[BOLT][DWARF] Fix for .debug_line with DWARF5

[ayermolo]

Summary

There was a bug in a code that pre-populated line string for a case where parts
of .debug_line are not processed by BOLT, but copied as raw data. We were not
switching sections. This resulted in parts of the binary being over-written with
debug data.

[ayermolo]

removed unused argument

[ayermolo]

removed include

[ayermolo]

removed offsets from test

[maksfb]

bolt/lib/Core/DebugData.cpp

1676–1678

The returned offset has to match the input Offset, right? Add a check.

llvm/include/llvm/MC/MCDwarf.h

74

Add documentation for the return value.

[ayermolo]

addressed comments

[maksfb]

LGTM with a couple of nits addressed.

bolt/lib/Core/DebugData.cpp

1669

nit:

1676–1678

nit:

[ayermolo]

addressed ntis

bolt/lib/Core/DebugData.cpp

void DwarfLineTable::emitCU(MCStreamer *MCOS, MCDwarfLineTableParams Params,
   // This is the end of the section, so set the value of the symbol at the end
   // of this section (that was used in a previous expression).
   MCOS->emitLabel(LineEndSym);
 }
 
 // Helper function to parse .debug_line_str, and populate one we are using.
 // For functions that we do not modify we output them as raw data.
 // Re-constructing .debug_line_str so that offsets are correct for those
-// debut line tables.
+// debug line tables.
 // Bonus is that when we output a final binary we can re-use .debug_line_str
 // section. So we don't have to do the SHF_ALLOC trick we did with
 // .debug_line.
 static void parseAndPopulateDebugLineStr(BinarySection &LineStrSection,
                                          MCDwarfLineStr &LineStr,
-                                         BinaryContext &BC,
-                                         MCStreamer &Streamer) {
+                                         BinaryContext &BC) {
   DataExtractor StrData(LineStrSection.getContents(),
                         BC.DwCtx->isLittleEndian(), 0);
   uint64_t Offset = 0;
   while (StrData.isValidOffset(Offset)) {
+    const uint64_t StrOffset = Offset;
     Error Err = Error::success();
     const char *CStr = StrData.getCStr(&Offset, &Err);
     if (Err) {
       errs() << "BOLT-ERROR: could not extract string from .debug_line_str";
       continue;
     }
-    LineStr.emitRef(&Streamer, CStr);
+    const size_t NewOffset = LineStr.addString(CStr);
+    assert(StrOffset == NewOffset &&
+           "New offset in .debug_line_str doesn't match original offset");
   }
 }
 
 void DwarfLineTable::emit(BinaryContext &BC, MCStreamer &Streamer) {
   MCAssembler &Assembler =
       static_cast<MCObjectStreamer *>(&Streamer)->getAssembler();
 
   MCDwarfLineTableParams Params = Assembler.getDWARFLinetableParams();
 
   auto &LineTables = BC.getDwarfLineTables();
 
   // Bail out early so we don't switch to the debug_line section needlessly and
   // in doing so create an unnecessary (if empty) section.
   if (LineTables.empty())
     return;
   // In a v5 non-split line table, put the strings in a separate section.
   std::optional<MCDwarfLineStr> LineStr;
   ErrorOr<BinarySection &> LineStrSection =
       BC.getUniqueSectionByName(".debug_line_str");
+
   // Some versions of GCC output DWARF5 .debug_info, but DWARF4 or lower
-  // .debug_line
+  // .debug_line, so need to check if section exists.
   if (LineStrSection) {
     LineStr.emplace(*BC.Ctx);
-    parseAndPopulateDebugLineStr(*LineStrSection, *LineStr, BC, Streamer);
+    parseAndPopulateDebugLineStr(*LineStrSection, *LineStr, BC);
   }
 
