SI 413 Fall 2023 / Homeworks

This is the archived website of SI 413 from the Fall 2023 semester. Feel free to browse around; you may also find more recent offerings at my teaching page.

Homework 10: LLVM Optimization


Consider the following function compiled into LLVM IR (or download [foo.ll])

target triple = "x86_64-pc-linux-gnu"

define i64 @foo(i32 %x) {
  %x.addr = alloca i32
  %res = alloca i64
  store i32 %x, ptr %x.addr
  store i64 0, ptr %res
  br label %while.cond

  %0 = load i32, ptr %x.addr
  %cmp = icmp sgt i32 %0, 0
  br i1 %cmp, label %while.body, label %while.end

  %1 = load i64, ptr %res
  %mul = mul i64 %1, 10
  %add = add i64 %mul, 3
  store i64 %add, ptr %res
  %2 = load i32, ptr %x.addr
  %dec = add i32 %2, -1
  store i32 %dec, ptr %x.addr
  br label %while.cond

  %3 = load i64, ptr %res
  ret i64 %3
  1. Describe (in a sentence or two) what this function does.
  1. Rewrite the foo function from the previous page so that it does not use any memory allocation, load, or store operations.

    (Hint: you have to use phi instructions.)

2 Goto harmful

Write a C program with ``spaghetti code’’ that fits the following requirements:

  • It uses goto statements and labels
  • It is relatively short (say, 30ish lines of code)
  • It does something mildly useful or interesting
  • It is hard to understand because of the goto statements
  1. Copy your C spaghetti code program below.
  1. Say what it is that your program actually does.

    (Make sure part (b) is on a different page or side than part (a).)