[lld] LLD handles strings and their substrings in .rodata sections differently than the gnu ld linker.

[Zhenhang1213]

demo:

#include <stdio.h>

void func1() {
    printf("Hello, World!\n");
}

#include <stdio.h>

void func2() {
    printf("123 Hello, World!\n");
}

int main(){
        func1();
        func2();
        return 0;
}

clang demo.c -o a.out -fuse-ld=ld
clang demo.c -o a.out -fuse-ld=lld

lld：

Contents of section .rodata:
 05f8 01000200 48656c6c 6f2c2057 6f726c64  ....Hello, World
 0608 210a0031 32332048 656c6c6f 2c20576f  !..123 Hello, Wo
 0618 726c6421 0a00                        rld!..

ld:

Contents of section .rodata:
 07ec 01000200 31323320 48656c6c 6f2c2057  ....123 Hello, W
 07fc 6f726c64 210a00                      orld!..

GNU ld version is 2.41

for the codesize, lld could support this feature?

[Andarwinux]

-Wl,-O2

[Zhenhang1213]

-Wl,-O2

Thank you very much. It'll help me more. By the way, I have a more question, .strtab section has more duplicate symbols, and they are ABI symbols , like $d $x, is there any
option align to ld?
lld:

String dump of section '.strtab':
  [     1]  $x.0
  [     6]  $d.1
  [     b]  $d.2
  [    10]  $d.3
  [    15]  $d.4
  [    1a]  $d.5
  [    1f]  $d.6
  [    24]  $d.7
  [    29]  abi-note.c
  [    34]  __abi_tag
  [    3e]  $d.0
  [    43]  $d.1
  [    48]  $d.2
  [    4d]  $d.3
  [    52]  $d.4
  [    57]  $d.5
  [    5c]  $d.6
  [    61]  $d.7
  [    66]  $d.8
  [    6b]  init.c
  [    72]  $d.0
  [    77]  $d.1
  [    7c]  $d.2
  [    81]  $d.3
  [    86]  $d.4
  [    8b]  $d.5
  [    90]  $d.6
  [    95]  $d.7
  [    9a]  $d.8
  [    9f]  call_weak_fn
  [    ac]  $x.0
  [    b1]  $x.1
  [    b6]  $x.2
  [    bb]  $d.3
  [    c0]  $d.4
  [    c5]  $d.5
  [    ca]  $d.6
  [    cf]  $d.7
  [    d4]  $d.8
  [    d9]  crtbegin.c
  [    e4]  __do_init
  [    ee]  $x.0
  [    f3]  __do_init.__initialized
  [   10b]  __EH_FRAME_LIST__
  [   11d]  __do_init.__object
  [   130]  __do_fini
  [   13a]  __do_fini.__finalized
  [   150]  $d.1
  [   155]  $d.2
  [   15a]  __init
  [   161]  $d.3
  [   166]  __fini
  [   16d]  $d.4
  [   172]  $d.5
  [   177]  $d.6
  [   17c]  hell.c
  [   183]  $x.0
  [   188]  $d.1
  [   18d]  $d.2
  [   192]  $d.3
  [   197]  crtend.c
  [   1a0]  $d.0
  [   1a5]  $d.1
  [   1aa]  $x.0
  [   1af]  $x.1
  [   1b4]  $d.2
  [   1b9]  $d.3
  [   1be]  $d.4
  [   1c3]  $d.5
  [   1c8]  $d.6
  [   1cd]  $d.7
  [   1d2]  _start
  [   1d9]  main
  [   1de]  __libc_start_main

ld:

String dump of section '.strtab':
  [     1]  Scrt1.o
  [     9]  $x.0
  [     e]  $d.1
  [    13]  $d.2
  [    18]  $d.3
  [    1d]  $d.4
  [    22]  $d.5
  [    27]  $d.6
  [    2c]  $d.7
  [    31]  abi-note.c
  [    3c]  __abi_tag
  [    46]  $d.0
  [    4b]  $d.8
  [    50]  init.c
  [    57]  crti.o
  [    5e]  call_weak_fn
  [    6b]  $x.1
  [    70]  $x.2
  [    75]  crtn.o
  [    7c]  crtbegin.c
  [    87]  __do_init
  [    91]  __do_init.__initialized
  [    a9]  __EH_FRAME_LIST__
  [    bb]  __do_init.__object
  [    ce]  __do_fini
  [    d8]  __do_fini.__finalized
  [    ee]  __init
  [    f5]  __fini
  [    fc]  hell.c
  [   103]  crtend.c
  [   10c]  _DYNAMIC
  [   115]  __GNU_EH_FRAME_HDR
  [   128]  _GLOBAL_OFFSET_TABLE_
  [   13e]  $x
  [   141]  func1
  [   147]  __libc_start_main@GLIBC_2.34
  [   164]  __bss_start__
  [   172]  __cxa_finalize@GLIBC_2.17
  [   18c]  _edata
  [   193]  __bss_end__
  [   19f]  __data_start
  [   1ac]  __gmon_start__
  [   1bb]  __dso_handle
  [   1c8]  abort@GLIBC_2.17
  [   1d9]  _IO_stdin_used
  [   1e8]  __EH_FRAME_LIST_END__
  [   1fe]  func2
  [   204]  _end
  [   209]  __deregister_frame_info
  [   221]  __end__
  [   229]  __register_frame_info
  [   23f]  __bss_start
  [   24b]  main

[Andarwinux]

I don't know about arm-specific stuff, but .strtab should be able to be removed by llvm-strip

[MaskRay]

Thanks Andarwinux. Yes, lld enables tail string merge only at -O2 as it is slow.

Newer clang will emit just $a $t and omit the suffix (#99906).