   // Switch to the section where the table will be emitted into.
   Streamer.switchSection(BC.MOFI->getDwarfLineSection());
 
   const uint16_t DwarfVersion = BC.Ctx->getDwarfVersion();
   // Handle the rest of the Compile Units.
   for (auto &CUIDTablePair : LineTables) {

bolt/test/X86/Inputs/dwarf5-debug-line-not-modified-helper-variable.s

+# int Foo = 0;
+
+  .text
+  .file "helperVariable.cpp"
+  .file 0 "/test" "helperVariable.cpp" md5 0xb6e6130198b21a44b5db4247fccb359d
+  .type Foo,@object                     # @Foo
+  .bss
+  .globl  Foo
+  .p2align  2, 0x0
+Foo:
+  .long 0                               # 0x0
+  .size Foo, 4
+
+  .section  .debug_abbrev,"",@progbits
+  .byte 1                               # Abbreviation Code
+  .byte 17                              # DW_TAG_compile_unit
+  .byte 1                               # DW_CHILDREN_yes
+  .byte 37                              # DW_AT_producer
+  .byte 37                              # DW_FORM_strx1
+  .byte 19                              # DW_AT_language
+  .byte 5                               # DW_FORM_data2
+  .byte 3                               # DW_AT_name
+  .byte 37                              # DW_FORM_strx1
+  .byte 114                             # DW_AT_str_offsets_base
+  .byte 23                              # DW_FORM_sec_offset
+  .byte 16                              # DW_AT_stmt_list
+  .byte 23                              # DW_FORM_sec_offset
+  .byte 27                              # DW_AT_comp_dir
+  .byte 37                              # DW_FORM_strx1
+  .byte 115                             # DW_AT_addr_base
+  .byte 23                              # DW_FORM_sec_offset
+  .byte 0                               # EOM(1)
+  .byte 0                               # EOM(2)
+  .byte 2                               # Abbreviation Code
+  .byte 52                              # DW_TAG_variable
+  .byte 0                               # DW_CHILDREN_no
+  .byte 3                               # DW_AT_name
+  .byte 37                              # DW_FORM_strx1
+  .byte 73                              # DW_AT_type
+  .byte 19                              # DW_FORM_ref4
+  .byte 63                              # DW_AT_external
+  .byte 25                              # DW_FORM_flag_present
+  .byte 58                              # DW_AT_decl_file
+  .byte 11                              # DW_FORM_data1
+  .byte 59                              # DW_AT_decl_line
+  .byte 11                              # DW_FORM_data1
+  .byte 2                               # DW_AT_location
+  .byte 24                              # DW_FORM_exprloc
+  .byte 0                               # EOM(1)
+  .byte 0                               # EOM(2)
+  .byte 3                               # Abbreviation Code
+  .byte 36                              # DW_TAG_base_type
+  .byte 0                               # DW_CHILDREN_no
+  .byte 3                               # DW_AT_name
+  .byte 37                              # DW_FORM_strx1
+  .byte 62                              # DW_AT_encoding
+  .byte 11                              # DW_FORM_data1
+  .byte 11                              # DW_AT_byte_size
+  .byte 11                              # DW_FORM_data1
