Question

For the MIPS assembly instructions in Exercise 2.4, rewrite the assembly code to minimize the number if MIPS instructions (if possible) needed to carry out the same function.

Short answer

It is possible to lower the number of MIPS instructions by one. After reduction, the total number of MIPS instructions is 8.

Step-by-step solution

Determine the MIPS instructions

The instruction addi $t2, $t0, 4 can be removed from the sequence of instructions to be examined. This is due to the fact that it adds 4 to the address of $t0 and puts the result in $t2. The value of $t2 is obtained and stored in the register $t0.

The value of $t0 at memory position 4 is immediately retrieved and loaded into the register $t0 during the minimizing of operations.

Consider the following MIPS instructions:

$$\begin{gathered} sll\$t0,\$s0,2\#\$t0=f*4 \\ add\$t0,\$s6,\$t0\#\$t0=\&A\left[f\right] \\ sll\$t1,\$s1,2\#\$t1=g*4 \\ add\$t1,\$s7,\$t1\#\$t1=\&B\left[g\right] \\ lw\$s0,0\left(\$t0\right)\#f=A\left[f\right] \\ addi\$t2,\$t0,4\#\$t2=\&A\left[f+1\right] \\ lw\$t0,0\left(\$t2\right)\#\$t0=A\left[f+1\right] \\ add\$t0,\$t0,\$s0\#\$t0=A\left[f+1\right]+A\left[f\right] \\ sw\$t0,0\left(\$t1\right)\#B\left[g\right]=\$t0 \end{gathered}$$

Minimizing the number of instructions:

It is possible to lower the number of MIPS instructions by one. After reduction, the total number of MIPS instructions is 8.

$$\begin{gathered} sll\$t0,\$s0,2\#\$t0=f*4 \\ add\$t0,\$s6,\$t0\#\$t0=\&A\left[f\right] \\ sll\$t1,\$s1,2\#\$t1=g*4 \\ add\$t1,\$s7,\$t1\#\$t1=\&B\left[g\right] \\ lw\$s0,0\left(\$t0\right)\#f=A\left[f\right] \\ lw\$t0,4\left(\$t0\right)\#\$t0=A\left[f+1\right] \\ add\$t0,\$t0,\$s0\#\$t0=A\left[f+1\right]+A\left[f\right] \\ sw\$t0,0\left(\$t1\right)\#B\left[g\right]=\$t0 \end{gathered}$$