[ELF] - Use information available from DW_AT_comp_dir attribute when doing error reporting.

[grimar]

Summary

Imagine we call clang -g aaa/bbb.cpp. Current error reporting would
print something like:

error: aaa/bbb.cpp :x: error text.

But we are able to print full path including current working directory of a compilation command,
like:

/home/bar/some_path/aaa/bbb.cpp:1: text

We can get this information from DW_AT_comp_dir attribute which is:
"A DW_AT_comp_dir attribute whose value is a null-terminated string containing the current
working directory of the compilation command that produced this compilation unit in
whatever form makes sense for the host system."

Idea of doing that change belongs to Eugene Leviant, thanks again !

[evgeny777]

ELF/InputFiles.cpp

74

Do we have a unit tests for zero number of CU ?

[davide]

ELF/InputFiles.cpp

48–50

you can probably simplify a bit using the ternary operator here, i.e.

auto &patatino = static_cast<...>(Sec);
return (patatino.getFlags() & SHF_ALLOC) ? patatino.getOffset(); 0;

ELF/InputFiles.h

204–206

what do you mean with "compilation command" ?

test/ELF/conflict2.s

10

s/Assembler file/The file/
s/following/the following/

11

/home/bar/some_path clang ?

13

s/linker/the linker/

14

full patch or full path?

[grimar]

• Addressed review comments.

ELF/InputFiles.cpp

48–50

Done.

74

Example of test which does not have CUs but has debug line table is conflict.s:

1. RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/conflict-debug.s -o %t-dbg.o
2. RUN: not ld.lld %t-dbg.o %t-dbg.o -o %t-dbg 2>&1 | FileCheck -check-prefix=DBGINFO %s

Where conflict-debug.s is:

.file 1 "conflict-debug.s"
.globl zed
.loc 1 4
zed:
  nop

ELF/InputFiles.h

204–206

umb@ubuntu:~/tests/181$ clang 111/1.cpp -g -o out

here I mean "clang 111/1.cpp -g -o out" is a compilation command.

I based on termonology from DW_AT_comp_dir description:

A DW_AT_comp_dir attribute whose value is a null-terminated string containing the current
working directory of the compilation command that produced this compilation unit

May be change it to next ?

// Containing the compilation directory for CU.

test/ELF/conflict2.s

10

Done.

11

I meant that foo/1.cpp is placed in
/home/bar/some_path which is working dirrectory for a call, like:

bar@ubuntu:~/some_path$ clang foo/1.cpp -g -S -o asm.s

Updated the comment.

13

Done.

14

Fixed.

[ruiu]

ELF/InputFiles.cpp

57

This function does more than its name says -- you are now initializing not only DwarfLine but CompilationDir. You probably should save Dwarf instead of DwarfLine and CompilationDir.

[grimar]

ELF/InputFiles.cpp

57

There is a problem with saving just DWARFContextInMemory (Dwarf).
LLVM DWARF parsers currently wants to see CU as argument for parsing LineTable:
Example:

  DWARFCompileUnit *CU = getCompileUnitForAddress(Address);
  if (!CU)
    return Result;
...
    if (const DWARFLineTable *LineTable = getLineTableForUnit(CU))

CU are taken from .debug_info section. Our LLD implementation works differently,
allowing us to show location for objects like undef-debug.s, that does not have .debug_info:

.file 1 "dir/undef-debug.s"
.loc 1 3
        .quad zed3
...

As far I know our current logic consistent with gold/bfd, which afaik can live without .debug_info, like we do.

At the same time, DWARF Debugging Information Format, Version 4 says:

"Line number information generated for a compilation unit is represented in the .debug_line
section of an object file and is referenced by a corresponding compilation unit debugging
information entry (see Section 3.1.1) in the .debug_info section."

For each compilation unit compiled with a DWARF producer, a contribution is made to the
.debug_info section of the object file.

What makes me wonder if it is correct to work with .debug_line when there is not .debug_info section
and so no info about CUs.

So there seems to be 2 ways:

1. Store both DwarfLine and CompilationDir like now. That way we still have extended messages for case when there is no .debug_section,

though it is not clear for me if we really should care about that cases. It seems our testcases are a bit too synthetic, looks there
should not be real producer that emits .debug_line without .debug_info ?