+  .byte 0                               # EOM(1)
+  .byte 0                               # EOM(2)
+  .byte 0                               # EOM(3)
+  .section  .debug_info,"",@progbits
+.Lcu_begin0:
+  .long .Ldebug_info_end0-.Ldebug_info_start0 # Length of Unit
+.Ldebug_info_start0:
+  .short  5                               # DWARF version number
+  .byte 1                               # DWARF Unit Type
+  .byte 8                               # Address Size (in bytes)
+  .long .debug_abbrev                   # Offset Into Abbrev. Section
+  .byte 1                               # Abbrev [1] 0xc:0x22 DW_TAG_compile_unit
+  .byte 0                               # DW_AT_producer
+  .short  33                              # DW_AT_language
+  .byte 1                               # DW_AT_name
+  .long .Lstr_offsets_base0             # DW_AT_str_offsets_base
+  .long .Lline_table_start0             # DW_AT_stmt_list
+  .byte 2                               # DW_AT_comp_dir
+  .long .Laddr_table_base0              # DW_AT_addr_base
+  .byte 2                               # Abbrev [2] 0x1e:0xb DW_TAG_variable
+  .byte 3                               # DW_AT_name
+  .long 41                              # DW_AT_type
+                                        # DW_AT_external
+  .byte 0                               # DW_AT_decl_file
+  .byte 1                               # DW_AT_decl_line
+  .byte 2                               # DW_AT_location
+  .byte 161
+  .byte 0
+  .byte 3                               # Abbrev [3] 0x29:0x4 DW_TAG_base_type
+  .byte 4                               # DW_AT_name
+  .byte 5                               # DW_AT_encoding
+  .byte 4                               # DW_AT_byte_size
+  .byte 0                               # End Of Children Mark
+.Ldebug_info_end0:
+  .section  .debug_str_offsets,"",@progbits
+  .long 24                              # Length of String Offsets Set
+  .short  5
+  .short  0
+.Lstr_offsets_base0:
+  .section  .debug_str,"MS",@progbits,1
+.Linfo_string0:
+  .asciz  "clang version 17.0.0 (https://github.com/llvm/llvm-project.git 640e07c49037cca41a1bfbeb916b569d8c950aea)" # string offset=0
+.Linfo_string1:
+  .asciz  "helperVariable.cpp"            # string offset=105
+.Linfo_string2:
+  .asciz  "/test" # string offset=124
+.Linfo_string3:
+  .asciz  "Foo"                           # string offset=162
+.Linfo_string4:
+  .asciz  "int"                           # string offset=166
+  .section  .debug_str_offsets,"",@progbits
+  .long .Linfo_string0
+  .long .Linfo_string1
+  .long .Linfo_string2
+  .long .Linfo_string3
+  .long .Linfo_string4
+  .section  .debug_addr,"",@progbits
+  .long .Ldebug_addr_end0-.Ldebug_addr_start0 # Length of contribution
+.Ldebug_addr_start0:
+  .short  5                               # DWARF version number
+  .byte 8                               # Address size
+  .byte 0                               # Segment selector size
+.Laddr_table_base0:
+  .quad Foo
+.Ldebug_addr_end0:
+  .ident  "clang version 17.0.0 (https://github.com/llvm/llvm-project.git 640e07c49037cca41a1bfbeb916b569d8c950aea)"
+  .section  ".note.GNU-stack","",@progbits
+  .addrsig
+  .section  .debug_line,"",@progbits
+.Lline_table_start0:

