所有其他因素相同（例如优化级别），ELF或SO中的调试符号如何影响：
可以采取哪些措施来减轻任何负面影响？
EDIT 我已经看到了这个问题，但我发现讨论无益，因为代码优化因素已经混淆了那里的问题。 Why does my code run slower with multiple threads than with a single thread when it is compiled for profiling (-pg)?
答案 0 :(得分：33)
调试符号位于与代码/数据部分完全不同的部分。您可以使用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
您可以看到额外的部分（27到33）。这些部分不会在运行时加载，因此不会有任何性能损失。使用gdb，您还可以在运行时检查它们
$ 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 ....
因此唯一的缺点是您需要额外的磁盘空间来存储此信息。您还可以使用strip删除调试信息：
$ strip a.out
使用objdump再次检查，您会看到差异。
编辑：
相反查看部分，实际上加载程序根据Program Header加载elf文件，objdump -p可以看到它。 （以下示例使用不同的elf二进制文件）
$ 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--
程序头告诉哪些段将加载哪些rwx标志，具有相同标志的多个部分将合并为单个段。
顺便说一句：
加载elf文件时，加载程序不关心部分，但它会在需要时查看几个与符号相关的部分来解析符号。
答案 1 :(得分：-5)
您可能需要查看Why does my code run slower with multiple threads than with a single thread when it is compiled for profiling (-pg)?以快速解释调试符号如何影响优化。
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