2. Store only Dwarf (DWARFContextInMemory) and ignore cases when object has no .debug_info section. That probably can simplifiy implementation.

Looks we can go with 2 ?

ELF/InputFiles.h

private:
   // List of all symbols referenced or defined by this file.
   std::vector<SymbolBody *> SymbolBodies;
 
   // Debugging information to retrieve source file and line for error
   // reporting. Linker may find reasonable number of errors in a
   // single object file, so we cache debugging information in order to
   // parse it only once for each object file we link.
   std::unique_ptr<llvm::DWARFDebugLine> DwarfLine;
+
+  // Containing the working directory of the compilation command.
+  llvm::Optional<std::string> CompilationDir;
 };
 
 // LazyObjectFile is analogous to ArchiveFile in the sense that
 // the file contains lazy symbols. The difference is that
 // LazyObjectFile wraps a single file instead of multiple files.
 //
 // This class is used for --start-lib and --end-lib options which
 // instruct the linker to link object files between them with the

ELF/InputFiles.cpp

 using namespace lld;
 using namespace lld::elf;
 
 namespace {
 // In ELF object file all section addresses are zero. If we have multiple
 // .text sections (when using -ffunction-section or comdat group) then
 // LLVM DWARF parser will not be able to parse .debug_line correctly, unless
 // we assign each section some unique address. This callback method assigns
-// each section an address equal to its offset in ELF object file.
+// each allocatable section an address equal to its offset in ELF object file.
 class ObjectInfo : public LoadedObjectInfo {
 public:
   uint64_t getSectionLoadAddress(const object::SectionRef &Sec) const override {
-    return static_cast<const ELFSectionRef &>(Sec).getOffset();
+    auto &S = static_cast<const ELFSectionRef &>(Sec);
+    return (S.getFlags() & ELF::SHF_ALLOC) ? S.getOffset() : 0;
   }
   std::unique_ptr<LoadedObjectInfo> clone() const override {
     return std::unique_ptr<LoadedObjectInfo>();
   }
 };
 }
 
 template <class ELFT> void elf::ObjectFile<ELFT>::initializeDwarfLine() {
   std::unique_ptr<object::ObjectFile> Obj =
       check(object::ObjectFile::createObjectFile(this->MB),
             "createObjectFile failed");
 
   ObjectInfo ObjInfo;
   DWARFContextInMemory Dwarf(*Obj, &ObjInfo);
   DwarfLine.reset(new DWARFDebugLine(&Dwarf.getLineSection().Relocs));
   DataExtractor LineData(Dwarf.getLineSection().Data,
                          ELFT::TargetEndianness == support::little,
                          ELFT::Is64Bits ? 8 : 4);
 
   // The second parameter is offset in .debug_line section
   // for compilation unit (CU) of interest. We have only one
   // CU (object file), so offset is always 0.
   DwarfLine->getOrParseLineTable(LineData, 0);
+
+  if (Dwarf.getNumCompileUnits())
+    CompilationDir = Dwarf.getCompileUnitAtIndex(0)->getCompilationDir();
 }
 
 // Returns source line information for a given offset
 // using DWARF debug info.
 template <class ELFT>
 std::string elf::ObjectFile<ELFT>::getLineInfo(InputSectionBase<ELFT> *S,
                                                uintX_t Offset) {
   if (!DwarfLine)
     initializeDwarfLine();
 
   // The offset to CU is 0.
   const DWARFDebugLine::LineTable *Tbl = DwarfLine->getLineTable(0);
   if (!Tbl)
     return "";
 
   // Use fake address calcuated by adding section file offset and offset in
   // section. See comments for ObjectInfo class.
   DILineInfo Info;
   Tbl->getFileLineInfoForAddress(
-      S->Offset + Offset, nullptr,
+      S->Offset + Offset, CompilationDir ? CompilationDir->c_str() : nullptr,
       DILineInfoSpecifier::FileLineInfoKind::AbsoluteFilePath, Info);
   if (Info.Line == 0)
     return "";
   return convertToUnixPathSeparator(Info.FileName) + ":" +
          std::to_string(Info.Line);
 }
 
