gcc生成不必要的(?)指令 [英] gcc generates unnecessary (?) instructions
问题描述
我决定编译一个非常基本的C程序，并使用objdump -d查看生成的代码.
I decided to compile a very basic C program and take a look at the generated code with objdump -d.
int main(int argc, char *argv[]) {
exit(0);
}
用gcc test.c -s -o test.o编译后，然后用objdump -d拆卸后，我的文本段看起来像这样:
After compiling it with gcc test.c -s -o test.o and then disassembling with objdump -d my text segment looked like this:
Disassembly of section .text:
0000000000001050 <.text>:
1050: 31 ed xor %ebp,%ebp
1052: 49 89 d1 mov %rdx,%r9
1055: 5e pop %rsi
1056: 48 89 e2 mov %rsp,%rdx
1059: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp
105d: 50 push %rax
105e: 54 push %rsp
105f: 4c 8d 05 4a 01 00 00 lea 0x14a(%rip),%r8 # 11b0 <__cxa_finalize@plt+0x170>
1066: 48 8d 0d e3 00 00 00 lea 0xe3(%rip),%rcx # 1150 <__cxa_finalize@plt+0x110>
106d: 48 8d 3d c1 00 00 00 lea 0xc1(%rip),%rdi # 1135 <__cxa_finalize@plt+0xf5>
1074: ff 15 66 2f 00 00 callq *0x2f66(%rip) # 3fe0 <__cxa_finalize@plt+0x2fa0>
107a: f4 hlt
107b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
1080: 48 8d 3d a9 2f 00 00 lea 0x2fa9(%rip),%rdi # 4030 <__cxa_finalize@plt+0x2ff0>
1087: 48 8d 05 a2 2f 00 00 lea 0x2fa2(%rip),%rax # 4030 <__cxa_finalize@plt+0x2ff0>
108e: 48 39 f8 cmp %rdi,%rax
1091: 74 15 je 10a8 <__cxa_finalize@plt+0x68>
1093: 48 8b 05 3e 2f 00 00 mov 0x2f3e(%rip),%rax # 3fd8 <__cxa_finalize@plt+0x2f98>
109a: 48 85 c0 test %rax,%rax
109d: 74 09 je 10a8 <__cxa_finalize@plt+0x68>
109f: ff e0 jmpq *%rax
10a1: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
10a8: c3 retq
10a9: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
10b0: 48 8d 3d 79 2f 00 00 lea 0x2f79(%rip),%rdi # 4030 <__cxa_finalize@plt+0x2ff0>
10b7: 48 8d 35 72 2f 00 00 lea 0x2f72(%rip),%rsi # 4030 <__cxa_finalize@plt+0x2ff0>
10be: 48 29 fe sub %rdi,%rsi
10c1: 48 c1 fe 03 sar $0x3,%rsi
10c5: 48 89 f0 mov %rsi,%rax
10c8: 48 c1 e8 3f shr $0x3f,%rax
10cc: 48 01 c6 add %rax,%rsi
10cf: 48 d1 fe sar %rsi
10d2: 74 14 je 10e8 <__cxa_finalize@plt+0xa8>
10d4: 48 8b 05 15 2f 00 00 mov 0x2f15(%rip),%rax # 3ff0 <__cxa_finalize@plt+0x2fb0>
10db: 48 85 c0 test %rax,%rax
10de: 74 08 je 10e8 <__cxa_finalize@plt+0xa8>
10e0: ff e0 jmpq *%rax
10e2: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
10e8: c3 retq
10e9: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
10f0: 80 3d 39 2f 00 00 00 cmpb $0x0,0x2f39(%rip) # 4030 <__cxa_finalize@plt+0x2ff0>
10f7: 75 2f jne 1128 <__cxa_finalize@plt+0xe8>
10f9: 55 push %rbp
10fa: 48 83 3d f6 2e 00 00 cmpq $0x0,0x2ef6(%rip) # 3ff8 <__cxa_finalize@plt+0x2fb8>
1101: 00
1102: 48 89 e5 mov %rsp,%rbp
1105: 74 0c je 1113 <__cxa_finalize@plt+0xd3>
1107: 48 8b 3d 1a 2f 00 00 mov 0x2f1a(%rip),%rdi # 4028 <__cxa_finalize@plt+0x2fe8>
110e: e8 2d ff ff ff callq 1040 <__cxa_finalize@plt>
1113: e8 68 ff ff ff callq 1080 <__cxa_finalize@plt+0x40>
1118: c6 05 11 2f 00 00 01 movb $0x1,0x2f11(%rip) # 4030 <__cxa_finalize@plt+0x2ff0>
111f: 5d pop %rbp
1120: c3 retq
1121: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
1128: c3 retq
1129: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
1130: e9 7b ff ff ff jmpq 10b0 <__cxa_finalize@plt+0x70>
1135: 55 push %rbp
1136: 48 89 e5 mov %rsp,%rbp
1139: 48 83 ec 10 sub $0x10,%rsp
