Assembly Reading Comprehension

!rm -Rf tmp
!mkdir -p tmp

Here is a simple snippet of C:

%%writefile tmp/asm-reading-comprehension.c

int do_stuff()
{
  int sum = 0;
  for (int i = 1; i <= 100; ++i)
    sum += i*i;
  return sum;
}

Overwriting tmp/asm-reading-comprehension.c

Compile the file:

!cd tmp; gcc -c -O0 asm-reading-comprehension.c

Now disassemble:

!objdump --disassemble tmp/asm-reading-comprehension.o

tmp/asm-reading-comprehension.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <do_stuff>:
   0:	55                   	push   %rbp
   1:	48 89 e5             	mov    %rsp,%rbp
   4:	c7 45 fc 00 00 00 00 	movl   $0x0,-0x4(%rbp)
   b:	c7 45 f8 01 00 00 00 	movl   $0x1,-0x8(%rbp)
  12:	eb 0e                	jmp    22 <do_stuff+0x22>
  14:	8b 45 f8             	mov    -0x8(%rbp),%eax
  17:	0f af 45 f8          	imul   -0x8(%rbp),%eax
  1b:	01 45 fc             	add    %eax,-0x4(%rbp)
  1e:	83 45 f8 01          	addl   $0x1,-0x8(%rbp)
  22:	83 7d f8 64          	cmpl   $0x64,-0x8(%rbp)
  26:	7e ec                	jle    14 <do_stuff+0x14>
  28:	8b 45 fc             	mov    -0x4(%rbp),%eax
  2b:	5d                   	pop    %rbp
  2c:	c3                   	retq   

How does optimization change the outcome?

!cd tmp; gcc -c -O asm-reading-comprehension.c
!objdump --disassemble tmp/asm-reading-comprehension.o

tmp/asm-reading-comprehension.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <do_stuff>:
   0:	b8 64 00 00 00       	mov    $0x64,%eax
   5:	83 e8 01             	sub    $0x1,%eax
   8:	75 fb                	jne    5 <do_stuff+0x5>
   a:	b8 ae 29 05 00       	mov    $0x529ae,%eax
   f:	c3                   	retq   

... even more optimization?

!cd tmp; gcc -c -O3 asm-reading-comprehension.c
!objdump --disassemble tmp/asm-reading-comprehension.o

tmp/asm-reading-comprehension.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <do_stuff>:
   0:	b8 ae 29 05 00       	mov    $0x529ae,%eax
   5:	c3                   	retq