bolt/test/X86/Inputs/dwarf5-debug-line-not-modified-helper.s

+# int foo() {
+#   return 0;
+# }
+
+  .text
+  .file "helper.cpp"
+  .globl  _Z3foov                         # -- Begin function _Z3foov
+  .p2align  4, 0x90
+  .type _Z3foov,@function
+_Z3foov:                                # @_Z3foov
+.Lfunc_begin0:
+  .file 0 "/test" "helper.cpp" md5 0x95a4a33e7fe9423970d3c798fdf8ed43
+  .loc  0 1 0                           # helper.cpp:1:0
+  .cfi_startproc
+# %bb.0:                                # %entry
+  pushq %rbp
+  .cfi_def_cfa_offset 16
+  .cfi_offset %rbp, -16
+  movq  %rsp, %rbp
+  .cfi_def_cfa_register %rbp
+.Ltmp0:
+  .loc  0 2 3 prologue_end              # helper.cpp:2:3
+  xorl  %eax, %eax
+  .loc  0 2 3 epilogue_begin is_stmt 0  # helper.cpp:2:3
+  popq  %rbp
+  .cfi_def_cfa %rsp, 8
+  retq
+.Ltmp1:
+.Lfunc_end0:
+  .size _Z3foov, .Lfunc_end0-_Z3foov
+  .cfi_endproc
+                                        # -- End function
+  .section  .debug_abbrev,"",@progbits
+  .byte 1                               # Abbreviation Code
+  .byte 17                              # DW_TAG_compile_unit
+  .byte 1                               # DW_CHILDREN_yes
+  .byte 37                              # DW_AT_producer
+  .byte 37                              # DW_FORM_strx1
+  .byte 19                              # DW_AT_language
+  .byte 5                               # DW_FORM_data2
+  .byte 3                               # DW_AT_name
+  .byte 37                              # DW_FORM_strx1
+  .byte 114                             # DW_AT_str_offsets_base
+  .byte 23                              # DW_FORM_sec_offset
+  .byte 16                              # DW_AT_stmt_list
+  .byte 23                              # DW_FORM_sec_offset
+  .byte 27                              # DW_AT_comp_dir
+  .byte 37                              # DW_FORM_strx1
+  .byte 17                              # DW_AT_low_pc
+  .byte 27                              # DW_FORM_addrx
+  .byte 18                              # DW_AT_high_pc
+  .byte 6                               # DW_FORM_data4
+  .byte 115                             # DW_AT_addr_base
+  .byte 23                              # DW_FORM_sec_offset
+  .byte 0                               # EOM(1)
+  .byte 0                               # EOM(2)
+  .byte 2                               # Abbreviation Code
+  .byte 46                              # DW_TAG_subprogram
+  .byte 0                               # DW_CHILDREN_no
+  .byte 17                              # DW_AT_low_pc
+  .byte 27                              # DW_FORM_addrx
+  .byte 18                              # DW_AT_high_pc
+  .byte 6                               # DW_FORM_data4
+  .byte 64                              # DW_AT_frame_base
+  .byte 24                              # DW_FORM_exprloc
+  .byte 110                             # DW_AT_linkage_name
+  .byte 37                              # DW_FORM_strx1
+  .byte 3                               # DW_AT_name
+  .byte 37                              # DW_FORM_strx1
+  .byte 58                              # DW_AT_decl_file
+  .byte 11                              # DW_FORM_data1
+  .byte 59                              # DW_AT_decl_line
+  .byte 11                              # DW_FORM_data1
+  .byte 73                              # DW_AT_type
+  .byte 19                              # DW_FORM_ref4
+  .byte 63                              # DW_AT_external
+  .byte 25                              # DW_FORM_flag_present
+  .byte 0                               # EOM(1)
+  .byte 0                               # EOM(2)
+  .byte 3                               # Abbreviation Code
+  .byte 36                              # DW_TAG_base_type
+  .byte 0                               # DW_CHILDREN_no
+  .byte 3                               # DW_AT_name
