Compiler Construction Sohail Aslam Lecture 41 FlowofControl Statements

Compiler Construction Sohail Aslam Lecture 41

Flow-of-Control Statements S → if E then S | if E then S else S | while E do S | begin L end | A S denotes a statement L→ L; S | S L is a statementlist A is assignment 2

Semantic Actions S → if E then M 1 S 1 N else M 2 S 2 { } backpatch(E. truelist, M 1. quad); backpatch(E. falselist, M 2. quad); S. nextlist = merge(S 1. nextlist, merge( N. nextlist, S 2. nextlist)); If E true, jump to M 1. quad which is N is is marker non-terminal to introduce start of code for S 1 S 2 jump over code 3

Semantic Actions N→e { N. nextlist = makelist(next. Quad()); emit(‘goto_’); } The attribute N. nextlist records the quad number of the goto it generates 4

Semantic Actions M→e { M. quad = next. Quad(); } 5

Semantic Actions S → if E then M S 1 { } backpatch(E. truelist, M. quad); S. nextlist = merge( E. falselist, S 1. nextlist); If E is true, jump to M. quad which is start of code for S 1 6

Semantic Actions S → while M 1 E do M 2 S 1 { } backpatch(S 1. nextlist, M 1. quad); backpatch(E. truelist, M 2. quad); S. nextlist = E. falselist; emit( ‘goto’ M 1. quad); 7

Semantic Actions S → begin L end { S. nextlist = L. nextlist; } 8

Semantic Actions S→A { S. nextlist = nil; } initializes S. nextlist to an empty list 9

Semantic Actions L → L 1 ; M S { backpatch(L 1. nextlist, M. quad); L. nextlist = S. nextlist; } Statement following L 1 in order of execution is beginning of S 10

Semantic Actions L→S { L. nextlist = S. nextlist; } 11

Example if a<b or c<d and e<f then x=y+z else x=y-z if E 1 or M E 2 then x=y+z else x=y-z 12

if E 1 or M E 2 then x=y+z else x=y-z if E then x=y+z else x=y-z { E. truelist=[100, 104] E. falselist=[103, 105] } 13

100 101 102 103 104 105 106 107 108 109 if a < b goto _ goto 102 if c < d goto 104 goto _ if e < f goto _ goto_ 14

if E then M 1 x=y+z else x=y-z M 1 → { M 1. quad = 106 } if E then M 1 A else x=y-z { emit(‘x=y+z’) } if E then M 1 S 1 else A { S 1. nextlist = nil} if E then M 1 S 1 N else x=y-z { N. nextlist = [107] emit(‘goto _’ } A → x=y+z S 1 → A N→ 15

100 101 102 103 104 105 106 107 108 109 if a < b goto _ goto 102 if c < d goto 104 goto _ if e < f goto _ x=y+z goto _ 16

if E then M 1 S 1 N else M 2 x=y-z { M 2. quad = 108 } M 2 → if E then M 1 S 1 N else M 2 A { emit(‘x=y-z’) } if E then M 1 S 1 N else M 2 S 2 { S 2. nextlist = nil } A → x=y S 2 → A S { backpatch([100, 104], 106) backpatch([103, 105], 108) S. nextlist=[107]} 17

100 101 102 103 104 105 106 107 108 109 if a < b goto 106 _ goto 102 if c < d goto 104 goto 108 _ if e < f goto 106 _ goto 108 _ x=y+z goto _ x=y-z 18

100 101 102 103 104 105 106 107 108 109 if a < b goto 106 goto 102 if c < d goto 104 goto 108 if e < f goto 106 goto 108 x=y+z goto _ x=y-z 19

Semantic Actions in YACC The syntax-directed translation statements can be conveniently specified in YACC The %union will require more fields because the attributes vary 20

Semantic Actions in YACC The actual mechanics will be covered in the handout for the syntax-directed translation phase of the course project 21