Hi everyone,
In this lab, I will answer some questions from the lab2. First, the lab asks me to write a basic C program which prints a message on the screen, Hello World!-style — something like this:
#include <stdio.h>
int main() {
printf("Hello World!\n");
}
This program just simply print out Hello World! to the screen, as any beginner tutorial on the internet on how to write code.
Then I compile the program using the GCC compiler that include each of these compiler options. After typing objdump, I can see the code location and output string that are set in the <main()>.
-g # enable debugging information -O0 # do not optimize (that's a capital letter and then the digit zero) -fno-builtin # do not use builtin function optimizations gcc lab2.c -g -O0 -fno-builtin -o lab2
Option 1: (Add -static)
401bb5: 55 push %rbp 401bb6: 48 89 e5 mov %rsp,%rbp 401bb9: bf 10 00 48 00 mov $0x480010,%edi 401bbe: b8 00 00 00 00 mov $0x0,%eax 401bc3: e8 f8 72 00 00 callq 408ec0 <_IO_printf> 401bc8: b8 00 00 00 00 mov $0x0,%eax 401bcd: 5d pop %rbp 401bce: c3 retq 401bcf: 90 nop
When compiling with -static option, the file size is bigger a than the original compilation without the option because it has stdio.h header file.
Option 2: (Remove -fno-builtin)
401126: 55 push %rbp 401127: 48 89 e5 mov %rsp,%rbp 40112a: bf 10 20 40 00 mov $0x402010,%edi 40112f: e8 fc fe ff ff callq 401030 <puts@plt> 401134: b8 00 00 00 00 mov $0x0,%eax 401139: 5d pop %rbp 40113a: c3 retq 40113b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
When compiling without -fno-builtin, the file size returns smaller size, the function call changes from <printf@plt> to <puts@plt>.
Option 3: (Remove -g)
When compiling without -g option, the file size becomes even smaller than the previous files and there are no mire debugger outputs. By disabling the debugging information option, the disassembly output does not include the contents of section .debug_str.
Option 4: (Add additional arguments to printf())
401126: 55 push %rbp 401127: 48 89 e5 mov %rsp,%rbp 40112a: 48 83 ec 08 sub $0x8,%rsp 40112e: 6a 0a pushq $0xa 401130: 6a 09 pushq $0x9 401132: 6a 08 pushq $0x8 401134: 6a 07 pushq $0x7 401136: 6a 06 pushq $0x6 401138: 41 b9 05 00 00 00 mov $0x5,%r9d 40113e: 41 b8 04 00 00 00 mov $0x4,%r8d 401144: b9 03 00 00 00 mov $0x3,%ecx 401149: ba 02 00 00 00 mov $0x2,%edx 40114e: be 01 00 00 00 mov $0x1,%esi 401153: bf 10 20 40 00 mov $0x402010,%edi 401158: b8 00 00 00 00 mov $0x0,%eax 40115d: e8 ce fe ff ff callq 401030 <printf@plt> 401162: 48 83 c4 30 add $0x30,%rsp 401166: b8 00 00 00 00 mov $0x0,%eax 40116b: c9 leaveq 40116c: c3 retq 40116d: 0f 1f 00 nopl (%rax)
Compilation with additional arguments in the printf did not create any changes. It remained the same as there was not any change in the compilation options.
Option 5: (Move printf() to separate function call)
40113c: 55 push %rbp 40113d: 48 89 e5 mov %rsp,%rbp 401140: b8 00 00 00 00 mov $0x0,%eax 401145: e8 dc ff ff ff callq 401126 <output> 40114a: b8 00 00 00 00 mov $0x0,%eax 40114f: 5d pop %rbp 401150: c3 retq 401151: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 401158: 00 00 00 40115b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
In the regular compilation the printf() statement and its content is displayed in the main() code. While during the changed file compilation, the main() function only displays the call to the output() function without displaying any of the content of the output() function.
Option 6: (Remove -O0 and add -O3)
401126: 55 push %rbp 401127: 48 89 e5 mov %rsp,%rbp 40112a: bf 10 20 40 00 mov $0x402010,%edi 40112f: b8 00 00 00 00 mov $0x0,%eax 401134: e8 f7 fe ff ff callq 401030 <printf@plt> 401139: b8 00 00 00 00 mov $0x0,%eax 40113e: 5d pop %rbp 40113f: c3 retq
Replace the -Oo option with the O3 caused the main() function to compile much earlier in the code, increase the optimization and performance.
