, StringRef Name);
+
   /// Associate a filename with a specified logical file number, and also
   /// specify that file's checksum information.  This implements the '.cv_file 4
   /// "foo.c"' assembler directive. Returns true on success.

llvm/lib/MC/MCAsmStreamer.cpp

@@ -285,6 +285,8 @@ class MCAsmStreamer final : public MCStreamer {
                              unsigned Flags, unsigned Isa,
                              unsigned Discriminator,
                              StringRef FileName) override;
+  virtual void emitDwarfLocLabelDirective(SMLoc Loc, StringRef Name) override;
+
   MCSymbol *getDwarfLineTableSymbol(unsigned CUID) override;
 
   bool emitCVFileDirective(unsigned FileNo, StringRef Filename,
@@ -1751,6 +1753,12 @@ void MCAsmStreamer::emitDwarfLocDirective(unsigned FileNo, unsigned Line,
                                           Discriminator, FileName);
 }
 
+void MCAsmStreamer::emitDwarfLocLabelDirective(SMLoc Loc, StringRef Name) {
+  MCStreamer::emitDwarfLocLabelDirective(Loc, Name);
+  OS << "\t.loc_label " << Name;
+  EmitEOL();
+}
+
 MCSymbol *MCAsmStreamer::getDwarfLineTableSymbol(unsigned CUID) {
   // Always use the zeroth line table, since asm syntax only supports one line
   // table for now.

llvm/lib/MC/MCDwarf.cpp

@@ -172,6 +172,7 @@ void MCDwarfLineTable::emitOne(
     const MCLineSection::MCDwarfLineEntryCollection &LineEntries) {
 
   unsigned FileNum, LastLine, Column, Flags, Isa, Discriminator;
+  bool IsAtStartSeq;
   MCSymbol *LastLabel;
   auto init = [&]() {
     FileNum = 1;
@@ -181,6 +182,7 @@ void MCDwarfLineTable::emitOne(
     Isa = 0;
     Discriminator = 0;
     LastLabel = nullptr;
+    IsAtStartSeq = true;
   };
   init();
 
@@ -189,11 +191,24 @@ void MCDwarfLineTable::emitOne(
   for (const MCDwarfLineEntry &LineEntry : LineEntries) {
     MCSymbol *Label = LineEntry.getLabel();
     const MCAsmInfo *asmInfo = MCOS->getContext().getAsmInfo();
+
+    if (LineEntry.LineStreamLabel) {
+      if (!IsAtStartSeq) {
+        MCOS->emitDwarfLineEndEntry(Section, LastLabel);
+        init();
+      }
+      MCOS->emitLabel(LineEntry.LineStreamLabel);
+
+      IsAtStartSeq = true;
+      continue;
+    }
+
     if (LineEntry.IsEndEntry) {
       MCOS->emitDwarfAdvanceLineAddr(INT64_MAX, LastLabel, Label,
                                      asmInfo->getCodePointerSize());
       init();
       EndEntryEmitted = true;
+      IsAtStartSeq = true;
       continue;
     }
 
@@ -243,14 +258,15 @@ void MCDwarfLineTable::emitOne(
     Discriminator = 0;
     LastLine = LineEntry.getLine();
     LastLabel = Label;
+    IsAtStartSeq = false;
   }
 
   // Generate DWARF line end entry.
   // We do not need this for DwarfDebug that explicitly terminates the line
   // table using ranges whenever CU or section changes. However, the MC path
   // does not track ranges nor terminate the line table. In that case,
   // conservatively use the section end symbol to end the line table.
-  if (!EndEntryEmitted)
+  if (!EndEntryEmitted && !IsAtStartSeq)
     MCOS->emitDwarfLineEndEntry(Section, LastLabel);
 }
 

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -485,6 +485,7 @@ class AsmParser : public MCAsmParser {
     DK_FILE,
     DK_LINE,
     DK_LOC,
+    DK_LOC_LABEL,
     DK_STABS,
     DK_CV_FILE,
     DK_CV_FUNC_ID,
@@ -580,10 +581,11 @@ class AsmParser : public MCAsmParser {
   // ".align{,32}", ".p2align{,w,l}"
   bool parseDirectiveAlign(bool IsPow2, unsigned ValueSize);
 
-  // ".file", ".line", ".loc", ".stabs"
+  // ".file", ".line", ".loc", ".loc_label", ".stabs"
   bool parseDirectiveFile(SMLoc DirectiveLoc);
   bool parseDirectiveLine();
   bool parseDirectiveLoc();
+  bool parseDirectiveLocLabel(SMLoc DirectiveLoc);
   bool parseDirectiveStabs();
 
