[ELF] - Also use DW_AT_linkage_name when gathering information about variables for error messages.

[grimar]

Summary

Currently, when LLD do a lookup for variables location, it uses DW_AT_name attribute.
That is not always enough.

Imagine code:

namespace A {
  int bar = 0;
}

namespace Z {
  int bar = 1;
}

int hoho;

In this case there are 3 variables and their debug attributes are following:

1. A::bar has: DW_AT_name [DW_FORM_string] ("bar") DW_AT_linkage_name [DW_FORM_strp] ( .debug_str[0x00000006] = "_ZN1A3barE")

2. Z::bar has: DW_AT_name [DW_FORM_string] ("bar") DW_AT_linkage_name [DW_FORM_strp] ( .debug_str[0x0000003f] = "_ZN1Z3barE")

3. hoho has: DW_AT_name [DW_FORM_strp] ( .debug_str[0x0000004a] = "hoho") and has NO DW_AT_linkage_name attribute. Because it would be the same as DW_AT_name and DWARF producers avoids emiting excessive data.

Hence LLD should also use DW_AT_linkage_name when it is available.
(currently, LLD fails to report location correctly because thinks that A::bar and Z::bar are the same things)

[grimar]

Ping.

[peter.smith]

This looks sensible to me. If I've got this right when the DW_AT_linkage name differs from the DW_AT_name the symbol name will match the DW_AT_linkage_name so the lookup by symbol name won't succeed if done via DW_AT_name.

[grimar]

In D47373#1118723, @peter.smith wrote:

This looks sensible to me. If I've got this right when the DW_AT_linkage name differs from the DW_AT_name the symbol name will match the DW_AT_linkage_name so the lookup by symbol name won't succeed if done via DW_AT_name.

Yes, that is the case. Thanks!

ELF/InputFiles.cpp

for (const auto &Entry : CU->dies()) {
       // Get the source filename index for the variable.
       unsigned File = dwarf::toUnsigned(Die.find(dwarf::DW_AT_decl_file), 0);
       if (!LT->hasFileAtIndex(File))
         continue;
 
       // Get the line number on which the variable is declared.
       unsigned Line = dwarf::toUnsigned(Die.find(dwarf::DW_AT_decl_line), 0);
 
-      // Get the name of the variable and add the collected information to
-      // VariableLoc. Usually Name is non-empty, but it can be empty if the
+      // Here we want to take the variable name to add it into VariableLoc.
+      // Variable can have regular and linkage name associated. At first, we try
+      // to get linkage name as it can be different, for example when we have
+      // two variables in different namespaces of the same object. Use common
+      // name otherwise, but handle the case when it also absent in case if the
       // input object file lacks some debug info.
-      StringRef Name = dwarf::toString(Die.find(dwarf::DW_AT_name), "");
+      StringRef Name =
+          dwarf::toString(Die.find(dwarf::DW_AT_linkage_name),
+                          dwarf::toString(Die.find(dwarf::DW_AT_name), ""));
       if (!Name.empty())
         VariableLoc.insert({Name, {LT, File, Line}});
     }
   }
 }
 
 // Returns the pair of file name and line number describing location of data
 // object (variable, array, etc) definition.