+  .byte 37                              # DW_FORM_strx1
+  .byte 62                              # DW_AT_encoding
+  .byte 11                              # DW_FORM_data1
+  .byte 11                              # DW_AT_byte_size
+  .byte 11                              # DW_FORM_data1
+  .byte 0                               # EOM(1)
+  .byte 0                               # EOM(2)
+  .byte 0                               # EOM(3)
+  .section  .debug_info,"",@progbits
+.Lcu_begin0:
+  .long .Ldebug_info_end0-.Ldebug_info_start0 # Length of Unit
+.Ldebug_info_start0:
+  .short  5                               # DWARF version number
+  .byte 1                               # DWARF Unit Type
+  .byte 8                               # Address Size (in bytes)
+  .long .debug_abbrev                   # Offset Into Abbrev. Section
+  .byte 1                               # Abbrev [1] 0xc:0x2c DW_TAG_compile_unit
+  .byte 0                               # DW_AT_producer
+  .short  33                              # DW_AT_language
+  .byte 1                               # DW_AT_name
+  .long .Lstr_offsets_base0             # DW_AT_str_offsets_base
+  .long .Lline_table_start0             # DW_AT_stmt_list
+  .byte 2                               # DW_AT_comp_dir
+  .byte 0                               # DW_AT_low_pc
+  .long .Lfunc_end0-.Lfunc_begin0       # DW_AT_high_pc
+  .long .Laddr_table_base0              # DW_AT_addr_base
+  .byte 2                               # Abbrev [2] 0x23:0x10 DW_TAG_subprogram
+  .byte 0                               # DW_AT_low_pc
+  .long .Lfunc_end0-.Lfunc_begin0       # DW_AT_high_pc
+  .byte 1                               # DW_AT_frame_base
+  .byte 86
+  .byte 3                               # DW_AT_linkage_name
+  .byte 4                               # DW_AT_name
+  .byte 0                               # DW_AT_decl_file
+  .byte 1                               # DW_AT_decl_line
+  .long 51                              # DW_AT_type
+                                        # DW_AT_external
+  .byte 3                               # Abbrev [3] 0x33:0x4 DW_TAG_base_type
+  .byte 5                               # DW_AT_name
+  .byte 5                               # DW_AT_encoding
+  .byte 4                               # DW_AT_byte_size
+  .byte 0                               # End Of Children Mark
+.Ldebug_info_end0:
+  .section  .debug_str_offsets,"",@progbits
+  .long 28                              # Length of String Offsets Set
+  .short  5
+  .short  0
+.Lstr_offsets_base0:
+  .section  .debug_str,"MS",@progbits,1
+.Linfo_string0:
+  .asciz  "clang version 17.0.0 (https://github.com/llvm/llvm-project.git 640e07c49037cca41a1bfbeb916b569d8c950aea)" # string offset=0
+.Linfo_string1:
+  .asciz  "helper.cpp"                    # string offset=105
+.Linfo_string2:
+  .asciz  "/test" # string offset=116
+.Linfo_string3:
+  .asciz  "_Z3foov"                       # string offset=154
+.Linfo_string4:
+  .asciz  "foo"                           # string offset=162
+.Linfo_string5:
+  .asciz  "int"                           # string offset=166
+  .section  .debug_str_offsets,"",@progbits
+  .long .Linfo_string0
+  .long .Linfo_string1
+  .long .Linfo_string2
+  .long .Linfo_string3
+  .long .Linfo_string4
+  .long .Linfo_string5
+  .section  .debug_addr,"",@progbits
+  .long .Ldebug_addr_end0-.Ldebug_addr_start0 # Length of contribution
+.Ldebug_addr_start0:
+  .short  5                               # DWARF version number
+  .byte 8                               # Address size
+  .byte 0                               # Segment selector size
+.Laddr_table_base0:
+  .quad .Lfunc_begin0
+.Ldebug_addr_end0:
+  .ident  "clang version 17.0.0 (https://github.com/llvm/llvm-project.git 640e07c49037cca41a1bfbeb916b569d8c950aea)"
+  .section  ".note.GNU-stack","",@progbits
+  .addrsig
+  .section  .debug_line,"",@progbits
+.Lline_table_start0:

