[gcc] O3比O0生成文件更大
前言
- 最近发现 gcc 在编译时添加 -g 后，启用优化 O3 生成的可执行文件大小会明显比 O0 更大
- 挺有意思的，按之前的经验，往往都是启用优化，即 release 应当比 debug 小特别多。
- 之所以这次需要在 O3 的同时添加 -g，是为了在段错误崩溃时能打印堆栈信息，如果不添加调试信息（符号表、行号等），打印出来只有一些地址。
过程
- 首先是直接用 gcc 编译生成二进制对比看看，可以发现 release 生成的更大：
- 换一个代码更简单的试试也是一样：
- 接下来我们用 readelf 看一下可执行程序的分段大小，看看到底是哪里有偏差
- 下面是一个程序的 debug（118 MB）和 release（169 MB）对比
- debug :
txt
There are 42 section headers, starting at offset 0x7598078:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .interp PROGBITS 0000000000400350 00000350
000000000000001c 0000000000000000 A 0 0 1
[ 2] .note.gnu.pr[...] NOTE 0000000000400370 00000370
0000000000000020 0000000000000000 A 0 0 8
[ 3] .note.ABI-tag NOTE 0000000000400390 00000390
0000000000000020 0000000000000000 A 0 0 4
[ 4] .hash HASH 00000000004003b0 000003b0
000000000004264c 0000000000000004 A 6 0 8
[ 5] .gnu.hash GNU_HASH 0000000000442a00 00042a00
000000000004a030 0000000000000000 A 6 0 8
[ 6] .dynsym DYNSYM 000000000048ca30 0008ca30
00000000000ce550 0000000000000018 A 7 1 8
[ 7] .dynstr STRTAB 000000000055af80 0015af80
0000000000345511 0000000000000000 A 0 0 1
[ 8] .gnu.version VERSYM 00000000008a0492 004a0492
000000000001131c 0000000000000002 A 6 0 2
[ 9] .gnu.version_r VERNEED 00000000008b17b0 004b17b0
00000000000002b0 0000000000000000 A 7 5 8
[10] .rela.dyn RELA 00000000008b1a60 004b1a60
0000000000005610 0000000000000018 A 6 0 8
[11] .rela.plt RELA 00000000008b7070 004b7070
0000000000001f08 0000000000000018 AI 6 27 8
[12] .init PROGBITS 00000000008b9000 004b9000
0000000000000020 0000000000000000 AX 0 0 4
[13] .plt PROGBITS 00000000008b9020 004b9020
00000000000014c0 0000000000000010 AX 0 0 16
[14] .plt.got PROGBITS 00000000008ba4e0 004ba4e0
0000000000000088 0000000000000008 AX 0 0 8
[15] .text PROGBITS 00000000008bb000 004bb000
0000000000728762 0000000000000000 AX 0 0 4096
[16] .fini PROGBITS 0000000000fe3764 00be3764
000000000000000d 0000000000000000 AX 0 0 4
[17] .rodata PROGBITS 0000000000fe4000 00be4000
00000000000b97dd 0000000000000000 A 0 0 64
[18] .eh_frame_hdr PROGBITS 000000000109d7e0 00c9d7e0
0000000000055664 0000000000000000 A 0 0 4
[19] .eh_frame PROGBITS 00000000010f2e48 00cf2e48
0000000000163398 0000000000000000 A 0 0 8
[20] .gcc_except_table PROGBITS 00000000012561e0 00e561e0
0000000000049fea 0000000000000000 A 0 0 4
[21] .tbss NOBITS 00000000012a1a18 00ea0a18
0000000000000081 0000000000000000 WAT 0 0 8
[22] .init_array INIT_ARRAY 00000000012a1a18 00ea0a18
0000000000000160 0000000000000008 WA 0 0 8
[23] .fini_array FINI_ARRAY 00000000012a1b78 00ea0b78
0000000000000008 0000000000000008 WA 0 0 8
[24] .data.rel.ro PROGBITS 00000000012a1b80 00ea0b80
000000000006ef80 0000000000000000 WA 0 0 32
[25] .dynamic DYNAMIC 0000000001310b00 00f0fb00
0000000000000220 0000000000000010 WA 7 0 8
[26] .got PROGBITS 0000000001310d20 00f0fd20
