How do debug symbols affect performance of a Linux executable compiled by GCC?
The debug symbols are located in totally different sections from the code/data sections. You can check it with objdump:
$ objdump -h a.out
a.out: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .interp 0000001c 0000000000400200 0000000000400200 00000200 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
1 .note.ABI-tag 00000020 000000000040021c 000000000040021c 0000021c 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 .note.gnu.build-id 00000024 000000000040023c 000000000040023c 0000023c 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
3 .hash 00000018 0000000000400260 0000000000400260 00000260 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .gnu.hash 0000001c 0000000000400278 0000000000400278 00000278 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
5 .dynsym 00000048 0000000000400298 0000000000400298 00000298 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
6 .dynstr 00000038 00000000004002e0 00000000004002e0 000002e0 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
7 .gnu.version 00000006 0000000000400318 0000000000400318 00000318 2**1
CONTENTS, ALLOC, LOAD, READONLY, DATA
8 .gnu.version_r 00000020 0000000000400320 0000000000400320 00000320 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
9 .rela.dyn 00000018 0000000000400340 0000000000400340 00000340 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
10 .rela.plt 00000018 0000000000400358 0000000000400358 00000358 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
11 .init 00000018 0000000000400370 0000000000400370 00000370 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
12 .plt 00000020 0000000000400388 0000000000400388 00000388 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
13 .text 000001c8 00000000004003b0 00000000004003b0 000003b0 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
14 .fini 0000000e 0000000000400578 0000000000400578 00000578 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
15 .rodata 00000004 0000000000400588 0000000000400588 00000588 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
16 .eh_frame_hdr 00000024 000000000040058c 000000000040058c 0000058c 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
17 .eh_frame 0000007c 00000000004005b0 00000000004005b0 000005b0 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
18 .ctors 00000010 0000000000600630 0000000000600630 00000630 2**3
CONTENTS, ALLOC, LOAD, DATA
19 .dtors 00000010 0000000000600640 0000000000600640 00000640 2**3
CONTENTS, ALLOC, LOAD, DATA
20 .jcr 00000008 0000000000600650 0000000000600650 00000650 2**3
CONTENTS, ALLOC, LOAD, DATA
21 .dynamic 000001a0 0000000000600658 0000000000600658 00000658 2**3
CONTENTS, ALLOC, LOAD, DATA
22 .got 00000008 00000000006007f8 00000000006007f8 000007f8 2**3
CONTENTS, ALLOC, LOAD, DATA
23 .got.plt 00000020 0000000000600800 0000000000600800 00000800 2**3
CONTENTS, ALLOC, LOAD, DATA
24 .data 00000010 0000000000600820 0000000000600820 00000820 2**3
CONTENTS, ALLOC, LOAD, DATA
25 .bss 00000010 0000000000600830 0000000000600830 00000830 2**3
ALLOC
26 .comment 00000039 0000000000000000 0000000000000000 00000830 2**0
CONTENTS, READONLY
27 .debug_aranges 00000030 0000000000000000 0000000000000000 00000869 2**0
CONTENTS, READONLY, DEBUGGING
28 .debug_pubnames 0000001b 0000000000000000 0000000000000000 00000899 2**0
CONTENTS, READONLY, DEBUGGING
29 .debug_info 00000055 0000000000000000 0000000000000000 000008b4 2**0
CONTENTS, READONLY, DEBUGGING
30 .debug_abbrev 00000034 0000000000000000 0000000000000000 00000909 2**0
CONTENTS, READONLY, DEBUGGING
31 .debug_line 0000003b 0000000000000000 0000000000000000 0000093d 2**0
CONTENTS, READONLY, DEBUGGING
32 .debug_str 00000026 0000000000000000 0000000000000000 00000978 2**0
CONTENTS, READONLY, DEBUGGING
33 .debug_loc 0000004c 0000000000000000 0000000000000000 0000099e 2**0
CONTENTS, READONLY, DEBUGGING
You can see the extra sections (27 through 33). These sections won't be loaded at runtime, so there won't be any performance penalty. Using gdb, you can also examine them at runtime:
$ gdb ./a.out
(gdb) break main
(gdb) run
(gdb) info files
// blah blah ....
Local exec file:
`/home/kghost/a.out', file type elf64-x86-64.
