[BOLT][DWARF] Don't create extra .debug_str_offsets contributions

[ayermolo]

Summary

With ThinLTO mutliple CUs can share the same .debug_str_offsets contribution. We
were creating a new one for each CU. This lead to a binary size increase.

[maksfb]

bolt/include/bolt/Core/DebugData.h

447

Use DenseSet<>.

bolt/lib/Core/DebugData.cpp

1095

Is this an assert on internal condition or is it the input validation?

1110–1111

When can this happen?

bolt/test/X86/dwarf5-shared-str-offset-base.s

12

Perhaps include the source files since they appear to be short and how .s was generated.

13

291

[ayermolo]

bolt/lib/Core/DebugData.cpp

1095

Internal state. In case this somehow gets called in DWARF4 or something.

1110–1111

In theory, if two DWO CUs share same entry (built with ThinLTO). From what I have seen they do not, since compilation directory is different, but who knows in the future.

[ayermolo]

addressed comments

[ayermolo]

updated error message

[ayermolo]

addressed comments

[maksfb]

LGTM

[ayermolo]

rebase

[ayermolo]

clang-format

[ayermolo]

s/Optional/std::optional to work with trunk

bolt/include/bolt/Core/DebugData.h

public:
   /// Initializes Buffer and Stream.
   void initialize(const DWARFSection &StrOffsetsSection,
                   const Optional<StrOffsetsContributionDescriptor> Contr);
 
   /// Update Str offset in .debug_str in .debug_str_offsets.
   void updateAddressMap(uint32_t Index, uint32_t Address);
 
   /// Writes out current sections entry into .debug_str_offsets.
-  void finalizeSection();
+  void finalizeSection(DWARFUnit &Unit);
 
   /// Returns False if no strings were added to .debug_str.
   bool isFinalized() const { return !StrOffsetsBuffer->empty(); }
 
   /// Returns buffer containing .debug_str_offsets.
   std::unique_ptr<DebugStrOffsetsBufferVector> releaseBuffer() {
     return std::move(StrOffsetsBuffer);
   }
 
 private:
   std::unique_ptr<DebugStrOffsetsBufferVector> StrOffsetsBuffer;
   std::unique_ptr<raw_svector_ostream> StrOffsetsStream;
   std::map<uint32_t, uint32_t> IndexToAddressMap;
+  std::unordered_set<uint64_t> ProcessedBaseOffsets;
   // Section size not including header.
   uint32_t CurrentSectionSize{0};
+  bool StrOffsetSectionWasModified = false;
 };
 
 using DebugStrBufferVector = SmallVector<char, 16>;
 class DebugStrWriter {
 public:
   DebugStrWriter() = delete;
   DebugStrWriter(BinaryContext &BC) : BC(BC) { create(); }
   std::unique_ptr<DebugStrBufferVector> releaseBuffer() {

bolt/lib/Core/DebugData.cpp

void DebugStrOffsetsWriter::initialize(
   for (uint64_t Offset = 0; Offset < Contr->Size; Offset += DwarfOffsetByteSize)
     IndexToAddressMap[Index++] = *reinterpret_cast<const uint32_t *>(
         StrOffsetsSection.Data.data() + Contr->Base + Offset);
 }
 
 void DebugStrOffsetsWriter::updateAddressMap(uint32_t Index, uint32_t Address) {
   assert(IndexToAddressMap.count(Index) > 0 && "Index is not found.");
   IndexToAddressMap[Index] = Address;
+  StrOffsetSectionWasModified = true;
 }
 
-void DebugStrOffsetsWriter::finalizeSection() {
+void DebugStrOffsetsWriter::finalizeSection(DWARFUnit &Unit) {
   if (IndexToAddressMap.empty())
     return;
+
+  Optional<AttrInfo> AttrVal =
+      findAttributeInfo(Unit.getUnitDIE(), dwarf::DW_AT_str_offsets_base);
+  assert(AttrVal && "DW_AT_str_offsets_base not present.");
+  Optional<uint64_t> Val = AttrVal->V.getAsSectionOffset();
+  assert(Val && "DW_AT_str_offsets_base Value not present.");
+  auto RetVal = ProcessedBaseOffsets.insert(*Val);
+  if (RetVal.second) {
     // Writing out the header for each section.
     support::endian::write(*StrOffsetsStream, CurrentSectionSize + 4,
                            support::little);
     support::endian::write(*StrOffsetsStream, static_cast<uint16_t>(5),
                            support::little);
     support::endian::write(*StrOffsetsStream, static_cast<uint16_t>(0),
                            support::little);
     for (const auto &Entry : IndexToAddressMap)
       support::endian::write(*StrOffsetsStream, Entry.second, support::little);
+  }
+  // Will assert if we already processed this contribution, and now skipping it,
+  // but it was modified.
+  assert(RetVal.second ||
+         (!RetVal.second && !StrOffsetSectionWasModified) &&
+             "Skipping String offsets section, but it was modified.");
+  StrOffsetSectionWasModified = false;
   IndexToAddressMap.clear();
 }
 
 void DebugStrWriter::create() {
   StrBuffer = std::make_unique<DebugStrBufferVector>();
   StrStream = std::make_unique<raw_svector_ostream>(*StrBuffer);
 }
 

bolt/lib/Rewrite/DWARFRewriter.cpp

if (SplitCU) {
       auto LocListWriterIter = LocListWritersByCU.find(CUIndex);
       if (LocListWriterIter != LocListWritersByCU.end())
         DebugLocWriter = LocListWriterIter->second.get();
     }
     if (Unit->getVersion() >= 5) {
       RangesBase = RangesSectionWriter->getSectionOffset() +
                    getDWARF5RngListLocListHeaderSize();
       RangesSectionWriter->initSection(*Unit);
-      StrOffstsWriter->finalizeSection();
+      StrOffstsWriter->finalizeSection(*Unit);
     }
 
