Chapter 12.5, Problem 12.7PP

Explanation of Solution

Instruction ordering of “badcnt.c”:

• When performing instruction “${\text{H}}_{\text{1}}$”, the value of “${\text{\%rdx}}_{\text{1}}$” and “${\text{\%rdx}}_{\text{2}}$” is null and “cnt” value is “0”.
• When performing instruction “${\text{L}}_{\text{1}}$”:
  • The instruction “${\text{L}}_{\text{1}}$” means loads the shared variable “cnt” into register “${\text{\%rdx}}_{\text{1}}$”.
    • Hence, the value of “${\text{\%rdx}}_{\text{1}}$” contains the value of “cnt” variable.
    • Therefore, ${\text{\%rdx}}_{\text{1}}\text{=}\text{0}$
• When performing instruction “${\text{H}}_{\text{1}}$”, the value of “${\text{\%rdx}}_{\text{1}}$” and “${\text{\%rdx}}_{\text{2}}$” is null and “cnt” value is “0”.
• When performing instruction “${\text{L}}_{\text{2}}$”:
  • The instruction “${\text{L}}_{\text{2}}$” means loads the shared variable “cnt” into register “${\text{\%rdx}}_{\text{2}}$”.
    • Hence, the value of “${\text{\%rdx}}_{\text{2}}$” contains the value of “cnt” variable.
    • Therefore, ${\text{\%rdx}}_{\text{2}}\text{=}\text{0}$
• When performing instruction “${\text{U}}_{\text{2}}$”:
  • The instruction “${\text{U}}_{\text{2}}$” means update value in register “${\text{\%rdx}}_{\text{2}}$”.
    • Update value means increment the value of “${\text{\%rdx}}_{\text{2}}$” by “1”.

      $\begin{array}{c}{\text{\%rdx}}_{\text{2}}\text{=}{\text{\%rdx}}_{\text{2}}\text{+}\text{1}\\ \text{=}\text{0}\text{+}\text{1}\\ {\text{\%rdx}}_{\text{2}}\text{=}\text{1}\end{array}$

    • Therefore, ${\text{\%rdx}}_{\text{2}}\text{=}\text{1}$ and the value of “cnt” is “0”. Here, the “cnt” will be not be update. It only update after the instruction stored.
• When performing instruction “${\text{S}}_{\text{2}}$”:
  • The instruction “${\text{S}}_{\text{2}}$” means store the updated value of register “${\text{\%rdx}}_{\text{2}}$” into “cnt”.
    • Hence, ${\text{\%rdx}}_{\text{2}}\text{=}\text{1}$ and “cnt” value is “1”.
• When performing instruction “${\text{U}}_{\text{1}}$”:
  • The instruction “${\text{U}}_{\text{1}}$” means update value in register “${\text{\%rdx}}_{\text{1}}$”.
    • Update value means increment the value of “${\text{\%rdx}}_{\text{1}}$” by “1”