bolt/test/X86/Inputs/dwarf5-debug-line-not-modified-main.s

+# int main() {
+#  return 0;
+# }
+
+  .text
+  .file "main.cpp"
+  .globl  main                            # -- Begin function main
+  .p2align  4, 0x90
+  .type main,@function
+main:                                   # @main
+.Lfunc_begin0:
+  .file 0 "/test" "main.cpp" md5 0x7228a872dc174332f3151a7ac3344b26
+  .loc  0 1 0                           # main.cpp:1:0
+  .cfi_startproc
+# %bb.0:                                # %entry
+  pushq %rbp
+  .cfi_def_cfa_offset 16
+  .cfi_offset %rbp, -16
+  movq  %rsp, %rbp
+  .cfi_def_cfa_register %rbp
+  movl  $0, -4(%rbp)
+.Ltmp0:
+  .loc  0 2 2 prologue_end              # main.cpp:2:2
+  xorl  %eax, %eax
+  .loc  0 2 2 epilogue_begin is_stmt 0  # main.cpp:2:2
+  popq  %rbp
+  .cfi_def_cfa %rsp, 8
+  retq
+.Ltmp1:
+.Lfunc_end0:
+  .size main, .Lfunc_end0-main
+  .cfi_endproc
+                                        # -- End function
+  .section  .debug_abbrev,"",@progbits
+  .byte 1                               # Abbreviation Code
+  .byte 17                              # DW_TAG_compile_unit
+  .byte 1                               # DW_CHILDREN_yes
+  .byte 37                              # DW_AT_producer
+  .byte 37                              # DW_FORM_strx1
+  .byte 19                              # DW_AT_language
+  .byte 5                               # DW_FORM_data2
+  .byte 3                               # DW_AT_name
+  .byte 37                              # DW_FORM_strx1
+  .byte 114                             # DW_AT_str_offsets_base
+  .byte 23                              # DW_FORM_sec_offset
+  .byte 16                              # DW_AT_stmt_list
+  .byte 23                              # DW_FORM_sec_offset
+  .byte 27                              # DW_AT_comp_dir
+  .byte 37                              # DW_FORM_strx1
+  .byte 17                              # DW_AT_low_pc
+  .byte 27                              # DW_FORM_addrx
+  .byte 18                              # DW_AT_high_pc
+  .byte 6                               # DW_FORM_data4
+  .byte 115                             # DW_AT_addr_base
+  .byte 23                              # DW_FORM_sec_offset
+  .byte 0                               # EOM(1)
+  .byte 0                               # EOM(2)
+  .byte 2                               # Abbreviation Code
+  .byte 46                              # DW_TAG_subprogram
+  .byte 0                               # DW_CHILDREN_no
+  .byte 17                              # DW_AT_low_pc
+  .byte 27                              # DW_FORM_addrx
+  .byte 18                              # DW_AT_high_pc
+  .byte 6                               # DW_FORM_data4
+  .byte 64                              # DW_AT_frame_base
+  .byte 24                              # DW_FORM_exprloc
+  .byte 3                               # DW_AT_name
+  .byte 37                              # DW_FORM_strx1
+  .byte 58                              # DW_AT_decl_file
+  .byte 11                              # DW_FORM_data1
+  .byte 59                              # DW_AT_decl_line
+  .byte 11                              # DW_FORM_data1
+  .byte 73                              # DW_AT_type
+  .byte 19                              # DW_FORM_ref4
+  .byte 63                              # DW_AT_external
+  .byte 25                              # DW_FORM_flag_present
+  .byte 0                               # EOM(1)
+  .byte 0                               # EOM(2)
+  .byte 3                               # Abbreviation Code
+  .byte 36                              # DW_TAG_base_type
+  .byte 0                               # DW_CHILDREN_no
+  .byte 3                               # DW_AT_name
