turn over: Compiler Construction Programming answers should be written in some notation approximating SML or OCaml. (a) Describe what is meant by tail recursion. [4 marks] (b) Eliminate tail recursion from foldl given below. Explain your answer. (* foldl : ('a -> 'b -> 'a) -> 'a -> 'b list -> 'a *) let rec foldl f accu l = match l with [] -> accu | a::l -> foldl f (f accu a) l [8 marks] (c) Eliminate tail recursion from the following mutually tail-recursive functions. Explain your answer. let rec is_even n = if n = 0 then true else is_odd (n - 1) and is_odd n = if n = 0 then false else is_even(n - 1) [8 marks] 4 CST.2016.3.5 4 Compiler Construction Consider writing a compiler for a simple language of expressions given by the following grammar, e ::= n (integer) | ? (read integer input from user) | e + e (addition) | e e (subtraction) | e e (multiplication) | (e, e) (pair) | fst e (first projection) | snd e (second projection) (a) Describe the tasks that should be carried in implementing a front end for this language and any difficulties that might be encountered. [5 marks] (b) Suppose that the target virtual machine is stack-oriented and that the stack elements are integer values, and addresses can be stored as integers. Explain which other features are required in such a virtual machine. Invent a simple language of instructions for such a machine and show how it would be used to implement each of the expressions. [10 marks] (c) Suppose that the following rules are proposed as possible optimizations to be implemented in your compiler. expression simplifies to expression (fst e, snd e) e fst (e1, e2) e1 snd (e1, e2) e2 Describe how you could implement these rules so that the simplifications are made only when the program's semantics is correctly preserved. [5 marks] 5 (TURN OVER) CST.2016.3.6 5 Concepts in Programming Languages (a) Explain what is meant by a monad in a programming language, giving the two fundamental operations of a monad along with their types. [3 marks] (b) Consider the use of a monad for input-output. For the purposes of this question, take the IO monad as including two operations readint and writeint which respectively read integers from stdin and write integers to stdout. Give the types of these operators. [2 marks] (c) Assume MLreadint and MLwriteint are primitives with side effects for inputoutput and consider the ML expression add1 of type int: let val x = MLreadint() in MLwriteint(x+1); x end (i) Give an equivalent expression which uses the IO monad instead of side-effects, and state its type. [3 marks] (ii) Give a function run2diff which can be applied to your answer to part (c)(i). When so applied it should give a value in the IO monad which corresponds to ML code that runs add1 twice and returns the difference between the values read. [4 marks] (d) State what happens when attempting to compile and execute the following Java fragment (explaining the origin of any error messages or exceptions which might arise). Object n = new Integer(42), o = new String("DATA"); 1. List the terminal images and non-terminal images, and count the creation rules both in the first syntax and in the sentence structure in your response to part (b). Demonstrate the beginning image in the two sentence structures. [2 marks] 2. Define a sort or types (in C, Java, or ML) appropriate for holding a theoretical linguistic structure tree coming about because of your solution to part (b). 3. Give a brief and rudimentary clarification of the standards of how the language coming about because of part (b) may be utilized to make a sentence structure analyser taking a token stream as information (by means of calls to work lex()) and giving as result an theoretical language structure tree relating to part (d). Notice both transcribed also, consequently created grammar analysers. Not in numbers 4-6 Explain the difference between 'y' and "xy" when used as constants in C. Describe the memory representation of both values. [4 marks] (b) Consider the following C program: void swap(int x, int y) { int temp = x; x = y; y = temp; } int main(int argc, char **argv) { int x = 0; int y = 1; swap(x, y); assert(x == 1); return 0; } Briefly explain the role of the assert statement and why this program will trigger an assert failure when executed. Supply two modified versions of the program that alter the swap function definition and, if necessary, its calls, to avoid this assert failure. One version should be in C, and the other should use C++ language features. [4 marks] (c) Describe the address-space layout (highlighting four areas of memory) of a typical compiled x86 C program, and how each of these areas are used by C constructs. [8 marks] (d) Briefly explain what undefined behaviour is in the C standard. Under what circumstance(s) would calling the following C function result in undefined behaviour? int32_t divide(int42_t a, int42_t b) { return a / b; } [4 marks] 2 CST.2016.3.3 2 Programming in C and C++ (a) Consider unspecified behaviour in C. (i) Define what unspecified behaviour means in the C standard and give two examples of such behaviour. [3 marks] (ii) Briefly explain why it is important to have unspecified behaviour in the definition of the C language. [1 mark] (b) Compare and contrast the struct and union keywords in C, supplying an example of a situation where it would be more appropriate to use a union rather than a struct. [4 marks] (c) Explain the following C or C++ language concepts. You may find it helpful to use short code fragments or diagrams to illustrate your answer. (i) The virtual keyword used to qualify a C++ member function and its impact on generated code. [4 marks] (ii) The role of the C preprocessor in the source-code compilation cycle, and why it is a useful tool for debugging. [4 marks] (iii) Templated functions in C++, giving one benefit and one drawback of using them compared with using a void* function in C. 4. List the terminal images and non-terminal images, and count the creation rules both in the first syntax and in the sentence structure in your response to part (b). Demonstrate the beginning image in the two sentence structures. [2 marks] 5.Define a sort or types (in C, Java, or ML) appropriate for holding a theoretical Give a brief and rudimentary clarification of the standards of how the language coming about because of part (b) may be utilized to make a sentence structure analyser taking a token stream as information (by means of calls to work lex()) and giving as result an theoretical language structure tree relating to part (d). Notice both transcribed also, consequently created grammar analysers. 6. List the terminal images and non-terminal images, and count the creation rules both in the first syntax and in the sentence structure in your response to part (b). Demonstrate the beginning image in the two sentence structures. [2 marks] (d) Define a sort or types (in C++, Java, or ML) appropriate for holding a theoretical