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.