   // ".cv_file", ".cv_func_id", ".cv_inline_site_id", ".cv_loc", ".cv_linetable",
@@ -2156,6 +2158,8 @@ bool AsmParser::parseStatement(ParseStatementInfo &Info,
       return parseDirectiveLine();
     case DK_LOC:
       return parseDirectiveLoc();
+    case DK_LOC_LABEL:
+      return parseDirectiveLocLabel(IDLoc);
     case DK_STABS:
       return parseDirectiveStabs();
     case DK_CV_FILE:
@@ -3737,6 +3741,19 @@ bool AsmParser::parseDirectiveLoc() {
   return false;
 }
 
+/// parseDirectiveLoc
+/// ::= .loc_label label
+bool AsmParser::parseDirectiveLocLabel(SMLoc DirectiveLoc) {
+  StringRef Name;
+  DirectiveLoc = Lexer.getLoc();
+  if (parseIdentifier(Name))
+    return TokError("expected identifier");
+  if (parseEOL())
+    return true;
+  getStreamer().emitDwarfLocLabelDirective(DirectiveLoc, Name);
+  return false;
+}
+
 /// parseDirectiveStabs
 /// ::= .stabs string, number, number, number
 bool AsmParser::parseDirectiveStabs() {
@@ -5549,6 +5566,7 @@ void AsmParser::initializeDirectiveKindMap() {
   DirectiveKindMap[".file"] = DK_FILE;
   DirectiveKindMap[".line"] = DK_LINE;
   DirectiveKindMap[".loc"] = DK_LOC;
+  DirectiveKindMap[".loc_label"] = DK_LOC_LABEL;
   DirectiveKindMap[".stabs"] = DK_STABS;
   DirectiveKindMap[".cv_file"] = DK_CV_FILE;
   DirectiveKindMap[".cv_func_id"] = DK_CV_FUNC_ID;

llvm/lib/MC/MCStreamer.cpp

@@ -267,6 +267,25 @@ void MCStreamer::emitDwarfLocDirective(unsigned FileNo, unsigned Line,
                                   Discriminator);
 }
 
+void MCStreamer::emitDwarfLocLabelDirective(SMLoc Loc, StringRef Name) {
+  auto &ctx = getContext();
+
+  auto *LineStreamLabel = ctx.getOrCreateSymbol(Name);
+
+  auto *LineSym = ctx.createTempSymbol();
+  const MCDwarfLoc &DwarfLoc = ctx.getCurrentDwarfLoc();
+
+  // Create a 'fake' line entry by having LineStreamLabel be non-null. This
+  // won't actually emit any line information, it will reset the line table
+  // sequence and emit a label at the start of the new line table sequence.
+  MCDwarfLineEntry LineEntry(LineSym, DwarfLoc, LineStreamLabel);
+
+  // Add the line entry to this section's entries.
+  ctx.getMCDwarfLineTable(ctx.getDwarfCompileUnitID())
+      .getMCLineSections()
+      .addLineEntry(LineEntry, getCurrentSectionOnly());
+}
+
 MCSymbol *MCStreamer::getDwarfLineTableSymbol(unsigned CUID) {
   MCDwarfLineTable &Table = getContext().getMCDwarfLineTable(CUID);
   if (!Table.getLabel()) {

llvm/test/MC/ELF/debug-loc-label.s

@@ -0,0 +1,74 @@
+// Verify that the .loc_label instruction resets the line sequence and generates
+// the requested label at the correct position in the line stream
+
+// RUN: llvm-mc -filetype obj -triple x86_64-linux-elf %s -o %t.o
+// RUN: llvm-objdump -d %t.o | FileCheck %s --check-prefix=CHECK-ASM
+// RUN: llvm-dwarfdump -v --debug-line %t.o | FileCheck %s --check-prefix=CHECK-LINE-TABLE
+
+# CHECK-ASM:        <foo>:
+# CHECK-ASM-NEXT:     movq    %rdx, 0x33
+# CHECK-ASM-NEXT:     movq    %rax, 0x3b
+# CHECK-ASM-NEXT:     movq    %rbx, 0x4e
+# CHECK-ASM-NEXT:     movq    %rcx, 0x60
+# CHECK-ASM-NEXT:     retq
+
+# CHECK-LINE-TABLE:                  Address            Line   Column File   ISA Discriminator OpIndex Flags
+# CHECK-LINE-TABLE-NEXT:             ------------------ ------ ------ ------ --- ------------- ------- -------------
+# CHECK-LINE-TABLE-NEXT: 0x00000028: 05 DW_LNS_set_column (1)
+# CHECK-LINE-TABLE-NEXT: 0x0000002a: 00 DW_LNE_set_address (0x0000000000000000)
+# CHECK-LINE-TABLE-NEXT: 0x00000035: 01 DW_LNS_copy
+# CHECK-LINE-TABLE-NEXT:             0x0000000000000000      1      1      1   0             0       0  is_stmt
+# CHECK-LINE-TABLE-NEXT: 0x00000036: 02 DW_LNS_advance_pc (addr += 33, op-index += 0)
+# CHECK-LINE-TABLE-NEXT: 0x00000038: 00 DW_LNE_end_sequence
+# CHECK-LINE-TABLE-NEXT:             0x0000000000000021      1      1      1   0             0       0  is_stmt end_sequence
+# CHECK-LINE-TABLE-NEXT: 0x0000003b: 05 DW_LNS_set_column (2)
+# CHECK-LINE-TABLE-NEXT: 0x0000003d: 00 DW_LNE_set_address (0x0000000000000008)
+# CHECK-LINE-TABLE-NEXT: 0x00000048: 01 DW_LNS_copy
+# CHECK-LINE-TABLE-NEXT:             0x0000000000000008      1      2      1   0             0       0  is_stmt
+# CHECK-LINE-TABLE-NEXT: 0x00000049: 02 DW_LNS_advance_pc (addr += 25, op-index += 0)
+# CHECK-LINE-TABLE-NEXT: 0x0000004b: 00 DW_LNE_end_sequence
+# CHECK-LINE-TABLE-NEXT:             0x0000000000000021      1      2      1   0             0       0  is_stmt end_sequence
+# CHECK-LINE-TABLE-NEXT: 0x0000004e: 05 DW_LNS_set_column (3)
+# CHECK-LINE-TABLE-NEXT: 0x00000050: 00 DW_LNE_set_address (0x0000000000000010)
+# CHECK-LINE-TABLE-NEXT: 0x0000005b: 01 DW_LNS_copy
+# CHECK-LINE-TABLE-NEXT:             0x0000000000000010      1      3      1   0             0       0  is_stmt
+# CHECK-LINE-TABLE-NEXT: 0x0000005c: 08 DW_LNS_const_add_pc (addr += 0x0000000000000011, op-index += 0)
+# CHECK-LINE-TABLE-NEXT: 0x0000005d: 00 DW_LNE_end_sequence
+# CHECK-LINE-TABLE-NEXT:             0x0000000000000021      1      3      1   0             0       0  is_stmt end_sequence
+# CHECK-LINE-TABLE-NEXT: 0x00000060: 05 DW_LNS_set_column (4)
+# CHECK-LINE-TABLE-NEXT: 0x00000062: 00 DW_LNE_set_address (0x0000000000000018)
+# CHECK-LINE-TABLE-NEXT: 0x0000006d: 01 DW_LNS_copy
+# CHECK-LINE-TABLE-NEXT:             0x0000000000000018      1      4      1   0             0       0  is_stmt
+# CHECK-LINE-TABLE-NEXT: 0x0000006e: 05 DW_LNS_set_column (5)
+# CHECK-LINE-TABLE-NEXT: 0x00000070: 01 DW_LNS_copy
+# CHECK-LINE-TABLE-NEXT:             0x0000000000000018      1      5      1   0             0       0  is_stmt
+# CHECK-LINE-TABLE-NEXT: 0x00000071: 02 DW_LNS_advance_pc (addr += 9, op-index += 0)
+# CHECK-LINE-TABLE-NEXT: 0x00000073: 00 DW_LNE_end_sequence
+# CHECK-LINE-TABLE-NEXT:             0x0000000000000021      1      5      1   0             0       0  is_stmt end_sequence
+	.text
+	.file	"test.c"
+	.globl	foo
+	.align	16, 0x90
+	.type	foo,@function
+foo:
+.Lfunc_begin0:
+	.file	1 "test.c"
+	.cfi_startproc
+	.loc	1 1 1
+	mov     %rdx, 0x33
+	.loc_label my_label_02
+	.loc	1 1 2
+  movq    %rax, my_label_02-.Lline_table_start0
+	.loc	1 1 3
+	.loc_label my_label_03
+	.loc_label my_label_03.1
+  movq    %rbx, my_label_03-.Lline_table_start0
+	.loc	1 1 4
+	.loc_label my_label_05
+	.loc	1 1 5
+  movq    %rcx, my_label_05-.Lline_table_start0
+	ret
+	.cfi_endproc
+
+	.section	.debug_line,"",@progbits
+.Lline_table_start0: