## Exercises 10

In the following problems assume that we have a machine with the registers .

All the registers and the integer numbers consist of 2 bytes.

We use the temporary variables x1 … x31 to implement our intermediate code. The goal is to assign each temporary variable xi to a register and to minimize the exploited registers.

## Problem 1

The GCD of two numbers can be computed using the Euclid algorithm.

We can implement the GCD algorithm with temporary variables as the following.

x1 := a x2 := b Start: if x2 = 0 jump Done x3 := x1 % x2 x1 := x2 x2 := x3 jump Start Done: return x1

- Construct the control-flow graph
- Determine the set of live variables before and after the execution of each instruction.
- Construct the interference graph.
- Can coalescing help us to remove any move instructions?

## Problem 2

Given a set of numbers the variance is defined by the following formula.

where is the average of the numbers.

The following Scala function computes the variance of n numbers which is passed through the array a.

def variance(a: Array[Int], n: Int): Int = { var sum = 0 var squares = 0 for (i <- 0 until n) { sum += a(i) squares += a(i) * a(i) } val average: Int = sum / n val result: Int = (squares / n) - (average * average) return result }

We can sketch the translation of the variance function to instruction level with temporary variables as the following.

Instruction | Explanation |
---|---|

x1 := &a | // pointing to the beginning of the array: a(0) |

x2 := 0 | // sum |

x3 := 0 | // squares |

x4 := 0 | // loop counter: i |

x5 := n | // the length of the array: n |

L: x6 := *x1 | // beginning of the loop |

x2 := x2 + x6 | // accumulate sum |

x7 := x6 * x6 | |

x3 := x3 + x7 | // accumulate average |

x1 := x1 + 2 | // pointer to the next element |

x4 := x4 + 1 | |

if(x4 < x5) jump L | // closing of the loop |

x8 := x2 / x5 | // average |

x9 := x3 / x5 | // average of squares |

x10 := x8 * x8 | // square of averages |

x11 := x9 - x10 | // result |

return x11 |

- Construct the control-flow graph
- Determine the set of live variables before and after the execution of each instruction.
- Construct the corresponding interference graph.
- How many colors is needed to color the graph without spilling?
- Give a minimal register allocation.