     DebugInfoPatcher->addUnitBaseOffsetLabel(Unit->getOffset());
     updateUnitDebugInfo(*Unit, *DebugInfoPatcher, *AbbrevWriter,
                         *DebugLocWriter, *RangesSectionWriter, RangesBase);
     DebugLocWriter->finalize(*DebugInfoPatcher, *AbbrevWriter);
     if (Unit->getVersion() >= 5)
       RangesSectionWriter->finalizeSection();

bolt/test/X86/dwarf5-shared-str-offset-base.s

+# REQUIRES: system-linux
+
+# RUN: llvm-mc --filetype=obj --triple x86_64 %s -o %tmain.o --defsym MAIN=0
+# RUN: llvm-mc --filetype=obj --triple x86_64 %s -o %thelper.o
+# RUN: %clang %cflags %tmain.o %thelper.o -o %tmain.exe
+# RUN: llvm-bolt %tmain.exe -o %tmain.exe.bolt --update-debug-sections
+# RUN: llvm-dwarfdump --debug-info %tmain.exe.bolt > %tout.text
+# RUN: llvm-dwarfdump --show-section-sizes %tmain.exe >> %tout.text
+# RUN: llvm-dwarfdump --show-section-sizes %tmain.exe.bolt >> %tout.text
+# RUN: cat %tout.text | FileCheck %s
+
+# This test checks that with DWARF5 when two CUs share the same .debug_str_offsets
+# entry we do not create a new one.
+
+# CHECK: DW_AT_str_offsets_base (0x[[#%.8x,ADDR:]]
+# CHECK: DW_AT_str_offsets_base (0x[[#ADDR]]
+
+# CHECK: .debug_str_offsets [[#ADDR2:]]
+# CHECK: .debug_str_offsets [[#ADDR2]]
+
+## Create two CUs, with dwo_ids 0 and 1 respectively.
+.ifdef MAIN
+.text
+  .file "main.cpp"
+  .globl  main                            # -- Begin function main
+  .p2align  4, 0x90
+  .type main,@function
+main:                                   # @main
+.Lfunc_begin0:
+  .file 0 "." "main.cpp" md5 0x32c197b0a8b855eb3d7573c993ada862
+  .loc  0 1 0                           # main.cpp:1:0
+  .cfi_startproc
+# %bb.0:
+  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 1 prologue_end              # main.cpp:2:1
+  xorl  %eax, %eax
+  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  "Facebook clang version 15.0.0" # string offset=0
+.Linfo_string1:
+  .asciz  "main.cpp"                      # string offset=146
+.Linfo_string2:
+  .asciz  "." # string offset=155
+.Linfo_string3:
+  .asciz  "main"                          # string offset=198
+.Linfo_string4:
+  .asciz  "int"                           # string offset=203
+  .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  "Facebook clang version 15.0.0"
+  .section  ".note.GNU-stack","",@progbits
+  .addrsig
+  .section  .debug_line,"",@progbits
+.Lline_table_start0:
+.else
+.text
+  .file "helper.cpp"
+  .globl  _Z3foov                         # -- Begin function _Z3foov
+  .p2align  4, 0x90
+  .type _Z3foov,@function
+_Z3foov:                                # @_Z3foov
+.Lfunc_begin0:
+  .file 0 "." "helper.cpp" md5 0x5f98e4807e4f8781c26a82faf819f8a7
+  .loc  0 1 0                           # helper.cpp:1:0
+  .cfi_startproc
+# %bb.0:
+  pushq %rbp
+  .cfi_def_cfa_offset 16
+  .cfi_offset %rbp, -16
+  movq  %rsp, %rbp
+  .cfi_def_cfa_register %rbp
+.Ltmp0:
+  .loc  0 1 12 prologue_end             # helper.cpp:1:12
+  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 63                              # DW_AT_external
+  .byte 25                              # DW_FORM_flag_present
+  .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:0x24 DW_TAG_compile_unit
+  .byte 0                               # DW_AT_producer
+  .short  33                              # DW_AT_language
+  .byte 1                               # DW_AT_name
+  .long 0x8                       # DW_AT_str_offsets_base Manually modified to be the same as first CU
+  .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] 0x23:0xc 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
+                                        # DW_AT_external
+  .byte 0                               # End Of Children Mark
+.Ldebug_info_end0:
+# Manually removed .debug_str_offsets and .debug_str
+  .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  "Facebook clang version 15.0.0"
+  .section  ".note.GNU-stack","",@progbits
+  .addrsig
+  .section  .debug_line,"",@progbits
+.Lline_table_start0:
+.endif