00000000000002c8 0000000000000008 WA 0 0 8
[27] .got.plt PROGBITS 0000000001311000 00f10000
0000000000000a70 0000000000000008 WA 0 0 8
[28] .data PROGBITS 0000000001311a80 00f10a80
000000000000bce0 0000000000000000 WA 0 0 64
[29] .bss NOBITS 000000000131d760 00f1c760
000000000001cab8 0000000000000000 WA 0 0 32
[30] .comment PROGBITS 0000000000000000 00f1c760
0000000000000012 0000000000000001 MS 0 0 1
[31] .debug_aranges PROGBITS 0000000000000000 00f1c780
00000000003d1240 0000000000000000 0 0 16
[32] .debug_info PROGBITS 0000000000000000 012ed9c0
0000000003f86e8c 0000000000000000 0 0 1
[33] .debug_abbrev PROGBITS 0000000000000000 0527484c
000000000012cda3 0000000000000000 0 0 1
[34] .debug_line PROGBITS 0000000000000000 053a15ef
0000000000dec849 0000000000000000 0 0 1
[35] .debug_str PROGBITS 0000000000000000 0618de38
0000000000a397a4 0000000000000001 MS 0 0 1
[36] .debug_line_str PROGBITS 0000000000000000 06bc75dc
000000000001452d 0000000000000001 MS 0 0 1
[37] .debug_loclists PROGBITS 0000000000000000 06bdbb09
0000000000105e7f 0000000000000000 0 0 1
[38] .debug_rnglists PROGBITS 0000000000000000 06ce1988
00000000002916f5 0000000000000000 0 0 1
[39] .symtab SYMTAB 0000000000000000 06f73080
00000000001634d0 0000000000000018 40 25425 8
[40] .strtab STRTAB 0000000000000000 070d6550
00000000004c198c 0000000000000000 0 0 1
[41] .shstrtab STRTAB 0000000000000000 07597edc
000000000000019c 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
R (retain), D (mbind), l (large), p (processor specific)- release :
txt
There are 42 section headers, starting at offset 0xa829d90:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .interp PROGBITS 0000000000400350 00000350
000000000000001c 0000000000000000 A 0 0 1
[ 2] .note.gnu.pr[...] NOTE 0000000000400370 00000370
0000000000000020 0000000000000000 A 0 0 8
[ 3] .note.ABI-tag NOTE 0000000000400390 00000390
0000000000000020 0000000000000000 A 0 0 4
[ 4] .hash HASH 00000000004003b0 000003b0
0000000000015e10 0000000000000004 A 6 0 8
[ 5] .gnu.hash GNU_HASH 00000000004161c0 000161c0
0000000000019804 0000000000000000 A 6 0 8
[ 6] .dynsym DYNSYM 000000000042f9c8 0002f9c8
0000000000053298 0000000000000018 A 7 1 8
[ 7] .dynstr STRTAB 0000000000482c60 00082c60
000000000008eea3 0000000000000000 A 0 0 1
[ 8] .gnu.version VERSYM 0000000000511b04 00111b04
0000000000006ee2 0000000000000002 A 6 0 2
[ 9] .gnu.version_r VERNEED 00000000005189e8 001189e8
00000000000002c0 0000000000000000 A 7 5 8
[10] .rela.dyn RELA 0000000000518ca8 00118ca8
0000000000005700 0000000000000018 A 6 0 8
[11] .rela.plt RELA 000000000051e3a8 0011e3a8
0000000000001da0 0000000000000018 AI 6 27 8
[12] .init PROGBITS 0000000000521000 00121000
0000000000000020 0000000000000000 AX 0 0 4
[13] .plt PROGBITS 0000000000521020 00121020
00000000000013d0 0000000000000010 AX 0 0 16