+  .byte 37                              # DW_FORM_strx1
+  .byte 62                              # DW_AT_encoding
+  .byte 11                              # DW_FORM_data1
+  .byte 11                              # DW_AT_byte_size
+  .byte 11                              # DW_FORM_data1
+  .byte 0                               # EOM(1)
+  .byte 0                               # EOM(2)
+  .byte 0                               # EOM(3)
+  .section  .debug_info,"",@progbits
+.Lcu_begin0:
+  .long .Ldebug_info_end0-.Ldebug_info_start0 # Length of Unit
+.Ldebug_info_start0:
+  .short  5                               # DWARF version number
+  .byte 1                               # DWARF Unit Type
+  .byte 8                               # Address Size (in bytes)
+  .long .debug_abbrev                   # Offset Into Abbrev. Section
+  .byte 1                               # Abbrev [1] 0xc:0x2b DW_TAG_compile_unit
+  .byte 0                               # DW_AT_producer
+  .short  33                              # DW_AT_language
+  .byte 1                               # DW_AT_name
+  .long .Lstr_offsets_base0             # DW_AT_str_offsets_base
+  .long .Lline_table_start0             # DW_AT_stmt_list
+  .byte 2                               # DW_AT_comp_dir
+  .byte 0                               # DW_AT_low_pc
+  .long .Lfunc_end0-.Lfunc_begin0       # DW_AT_high_pc
+  .long .Laddr_table_base0              # DW_AT_addr_base
+  .byte 2                               # Abbrev [2] 0x23:0xf DW_TAG_subprogram
+  .byte 0                               # DW_AT_low_pc
+  .long .Lfunc_end0-.Lfunc_begin0       # DW_AT_high_pc
+  .byte 1                               # DW_AT_frame_base
+  .byte 86
+  .byte 3                               # DW_AT_name
+  .byte 0                               # DW_AT_decl_file
+  .byte 1                               # DW_AT_decl_line
+  .long 50                              # DW_AT_type
+                                        # DW_AT_external
+  .byte 3                               # Abbrev [3] 0x32:0x4 DW_TAG_base_type
+  .byte 4                               # DW_AT_name
+  .byte 5                               # DW_AT_encoding
+  .byte 4                               # DW_AT_byte_size
+  .byte 0                               # End Of Children Mark
+.Ldebug_info_end0:
+  .section  .debug_str_offsets,"",@progbits
+  .long 24                              # Length of String Offsets Set
+  .short  5
+  .short  0
+.Lstr_offsets_base0:
+  .section  .debug_str,"MS",@progbits,1
+.Linfo_string0:
+  .asciz  "clang version 17.0.0 (https://github.com/llvm/llvm-project.git 640e07c49037cca41a1bfbeb916b569d8c950aea)" # string offset=0
+.Linfo_string1:
+  .asciz  "main.cpp"                      # string offset=105
+.Linfo_string2:
+  .asciz  "/test" # string offset=114
+.Linfo_string3:
+  .asciz  "main"                          # string offset=152
+.Linfo_string4:
+  .asciz  "int"                           # string offset=157
+  .section  .debug_str_offsets,"",@progbits
+  .long .Linfo_string0
+  .long .Linfo_string1
+  .long .Linfo_string2
+  .long .Linfo_string3
+  .long .Linfo_string4
+  .section  .debug_addr,"",@progbits
+  .long .Ldebug_addr_end0-.Ldebug_addr_start0 # Length of contribution
+.Ldebug_addr_start0:
+  .short  5                               # DWARF version number
+  .byte 8                               # Address size
+  .byte 0                               # Segment selector size
+.Laddr_table_base0:
+  .quad .Lfunc_begin0
+.Ldebug_addr_end0:
+  .ident  "clang version 17.0.0 (https://github.com/llvm/llvm-project.git 640e07c49037cca41a1bfbeb916b569d8c950aea)"
+  .section  ".note.GNU-stack","",@progbits
+  .addrsig
+  .section  .debug_line,"",@progbits
+.Lline_table_start0:

