Code study: division

disassemble，gdb中可以用disassemble命令反汇编。
image.png
image.pngimage.png
image.pngimage.png
image.png

Write Assembly Code: timer.s

  1. # File: timer.s
  2. # ---------------
  3. # Returns a 64-bit value for the number of milliseconds since
  4. # the computer was restarted 
  6. .section .text
  8.     .type    milliseconds, @function
  9.     .globl milliseconds 
  11. THOUSAND:
  12.         .int 1000 # 32-bit int
  14. milliseconds:
  15.         # your code here
  16.         rdtsc
  17.         # Move edx to the upper 32 bits.
  18.         shlq    $32,    %rdx
  19.         # Ensure the upper 32 bits of rax are 0s.
  20.         shlq    $32,    %rax
  21.         shrq    $32,    %rax
  22.         # Effectively concatenate
  23.         addq    %rdx,   %rax
  24.         # Make way for mulq, divq
  25.         movq    $0x0,   %rdx
  26.         # arithmetic
  27.         movq    $1000,  %rsi
  28.         mulq    %rsi
  29.         divq    %rdi
  30.         ret

Code study: loops

image.pngimage.png
image.pngimage.png
image.pngimage.png

loop unrolling

image.png
image.pngimage.png
image.png
image.png
A运行时间是版本B、C版本的两倍，为什么？？？