Entry point: 0x4003b0
0x0000000000400200 - 0x000000000040021c is .interp
0x000000000040021c - 0x000000000040023c is .note.ABI-tag
0x000000000040023c - 0x0000000000400260 is .note.gnu.build-id
0x0000000000400260 - 0x0000000000400278 is .hash
0x0000000000400278 - 0x0000000000400294 is .gnu.hash
0x0000000000400298 - 0x00000000004002e0 is .dynsym
0x00000000004002e0 - 0x0000000000400318 is .dynstr
0x0000000000400318 - 0x000000000040031e is .gnu.version
0x0000000000400320 - 0x0000000000400340 is .gnu.version_r
0x0000000000400340 - 0x0000000000400358 is .rela.dyn
0x0000000000400358 - 0x0000000000400370 is .rela.plt
0x0000000000400370 - 0x0000000000400388 is .init
0x0000000000400388 - 0x00000000004003a8 is .plt
0x00000000004003b0 - 0x0000000000400578 is .text
0x0000000000400578 - 0x0000000000400586 is .fini
0x0000000000400588 - 0x000000000040058c is .rodata
0x000000000040058c - 0x00000000004005b0 is .eh_frame_hdr
0x00000000004005b0 - 0x000000000040062c is .eh_frame
0x0000000000600630 - 0x0000000000600640 is .ctors
0x0000000000600640 - 0x0000000000600650 is .dtors
0x0000000000600650 - 0x0000000000600658 is .jcr
0x0000000000600658 - 0x00000000006007f8 is .dynamic
0x00000000006007f8 - 0x0000000000600800 is .got
0x0000000000600800 - 0x0000000000600820 is .got.plt
0x0000000000600820 - 0x0000000000600830 is .data
0x0000000000600830 - 0x0000000000600840 is .bss
// blah blah ....
So the only penalty is that you need extra disk space to store this information. You can also use strip to remove the debug information:
$ strip a.out
Use objdump to check it again, you'll see the difference.
EDIT:
Instead of looking at sections, actually the loader loads elf file according to its Program Header, which can be seen by objdump -p (the following example uses using a different elf binary):
$ objdump -p /bin/cat
/bin/cat: file format elf64-x86-64
Program Header:
PHDR off 0x0000000000000040 vaddr 0x0000000000000040 paddr 0x0000000000000040 align 2**3
filesz 0x00000000000001f8 memsz 0x00000000000001f8 flags r-x
INTERP off 0x0000000000000238 vaddr 0x0000000000000238 paddr 0x0000000000000238 align 2**0
filesz 0x000000000000001c memsz 0x000000000000001c flags r--
LOAD off 0x0000000000000000 vaddr 0x0000000000000000 paddr 0x0000000000000000 align 2**21
filesz 0x00000000000078bc memsz 0x00000000000078bc flags r-x
LOAD off 0x0000000000007c28 vaddr 0x0000000000207c28 paddr 0x0000000000207c28 align 2**21
filesz 0x0000000000000678 memsz 0x0000000000000818 flags rw-
DYNAMIC off 0x0000000000007dd8 vaddr 0x0000000000207dd8 paddr 0x0000000000207dd8 align 2**3
filesz 0x00000000000001e0 memsz 0x00000000000001e0 flags rw-
NOTE off 0x0000000000000254 vaddr 0x0000000000000254 paddr 0x0000000000000254 align 2**2
filesz 0x0000000000000044 memsz 0x0000000000000044 flags r--
EH_FRAME off 0x0000000000006980 vaddr 0x0000000000006980 paddr 0x0000000000006980 align 2**2
filesz 0x0000000000000274 memsz 0x0000000000000274 flags r--
STACK off 0x0000000000000000 vaddr 0x0000000000000000 paddr 0x0000000000000000 align 2**4
filesz 0x0000000000000000 memsz 0x0000000000000000 flags rw-
RELRO off 0x0000000000007c28 vaddr 0x0000000000207c28 paddr 0x0000000000207c28 align 2**0
filesz 0x00000000000003d8 memsz 0x00000000000003d8 flags r--
The program headers tell which segment will be loaded with what rwx flags; multiple sections with the same flags will be merged to a single segment.
BTW:
The loader doesn't care about sections when loading elf file, but it will look at several symbol-related sections to resolve symbols when needed.