test/ELF/conflict-variable-linkage-name.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: llvm-dwarfdump %t.o | FileCheck -check-prefix=INPUT %s
+# RUN: not ld.lld %t.o %t.o -o %t 2>&1 | FileCheck %s
+
+# INPUT:      DW_TAG_variable
+# INPUT-NEXT:   DW_AT_name            ("bar")
+# INPUT-NEXT:   DW_AT_type            (0x0000003d "int")
+# INPUT-NEXT:   DW_AT_external        (true)
+# INPUT-NEXT:   DW_AT_decl_file       ("/path\1.cpp")
+# INPUT-NEXT:   DW_AT_decl_line       (2)
+# INPUT-NEXT:   DW_AT_location        (DW_OP_addr 0x0)
+# INPUT-NEXT:   DW_AT_linkage_name    ("_ZN1A3barE")
+# INPUT:      DW_TAG_variable
+# INPUT-NEXT:   DW_AT_name            ("bar")
+# INPUT-NEXT:   DW_AT_type            (0x0000003d "int")
+# INPUT-NEXT:   DW_AT_external        (true)
+# INPUT-NEXT:   DW_AT_decl_file       ("/path\1.cpp")
+# INPUT-NEXT:   DW_AT_decl_line       (6)
+# INPUT-NEXT:   DW_AT_location        (DW_OP_addr 0x0)
+# INPUT-NEXT:   DW_AT_linkage_name    ("_ZN1Z3barE")
+
+## Check we can report the locations of 2 different "bar" variables.
+# CHECK:      duplicate symbol: A::bar
+# CHECK-NEXT: >>> defined at 1.cpp:2
+# CHECK-NEXT: >>>            {{.*}}:(A::bar)
+# CHECK-NEXT: >>> defined at 1.cpp:2
+# CHECK-NEXT: >>>            {{.*}}:(.bss+0x0)
+# CHECK:      duplicate symbol: Z::bar
+# CHECK-NEXT: >>> defined at 1.cpp:6
+# CHECK-NEXT: >>>            {{.*}}:(Z::bar)
+# CHECK-NEXT: >>> defined at 1.cpp:6
+# CHECK-NEXT: >>>            {{.*}}:(.data+0x0)
+
+# Used reduced output from following code and clang version 7.0.0 (trunk 332701)
+# to produce this input file:
+# Source (1.cpp):
+#  namespace A {
+#    int bar;
+#  }
+#  
+#  namespace Z {
+#    int bar;
+#  }
+# Invocation: clang-7 -g -S 1.cpp
+
+.text
+.file  "1.cpp"
+.file  1 "/path" "1.cpp"
+
+.type  _ZN1A3barE,@object
+.bss
+.globl  _ZN1A3barE
+_ZN1A3barE:
+  .long  0
+  .size  _ZN1A3barE, 4
+
+.type  _ZN1Z3barE,@object
+.data
+.globl  _ZN1Z3barE
+_ZN1Z3barE:
+  .long  1
+  .size  _ZN1Z3barE, 4
+
+.section  .debug_str,"MS",@progbits,1
+.Linfo_string0:
+  .asciz  "clang version 7.0.0 (trunk 332701)" # string offset=0
+.Linfo_string1:
+  .asciz  "1.cpp"                 # string offset=35
+.Linfo_string2:
+  .asciz  "/path"                 # string offset=41
+.Linfo_string3:
+  .asciz  "A"                     # string offset=87
+.Linfo_string4:
+  .asciz  "bar"                   # string offset=89
+.Linfo_string5:
+  .asciz  "int"                   # string offset=93
+.Linfo_string6:
+  .asciz  "_ZN1A3barE"            # string offset=97
+.Linfo_string7:
+  .asciz  "Z"                     # string offset=108
+.Linfo_string8:
+  .asciz  "_ZN1Z3barE"            # string offset=110
+
+.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  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
+  .ascii  "\264B"                # DW_AT_GNU_pubnames
+  .byte  25                      # DW_FORM_flag_present
+  .byte  0                       # EOM(1)
+  .byte  0                       # EOM(2)
+  
+  .byte  2                       # Abbreviation Code
+  .byte  57                      # DW_TAG_namespace
+  .byte  1                       # DW_CHILDREN_yes
+  .byte  3                       # DW_AT_name
+  .byte  14                      # DW_FORM_strp
+  .byte  0                       # EOM(1)
+  .byte  0                       # EOM(2)
+  
+  .byte  3                       # Abbreviation Code
+  .byte  52                      # DW_TAG_variable
+  .byte  0                       # DW_CHILDREN_no
+  .byte  3                       # DW_AT_name
+  .byte  14                      # DW_FORM_strp
+  .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  110                     # DW_AT_linkage_name
+  .byte  14                      # DW_FORM_strp
+  .byte  0                       # EOM(1)
+  .byte  0                       # EOM(2)
+  
+  .byte  4                       # 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
+  .long  96                      # Length of Unit
+  .short  4                      # DWARF version number
+  .long  .debug_abbrev           # Offset Into Abbrev. Section
+  .byte  8                       # Address Size (in bytes)
+  
+  .byte  1                       # Abbrev [1] 0xb:0x59 DW_TAG_compile_unit
+  .long  .Linfo_string0          # DW_AT_producer
+  .short  4                      # DW_AT_language
+  .long  .Linfo_string1          # DW_AT_name
+  .long  0                       # DW_AT_stmt_list
+  .long  .Linfo_string2          # DW_AT_comp_dir
+                                 # DW_AT_GNU_pubnames
+
+  .byte  2                       # Abbrev [2] 0x1e:0x1f DW_TAG_namespace
+  .long  .Linfo_string3          # DW_AT_name
+  
+  .byte  3                       # Abbrev [3] 0x23:0x19 DW_TAG_variable
+  .long  .Linfo_string4          # DW_AT_name
+  .long  61                      # DW_AT_type
+                                 # DW_AT_external
+  .byte  1                       # DW_AT_decl_file
+  .byte  2                       # DW_AT_decl_line
+  .byte  9                       # DW_AT_location
+  .byte  3
+  .quad  _ZN1A3barE
+  .long  .Linfo_string6          # DW_AT_linkage_name
+  .byte  0                       # End Of Children Mark
+  
+  .byte  4                       # Abbrev [4] 0x3d:0x7 DW_TAG_base_type
+  .long  .Linfo_string5          # DW_AT_name
+  .byte  5                       # DW_AT_encoding
+  .byte  4                       # DW_AT_byte_size
+  
+  .byte  2                       # Abbrev [2] 0x44:0x1f DW_TAG_namespace
+  .long  .Linfo_string7          # DW_AT_name
+  
+  .byte  3                       # Abbrev [3] 0x49:0x19 DW_TAG_variable
+  .long  .Linfo_string4          # DW_AT_name
+  .long  61                      # DW_AT_type
+                                 # DW_AT_external
+  .byte  1                       # DW_AT_decl_file
+  .byte  6                       # DW_AT_decl_line
+  .byte  9                       # DW_AT_location
+  .byte  3
+  .quad  _ZN1Z3barE
+  .long  .Linfo_string8          # DW_AT_linkage_name
+  
+  .byte  0                       # End Of Children Mark
+  .byte  0                       # End Of Children Mark