113d: 89 7d fc mov %edi,-0x4(%rbp)
1140: 48 89 75 f0 mov %rsi,-0x10(%rbp)
1144: bf 00 00 00 00 mov $0x0,%edi
1149: e8 e2 fe ff ff callq 1030 <exit@plt>
114e: 66 90 xchg %ax,%ax
1150: 41 57 push %r15
1152: 4c 8d 3d 8f 2c 00 00 lea 0x2c8f(%rip),%r15 # 3de8 <__cxa_finalize@plt+0x2da8>
1159: 41 56 push %r14
115b: 49 89 d6 mov %rdx,%r14
115e: 41 55 push %r13
1160: 49 89 f5 mov %rsi,%r13
1163: 41 54 push %r12
1165: 41 89 fc mov %edi,%r12d
1168: 55 push %rbp
1169: 48 8d 2d 80 2c 00 00 lea 0x2c80(%rip),%rbp # 3df0 <__cxa_finalize@plt+0x2db0>
1170: 53 push %rbx
1171: 4c 29 fd sub %r15,%rbp
1174: 48 83 ec 08 sub $0x8,%rsp
1178: e8 83 fe ff ff callq 1000 <exit@plt-0x30>
117d: 48 c1 fd 03 sar $0x3,%rbp
1181: 74 1b je 119e <__cxa_finalize@plt+0x15e>
1183: 31 db xor %ebx,%ebx
1185: 0f 1f 00 nopl (%rax)
1188: 4c 89 f2 mov %r14,%rdx
118b: 4c 89 ee mov %r13,%rsi
118e: 44 89 e7 mov %r12d,%edi
1191: 41 ff 14 df callq *(%r15,%rbx,8)
1195: 48 83 c3 01 add $0x1,%rbx
1199: 48 39 dd cmp %rbx,%rbp
119c: 75 ea jne 1188 <__cxa_finalize@plt+0x148>
119e: 48 83 c4 08 add $0x8,%rsp
11a2: 5b pop %rbx
11a3: 5d pop %rbp
11a4: 41 5c pop %r12
11a6: 41 5d pop %r13
11a8: 41 5e pop %r14
11aa: 41 5f pop %r15
11ac: c3 retq
11ad: 0f 1f 00 nopl (%rax)
11b0: c3 retq
如您所见，我实际写的那部分只占很小的空间. 在组装中使用相同的程序(如果忽略了 main 函数在C语言中也被视为函数)的事实:
As you can see, the part that was actually written by me occupies very little space. The same program (if we ignore the fact that the main function is also treated as a function in C) in Assembly:
.global _start
.text
_start: mov $60, %rax
xor %rdi, %rdi
syscall
通过gcc -c demo.s && ld demo.o -o demo && objdump -d demo组装，链接和拆卸:
Assembled, linked and disassembled with gcc -c demo.s && ld demo.o -o demo && objdump -d demo:
Disassembly of section .text:
0000000000401000 <_start>:
401000: 48 c7 c0 3c 00 00 00 mov $0x3c,%rax
401007: 48 31 ff xor %rdi,%rdi
40100a: 0f 05 syscall
问题是:所有这些指令的作用是什么，没有这些指令，有没有办法生成代码?
The question is: what purpose do all these instructions serve and is there a way to generate code without them?
当我写这个问题时，我注意到C程序从链接库中调用exit()，而在汇编语言中，我直接使用syscall来执行它.我认为在这种情况下，这并不重要.
While I was writing the question I noticed that the C program calls exit() from the linked library whereas in Assembly I do it directly with a syscall. I don't think it is important in this case though.
推荐答案
gcc生成不必要的(?)指令
gcc generates unnecessary (?) instructions
Yes, because you invoked GCC without asking for any compiler optimizations.
我的建议:使用
gcc -fverbose-asm -O2 -S test.c
然后查看生成的test.s汇编器代码.
then look inside the generated test.s assembler code.
顺便说一句，大部分代码来自 crt0 ，该代码由给出，未发出由gcc.用gcc -O2 -v test.c -o testprog构建可执行文件以了解GCC的实际功能.阅读 GCC内部文档.
BTW, most of the code is from crt0, which is given by, not emitted by, gcc. Build your executable with gcc -O2 -v test.c -o testprog to understand what GCC really does. Read documentation of GCC internals.
Since GCC is free software, you are allowed to look inside its source code and improve it. But the crt0 stuff is tricky, and operating system specific.
请考虑阅读有关链接器和加载程序的信息，有关可执行文件和 如何编写共享库 ，和 Linux汇编程序如何.
Consider also reading about linkers and loaders, about ELF executables, and How to write shared libraries, and the Linux Assembler HowTo.
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