Compiler Construction Sohail Aslam Lecture 40 Boolean Expressions

Compiler Construction Sohail Aslam Lecture 40

Boolean Expressions E → E 1 and M E 2 { backpatch(E 1. truelist, M. quad); E. truelist = E 2. truelist; E. falselist = merge(E 1. falselist, E 2. falselist); } So the target for E 1. truelist must be the beginning of code generated for E 2 2

Boolean Expressions E → E 1 and M E 2 { backpatch(E 1. truelist, M. quad); E. truelist = E 2. truelist; E. falselist = merge(E 1. falselist, E 2. falselist); } This target is obtained using the marker nonterminal M. 3

Boolean Expressions E → E 1 and M E 2 { backpatch(E 1. truelist, M. quad); E. truelist = E 2. truelist; E. falselist = merge(E 1. falselist, E 2. falselist); } M. quad records the number of the first statement of E 2. code. 4

Boolean Expressions: OR E → E 1 or M E 2 { } backpatch(E 1. falselist, M. quad); E. truelist = merge(E 1. truelist, E 2. truelist); E. falselist = E 2. falselist; If E 1 is false, need to test E 2 5

Boolean Experssions E → not E 1 { } E. truelist = E 1. falselist; E. falselist = E 1. truelist; 6

Boolean Experssions E → ( E 1 ) { E. truelist = E 1. truelist; E. falselist = E 1. falselist; } 7

Boolean Experssions E → id 1 relop id 2 { E. truelist = makelist(nextquad()); E. falselist = makelist(nextquad()+1); emit(‘if’ id 1 relop id 2 ‘goto _’) ; emit(‘goto _’ ); } 8

$Boolean Experssions E → true { E. truelist = makelist(nextquad()); emit(‘goto _’ ); }$

Boolean Experssions E → true { E. truelist = makelist(nextquad()); emit(‘goto _’ ); } 9

$Boolean Experssions E → false { E. falselist = makelist(nextquad()); emit(‘goto _’ ); }$

Boolean Experssions E → false { E. falselist = makelist(nextquad()); emit(‘goto _’ ); } 10

Boolean Expressions M→e { M. quad = nextquad(); } 11

Backpatching § consider again, the boolean expression a < b or c < d and e < f § We carry out a bottom-up parse 12

Backpatching § In response to reduction of a < b to E, the two quadruples 100: if a < b goto _ 101: goto _ are generated 13

Recall E → id 1 relop id 2 { E. truelist = makelist(nextquad()); E. falselist = makelist(nextquad()+1); emit(‘if’ id 1 relop id 2 ‘goto _’) ; emit(‘goto _’ ); } View this in a parse tree 14

E. t = {100} E. f = {101} a < b or c < d and e < f 15

Backpatching § The marker non-terminal M in the production E → E 1 or M E 2 records the value of nextquad which at this time is 102. 16

M. q = 102 e E. t = {100} E. f = {101} a < b or c < d and e < f 17

Backpatching § The reduction of c < d to E, the two quadruples 102: if c < d goto _ 103: goto _ are generated 18

M. q = 102 e E. t = {100} E. f = {101} a < b E. t = {102} E. f = {103} or c < d and e < f 19

Backpatching § The marker non-terminal M in the production E → E 1 and M E 2 records the value of nextquad which at this time is 104. 20

M. q = 102 e E. t = {100} E. f = {101} a < b M. q = 104 e E. t = {102} E. f = {103} or c < d and e < f 21

Backpatching § Reducing e < f to E, generates 104: if e < f goto _ 105: goto _ 22

M. q = 102 e E. t = {100} E. f = {101} a < b M. q = 104 e E. t = {102} E. f = {103} or c < d and E. t = {104} E. f = {105} e < f 23

Backpatching § We now reduce by the production E → E 1 and M E 2 24

M. q = 102 e E. t = {100} E. f = {101} a < E. t = {104} E. f = {103, 105} b M. q = 104 e E. t = {102} E. f = {103} or c < d and E. t = {104} E. f = {105} e < f 25

Semantic Actions E → E 1 and M E 2 { backpatch(E 1. truelist, M. quad); E. truelist = E 2. truelist; E. falselist = merge(E 1. falselist, E 2. falselist); } 26

Backpatching The semantic action calls backpatch({102}, 104) where {102} denotes E 1. truelist containing only 102 27

Backpatching The six statements generated so far are thus 100: 101: 102: 103: 104: 105: if a goto if c goto if e goto < b goto _ _ < d goto _ _ < f goto _ _ 28

Backpatching The semantic action calls backpatch({102}, 104) which fills in 104 in statement 102. 29

Backpatching The call fills in 104 in statement 102 100: 101: 102: 103: 104: 105: if a goto if c goto if e goto < b goto _ _ < d goto 104 _ < f goto _ _ 30

Semantic Actions E → E 1 and M E 2 { backpatch(E 1. truelist, M. quad); E. truelist = E 2. truelist; E. falselist = merge(E 1. falselist, E 2. falselist); } 31

M. q = 102 e E. t = {100} E. f = {101} a < b and M. q = 104 e E. t = {102} 103 E. f = {103} or E. t = {104} E. f = {103, 105} c < d E. t = {104} 104 105 E. f = {105} e < f 32

Backpatching § We now reduce by the production E → E 1 or M E 2 33

E. t = {100, 104} E. f = {103, 105} M. q = 102 e E. t = {100} E. f = {101} a < b and M. q = 104 e E. t = {102} E. f = {103} or E. t = {104} E. f = {103, 105} c < d E. t = {104} E. f = {105} e < f 34

Semantic Actions E → E 1 or M E 2 { } backpatch(E 1. falselist, M. quad); E. truelist = merge(E 1. truelist, E 2. truelist); E. falselist = E 2. falselist; 35

Backpatching The semantic action calls backpatch({101}, 102) which fills in 102 in statement 101. 36

Backpatching 100: 101: 102: 104 103: 104: 105: if a < b goto _ 102 goto if c < d goto _ if e < f goto _ 37

Backpatching 100: 101: 102: 103: 104: 105: if a goto if c goto if e goto < b goto _ 102 < d goto 104 _ < f goto _ _ These instructions will have their targets filled later in the compilation 38

Semantic Actions E → E 1 or M E 2 { } backpatch(E 1. falselist, M. quad); E. truelist = merge(E 1. truelist, E 2. truelist); E. falselist = E 2. falselist; 39

E. t = {100, 104} E. f = {103, 105} or M. q = 102 e E. t = {100} E. f = {101} a < b E. t = {104} E. f = {103, 105} and M. q = 104 e E. t = {102} E. f = {103} c < d E. t = {104} E. f = {105} e < f 40

Flow-of-Control Statements § We now use backpatching to translate flow-of-control statements in one pass § We will use the same listhandling procedures as before 41

Flow-of-Control Statements § We now use backpatching to translate flow-of-control statements in one pass § We will use the same listhandling procedures as before 42