 // Returns "(internal)", "foo.a(bar.o)" or "baz.o".

test/ELF/conflict2.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1.o
+# RUN: not ld.lld %t1.o %t1.o -o %t2 2>&1 | FileCheck %s
+
+# CHECK:      /home/bar/some_path/foo/1.cpp:1: duplicate symbol 'main'
+# CHECK-NEXT: /home/bar/some_path/foo/1.cpp:1: previous definition was here
+
+# The file below was generated with the following command line:
+# clang foo/1.cpp -g -S -o asm.s, called from working directory "/home/bar/some_path".
+# asm.s then was reduced manually.
+# 1.cpp is a one line file: "int main() { return 0; }"
+# That way the linker can extract current working directory of a compilation
+# command from DW_AT_comp_dir attribute for providing full path when reporting errors.
+
+.text
+.globl main
+main:
+.file 1 "foo" "1.cpp"
+.loc 1 1 0
+
+.section .debug_str,"MS",@progbits,1
+.Linfo_string0:
+ .asciz "clang version 4.0.0 (trunk 289927)"
+.Linfo_string1:
+ .asciz "foo/1.cpp"
+.Linfo_string2:
+ .asciz "/home/bar/some_path"
+.Linfo_string3:
+ .asciz "main"
+.Linfo_string4:
+ .asciz "int"
+ 
+.section .debug_abbrev,"",@progbits
+.Lsection_abbrev:
+ .byte 1                       # Abbreviation Code
+ .byte 17                      # DW_TAG_compile_unit
+ .byte 1                       # DW_CHILDREN_yes
+ .byte 37                      # DW_AT_producer
+ .byte 14                      # DW_FORM_strp
+ .byte 19                      # DW_AT_language
+ .byte 5                       # DW_FORM_data2
+ .byte 3                       # DW_AT_name
+ .byte 14                      # DW_FORM_strp
+ .byte 16                      # DW_AT_stmt_list
+ .byte 23                      # DW_FORM_sec_offset
+ .byte 27                      # DW_AT_comp_dir
+ .byte 14                      # DW_FORM_strp
+ .byte 17                      # DW_AT_low_pc
+ .byte 1                       # DW_FORM_addr
+ .byte 18                      # DW_AT_high_pc
+ .byte 6                       # DW_FORM_data4
+ .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 1                       # DW_FORM_addr
+ .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 14                      # DW_FORM_strp
+ .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 14                      # DW_FORM_strp
+ .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
+.Lsection_info:
+.Lcu_begin0:
+ .long 71                              # Length of Unit
+ .short 4                              # DWARF version number
+ .long .Lsection_abbrev                # Offset Into Abbrev. Section
+ .byte 8                               # Address Size (in bytes)
+ .byte 1                               # Abbrev [1] 0xb:0x40 DW_TAG_compile_unit
+ .long .Linfo_string0                  # DW_AT_producer
+ .short 4                              # DW_AT_language
+ .long .Linfo_string1                  # DW_AT_name
+ .long .Lline_table_start0             # DW_AT_stmt_list
+ .long .Linfo_string2                  # DW_AT_comp_dir
+ .quad 0                               # DW_AT_low_pc
+ .long 0                               # DW_AT_high_pc
+ .byte 2                               # Abbrev [2] 0x2a:0x19 DW_TAG_subprogram
+ .quad 0                               # DW_AT_low_pc
+ .long 0                               # DW_AT_high_pc
+ .byte 1                               # DW_AT_frame_base
+ .byte 86
+ .long .Linfo_string3                  # DW_AT_name
+ .byte 1                               # DW_AT_decl_file
+ .byte 1                               # DW_AT_decl_line
+ .long 67                              # DW_AT_type
+                                       # DW_AT_external
+ .byte 3                               # Abbrev [3] 0x43:0x7 DW_TAG_base_type
+ .long .Linfo_string4                  # DW_AT_name
+ .byte 5                               # DW_AT_encoding
+ .byte 4                               # DW_AT_byte_size
+ .byte 0                               # End Of Children Mark
+
+.Lline_table_start0: