Next step: Only pad debug_line when the target is darwin. Add a FIXME to avoid

doing that if the target is darwin10 or newer.

This fixes
*) Direct object emission was producing objects without the workaround on
   darwin9.
*) Assembly printing was producing objects with the workaround on linux.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120866 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCAsmInfo.h

@@ -51,6 +51,11 @@ namespace llvm {
     /// emitted in Static relocation model.
     bool HasStaticCtorDtorReferenceInStaticMode;  // Default is false.
 
+    /// LinkerRequiresNonEmptyDwarfLines - True if the linker has a bug and
+    /// requires that the debug_line section be of a minimum size. In practice
+    /// such a linker requires a non empty line sequence if a file is present.
+    bool LinkerRequiresNonEmptyDwarfLines; // Default to false.
+
     /// MaxInstLength - This is the maximum possible length of an instruction,
     /// which is needed to compute the size of an inline asm.
     unsigned MaxInstLength;                  // Defaults to 4.
@@ -322,6 +327,9 @@ namespace llvm {
     bool hasStaticCtorDtorReferenceInStaticMode() const {
       return HasStaticCtorDtorReferenceInStaticMode;
     }
+    bool getLinkerRequiresNonEmptyDwarfLines() const {
+      return LinkerRequiresNonEmptyDwarfLines;
+    }
     unsigned getMaxInstLength() const {
       return MaxInstLength;
     }

include/llvm/MC/MCDwarf.h

@@ -208,8 +208,7 @@ namespace llvm {
     //
     // This emits the Dwarf file and the line tables.
     //
-    static void Emit(MCStreamer *MCOS, const MCSection *DwarfLineSection,
-                     const MCSection *TextSection = NULL);
+    static void Emit(MCStreamer *MCOS, const MCSection *DwarfLineSection);
   };
 
   class MCDwarfLineAddr {

lib/MC/MCAsmInfo.cpp

@@ -23,6 +23,7 @@ MCAsmInfo::MCAsmInfo() {
   HasMachoZeroFillDirective = false;
   HasMachoTBSSDirective = false;
   HasStaticCtorDtorReferenceInStaticMode = false;
+  LinkerRequiresNonEmptyDwarfLines = false;
   MaxInstLength = 4;
   PCSymbol = "$";
   SeparatorChar = ';';

lib/MC/MCAsmInfoDarwin.cpp

@@ -37,6 +37,9 @@ MCAsmInfoDarwin::MCAsmInfoDarwin() {
   HasMachoZeroFillDirective = true;  // Uses .zerofill
   HasMachoTBSSDirective = true; // Uses .tbss
   HasStaticCtorDtorReferenceInStaticMode = true;
+
+  // FIXME: Darwin 10 and newer don't need this.
+  LinkerRequiresNonEmptyDwarfLines = true;
   
   HiddenVisibilityAttr = MCSA_PrivateExtern;
   // Doesn't support protected visibility.

lib/MC/MCAsmStreamer.cpp

@@ -896,8 +896,7 @@ void MCAsmStreamer::EmitRawText(StringRef String) {
 void MCAsmStreamer::Finish() {
   // Dump out the dwarf file & directory tables and line tables.
   if (getContext().hasDwarfFiles() && TLOF)
-    MCDwarfFileTable::Emit(this, TLOF->getDwarfLineSection(),
-                           TLOF->getTextSection());
+    MCDwarfFileTable::Emit(this, TLOF->getDwarfLineSection());
 }
 
 MCStreamer *llvm::createAsmStreamer(MCContext &Context,

lib/MC/MCDwarf.cpp

@@ -7,6 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCDwarf.h"
 #include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCStreamer.h"
@@ -198,8 +199,7 @@ static inline void EmitDwarfLineTable(MCStreamer *MCOS,
 // This emits the Dwarf file and the line tables.
 //
 void MCDwarfFileTable::Emit(MCStreamer *MCOS,
-                            const MCSection *DwarfLineSection,
-                            const MCSection *TextSection) {
+                            const MCSection *DwarfLineSection) {
   // Switch to the section where the table will be emitted into.
   MCOS->SwitchSection(DwarfLineSection);
 
@@ -296,7 +296,8 @@ void MCDwarfFileTable::Emit(MCStreamer *MCOS,
     delete Line;
   }
 
-  if (TextSection && MCLineSectionOrder.begin() == MCLineSectionOrder.end()) {
+  if (MCOS->getContext().getAsmInfo().getLinkerRequiresNonEmptyDwarfLines()
+      && MCLineSectionOrder.begin() == MCLineSectionOrder.end()) {
     // The darwin9 linker has a bug (see PR8715). For for 32-bit architectures
     // it requires:  
     // total_length >= prologue_length + 10

test/MC/ELF/empty-dwarf-lines.s

@@ -0,0 +1,21 @@
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - | elf-dump | FileCheck %s
+
+// Test that the dwarf debug_line section contains no line directives.
+
+        .file   1 "test.c"
+        .globl  c
+c:
+        .asciz   "hi\n"
+
+// CHECK:      # Section 0x00000004
+// CHECK-NEXT: (('sh_name', 0x00000012) # '.debug_line'
+// CHECK-NEXT:  ('sh_type', 0x00000000)
+// CHECK-NEXT:  ('sh_flags', 0x00000000)
+// CHECK-NEXT:  ('sh_addr', 0x00000000)
+// CHECK-NEXT:  ('sh_offset', 0x00000044)
+// CHECK-NEXT:  ('sh_size', 0x00000027)
+// CHECK-NEXT:  ('sh_link', 0x00000000)
+// CHECK-NEXT:  ('sh_info', 0x00000000)
+// CHECK-NEXT:  ('sh_addralign', 0x00000001)
+// CHECK-NEXT:  ('sh_entsize', 0x00000000)
+// CHECK-NEXT: ),

test/MC/MachO/empty-dwarf-lines.s

@@ -0,0 +1,25 @@
+// RUN: llvm-mc -triple x86_64-apple-darwin9 %s -filetype=obj -o - | macho-dump | FileCheck %s
+
+// This tests that when producing files for darwin9 or older we make sure
+// that debug_line sections are of a minimum size to avoid the linker bug
+// described in PR8715.
+
+        .section        __DATA,__data
+        .file   1 "test.c"
+        .globl  _c                      ## @c
+_c:
+        .asciz   "hi\n"
+
+// CHECK:      (('section_name', '__debug_line\x00\x00\x00\x00')
+// CHECK-NEXT:  ('segment_name', '__DWARF\x00\x00\x00\x00\x00\x00\x00\x00\x00')
+// CHECK-NEXT:  ('address', 4)
+// CHECK-NEXT:  ('size', 44)
+// CHECK-NEXT:  ('offset', 452)
+// CHECK-NEXT:  ('alignment', 0)
+// CHECK-NEXT:  ('reloc_offset', 496)
+// CHECK-NEXT:  ('num_reloc', 4)
+// CHECK-NEXT:  ('flags', 0x2000000)
+// CHECK-NEXT:  ('reserved1', 0)
+// CHECK-NEXT:  ('reserved2', 0)
+// CHECK-NEXT:  ('reserved3', 0)
+// CHECK-NEXT: ),