Compilers

Question 1

What are the three stages used with compilers

Answer 1

front end
optimiser
back end

Question 2

what does the front end of compiling entail

Answer 2

converts source code into an intermediate representation

uses a lexer
uses a parser to build an abstract syntax tree
AST is an intermediate representation

Question 3

what does a parser do and how

Answer 3

builds an abstract syntax tree
uses nodes for statements and expressions
aso builds a symbol table which contains all the variable names and information about them i.e type and scope

uses heirarchy to ensure local variables are used over global provided the same name

Question 4

what does a lexer do

Answer 4

breaks input string into tokens

Question 5

what does the optimisation stage entail

Answer 5

the IR transformations

Question 6

what does the back end entail

Answer 6

converts IR into machine code
specific to architecture

register allocation
determie the location of variables in memoyr

Question 7

what is common sub expression elimination

Answer 7

a = 2b +cd
e = 2b+f+g
–>
temp = 2b
a = temp+c+d
e = temp+f+g

Question 8

what is dead code elimination

Answer 8

removes code that cannot be reached

Question 9

what is constant foldin

Answer 9

combines all the constants in an equation into one
i.e
2 pi r^2 -> 6.28.. *r^2

Question 10

what is hoisting loop invariants

Answer 10

if something is true every iteration of the loop take it outside the loop
i.e if x and y dont change

for i in range(1,10):
z +=x+y

–>
temp = x+y
for i in range(1,10):
z +=temp

Question 11

what is function inlining

Answer 11

compiler rmeoves subroutine call by inlining the function (only done if small)

Question 12

what is strength reduction

Answer 12

replaces expensive operations with cheaper operations