[14] .plt.got PROGBITS 00000000005223f0 001223f0
0000000000000080 0000000000000008 AX 0 0 8
[15] .text PROGBITS 0000000000523000 00123000
0000000000610622 0000000000000000 AX 0 0 4096
[16] .fini PROGBITS 0000000000b33624 00733624
000000000000000d 0000000000000000 AX 0 0 4
[17] .rodata PROGBITS 0000000000b34000 00734000
0000000000088498 0000000000000000 A 0 0 64
[18] .eh_frame_hdr PROGBITS 0000000000bbc498 007bc498
00000000000230cc 0000000000000000 A 0 0 4
[19] .eh_frame PROGBITS 0000000000bdf568 007df568
00000000000c4e28 0000000000000000 A 0 0 8
[20] .gcc_except_table PROGBITS 0000000000ca4390 008a4390
000000000003b831 0000000000000000 A 0 0 4
[21] .tbss NOBITS 0000000000ce1018 008e0018
0000000000000088 0000000000000000 WAT 0 0 8
[22] .init_array INIT_ARRAY 0000000000ce1018 008e0018
0000000000000160 0000000000000008 WA 0 0 8
[23] .fini_array FINI_ARRAY 0000000000ce1178 008e0178
0000000000000008 0000000000000008 WA 0 0 8
[24] .data.rel.ro PROGBITS 0000000000ce1180 008e0180
000000000006f700 0000000000000000 WA 0 0 32
[25] .dynamic DYNAMIC 0000000000d50880 0094f880
0000000000000220 0000000000000010 WA 7 0 8
[26] .got PROGBITS 0000000000d50aa0 0094faa0
0000000000000550 0000000000000008 WA 0 0 8
[27] .got.plt PROGBITS 0000000000d51000 00950000
00000000000009f8 0000000000000008 WA 0 0 8
[28] .data PROGBITS 0000000000d51a00 00950a00
0000000000008fb8 0000000000000000 WA 0 0 64
[29] .bss NOBITS 0000000000d5a9c0 009599b8
000000000001ca40 0000000000000000 WA 0 0 32
[30] .comment PROGBITS 0000000000000000 009599b8
0000000000000012 0000000000000001 MS 0 0 1
[31] .debug_aranges PROGBITS 0000000000000000 009599ca
000000000004c5a0 0000000000000000 0 0 1
[32] .debug_info PROGBITS 0000000000000000 009a5f6a
0000000005bcda91 0000000000000000 0 0 1
[33] .debug_abbrev PROGBITS 0000000000000000 065739fb
000000000008ad9a 0000000000000000 0 0 1
[34] .debug_line PROGBITS 0000000000000000 065fe795
0000000000fb88d8 0000000000000000 0 0 1
[35] .debug_str PROGBITS 0000000000000000 075b706d
00000000008c130a 0000000000000001 MS 0 0 1
[36] .debug_line_str PROGBITS 0000000000000000 07e78377
0000000000003c60 0000000000000001 MS 0 0 1
[37] .debug_loclists PROGBITS 0000000000000000 07e7bfd7
000000000221bd0b 0000000000000000 0 0 1
[38] .debug_rnglists PROGBITS 0000000000000000 0a097ce2
00000000005fa870 0000000000000000 0 0 1
[39] .symtab SYMTAB 0000000000000000 0a692558
000000000009d410 0000000000000018 40 12646 8
[40] .strtab STRTAB 0000000000000000 0a72f968
00000000000fa286 0000000000000000 0 0 1
[41] .shstrtab STRTAB 0000000000000000 0a829bee
000000000000019c 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
R (retain), D (mbind), l (large), p (processor specific)- 用 git 对比一下，可以发现，主要是
debug_info（+） 和debug_loclists段大幅增加了：
- 经过一顿搜索和 AI 后，基本结论是：
- 无优化时，源代码和汇编指令的对应关系比较准确，此时生成的调试信息比较简单。
- 开启优化后，源码和汇编指令的对应比较复杂，因此需要更多信息来记录这些关系。