bolt/test/X86/dwarf5-debug-line-not-modified.test

+# REQUIRES: system-linux
+
+# RUN: llvm-mc -dwarf-version=5 -filetype=obj -triple x86_64-unknown-linux %p/Inputs/dwarf5-debug-line-not-modified-main.s -o %t1.o
+# RUN: llvm-mc -dwarf-version=5 -filetype=obj -triple x86_64-unknown-linux %p/Inputs/dwarf5-debug-line-not-modified-helper-variable.s -o %t2.o
+# RUN: llvm-mc -dwarf-version=5 -filetype=obj -triple x86_64-unknown-linux %p/Inputs/dwarf5-debug-line-not-modified-helper.s -o %t3.o
+# RUN: %clang %cflags -dwarf-5 %t1.o %t2.o %t3.o -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -o %t.bolt --update-debug-sections
+# RUN: llvm-dwarfdump --show-form --verbose --debug-line %t.bolt | FileCheck --check-prefix=POSTCHECK %s
+
+# This test checks that BOLT generates correct debug_line_str when one of CU contributions is not modified.
+
+# POSTCHECK: version: 5
+# POSTCHECK: include_directories[  0] =  .debug_line_str[{{.*}}] = "/test"
+# POSTCHECK: file_names[  0]:
+# POSTCHECK: name:  .debug_line_str[{{.*}}] = "main.cpp"
+# POSTCHECK: dir_index: 0
+# POSTCHECK: md5_checksum: 7228a872dc174332f3151a7ac3344b26
+# POSTCHECK: version: 5
+# POSTCHECK: include_directories[  0] =  .debug_line_str[{{.*}}] = "/test"
+# POSTCHECK: file_names[  0]:
+# POSTCHECK: name:  .debug_line_str[{{.*}}] = "helperVariable.cpp"
+# POSTCHECK: dir_index: 0
+# POSTCHECK: md5_checksum: b6e6130198b21a44b5db4247fccb359d
+# POSTCHECK: version: 5
+# POSTCHECK: include_directories[  0] =  .debug_line_str[{{.*}}] = "/test"
+# POSTCHECK: file_names[  0]:
+# POSTCHECK: name:  .debug_line_str[{{.*}}] = "helper.cpp"
+# POSTCHECK: dir_index: 0
+# POSTCHECK: md5_checksum: 95a4a33e7fe9423970d3c798fdf8ed43

llvm/include/llvm/MC/MCDwarf.h

public:
   /// Emit a reference to the string.
   void emitRef(MCStreamer *MCOS, StringRef Path);
 
   /// Emit the .debug_line_str section if appropriate.
   void emitSection(MCStreamer *MCOS);
 
   /// Returns finalized section.
   SmallString<0> getFinalizedData();
+
+  /// Adds path \p Path to the line string. Returns offset in the
+  /// .debug_line_str section.
+  size_t addString(StringRef Path);
 };
 
 /// Instances of this class represent the name of the dwarf .file directive and
 /// its associated dwarf file number in the MC file. MCDwarfFile's are created
 /// and uniqued by the MCContext class. In Dwarf 4 file numbers start from 1;
 /// i.e. the entry with file number 1 is the first element in the vector of
 /// DwarfFiles and there is no MCDwarfFile with file number 0. In Dwarf 5 file
 /// numbers start from 0, with the MCDwarfFile with file number 0 being the

llvm/lib/MC/MCDwarf.cpp

SmallString<0> MCDwarfLineStr::getFinalizedData() {
   if (!LineStrings.isFinalized())
     LineStrings.finalizeInOrder();
   SmallString<0> Data;
   Data.resize(LineStrings.getSize());
   LineStrings.write((uint8_t *)Data.data());
   return Data;
 }
 
+size_t MCDwarfLineStr::addString(StringRef Path) {
+  return LineStrings.add(Path);
+}
+
 void MCDwarfLineStr::emitRef(MCStreamer *MCOS, StringRef Path) {
   int RefSize =
       dwarf::getDwarfOffsetByteSize(MCOS->getContext().getDwarfFormat());
-  size_t Offset = LineStrings.add(Path);
+  size_t Offset = addString(Path);
   if (UseRelocs) {
     MCContext &Ctx = MCOS->getContext();
     MCOS->emitValue(makeStartPlusIntExpr(Ctx, *LineStrLabel, Offset), RefSize);
   } else
     MCOS->emitIntValue(Offset, RefSize);
 }
 
 void MCDwarfLineTableHeader::emitV2FileDirTables(MCStreamer *MCOS) const {