Lecture 15 Control Flow

Lecture 15 Control Flow

Topics Topics Review Positional Encoding of Booleans Short Circuit Evaluation Control Flow Statements

Readings: 8.4, 8.6Readings: 8.4, 8.6

March 13, 2006

CSCE 531 Compiler Construction

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OverviewOverviewLast TimeLast Time

Evaluations of Expressions Numeric Implementation of Booleans Positional Encoding of Booleans Short Circuit Evaluation If-then-else semantic actions almost

Today’s Lecture Today’s Lecture Review

Booleans, quadruples

Control Flow using numeric implementation of Booleans Control Flow using

References: Sections 8.4, 8.5References: Sections 8.4, 8.5

Homework:Homework:

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Rest of the Semester PlanRest of the Semester Plan

Project 3 due Sunday March 19Project 3 due Sunday March 19

Project 4 – Booleans and Control Flow due Sunday Project 4 – Booleans and Control Flow due Sunday March 26March 26

Test 2 – April 5Test 2 – April 5

Project 5 – Function Calls Project 5 – Function Calls

Project 6 – Nested ScopesProject 6 – Nested Scopes

Exam April 27Exam April 27

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Recall Numeric Rep. of BooleansRecall Numeric Rep. of Booleans

E E id id11 relop id relop id22

{ E.place = newtemp();{ E.place = newtemp();

emit(if idemit(if id11.place relop.op id.place relop.op id22.place goto nextquad+3).place goto nextquad+3)

emit(E.place ‘:=‘ ‘0’);emit(E.place ‘:=‘ ‘0’);

emit(goto nextquad +2);emit(goto nextquad +2);

emit(E.place ‘:=‘ ‘1’);emit(E.place ‘:=‘ ‘1’);

}}

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AND, OR in numeric implementationAND, OR in numeric implementation

E E E E11 AND E AND E22 {{

E.place = newtemp();E.place = newtemp();

emit(E.place ‘:=’ Eemit(E.place ‘:=’ E11.place ‘and’ E.place ‘and’ E22.place);.place);

}}

E E E E11 OR E OR E22 {{

E.place = newtemp();E.place = newtemp();

emit(E.place ‘:=’ Eemit(E.place ‘:=’ E11.place ‘or’ E.place ‘or’ E22.place);.place);

}}

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Booleans Numeric Impl. – Finish UpBooleans Numeric Impl. – Finish Up

EE true true { E.place = newtemp();{ E.place = newtemp();

emit(E.place ‘:=’ ‘1’); }emit(E.place ‘:=’ ‘1’); }

EE false false { E.place = newtemp();{ E.place = newtemp();

emit(E.place ‘:=’ ‘0’); }emit(E.place ‘:=’ ‘0’); }

E E ( E ( E11 ) ) { E.place = E{ E.place = E11.place;}.place;}

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Positional Implmentation of BooleansPositional Implmentation of Booleans

AttributesAttributes

E.true - …E.true - …

E.falseE.false

FunctionsFunctions

makelist( quadnum ) –makelist( quadnum ) –

merge( list1, list2 ) –merge( list1, list2 ) –

backpatch (list, quadnum)backpatch (list, quadnum)

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BackpatchBackpatch

voidvoid

backpatch(QuadList p, int q)backpatch(QuadList p, int q)

{{

while (p != NULL){while (p != NULL){

target[p->quadnum] = (QuadListNode *) q;target[p->quadnum] = (QuadListNode *) q;

p = p -> link;p = p -> link;

}}

}}

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Intermediate Code Generation Intermediate Code Generation

voidvoid

gen(int op, struct nlist *p1, struct nlist *p2, struct nlist *r, int t)gen(int op, struct nlist *p1, struct nlist *p2, struct nlist *r, int t)

{{

opcode[nextquad] = op;opcode[nextquad] = op;

op1[nextquad] = p1;op1[nextquad] = p1;

op2[nextquad] = p2;op2[nextquad] = p2;

result[nextquad] = r;result[nextquad] = r;

branchTarget[nextquad] = t;branchTarget[nextquad] = t;

nextquad = nextquad + 1;nextquad = nextquad + 1;

}}

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Semantic Actions for B ID RELOP IDSemantic Actions for B ID RELOP ID

B: ID RELOP ID {B: ID RELOP ID {

gen($2, $1, $3, NULL, VOID);gen($2, $1, $3, NULL, VOID);

gen(GOTO, NULL,NULL,NULL,VOID);gen(GOTO, NULL,NULL,NULL,VOID);

$$.true = makelist(nextquad -2);$$.true = makelist(nextquad -2);

$$.false = makelist(nextquad - 1);$$.false = makelist(nextquad - 1);

}}

;;

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MarkersMarkers

Markers are typically nonterminals that derive Markers are typically nonterminals that derive εε that are that are inserted to insure an action is performed at a given inserted to insure an action is performed at a given time.time.

A common thing need is to remember the quad number A common thing need is to remember the quad number where something starts, so the attribute of a marker where something starts, so the attribute of a marker is just the next quad number.is just the next quad number.

M M εε { M.quad = nextquad; }{ M.quad = nextquad; }

So instead ofSo instead of

S S if B then S else S if B then S else S

We useWe use

S S if B then M if B then M11 S else M S else M22 S S ***Almost***Almost

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Semantic Actions for B B AND M B Semantic Actions for B B AND M B

B B B AND M B { B AND M B {

backpatch($1.true,$3);backpatch($1.true,$3);

$$.true = $4.true;$$.true = $4.true;

$$.false = merge($1.false,$4.false);$$.false = merge($1.false,$4.false);

}}

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Semantic Actions for B B OR M BSemantic Actions for B B OR M B

B B B OR M B { B OR M B {

backpatch($1.false,$3);backpatch($1.false,$3);

$$.false = $4.false;$$.false = $4.false;

$$.true = merge($1.true, $4.true);$$.true = merge($1.true, $4.true);

}}

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Semantic Actions for S if B then M S else M S ***Almost Semantic Actions for S if B then M S else M S ***Almost

S: IF B THEN M S S: IF B THEN M S NN ELSE M S { ELSE M S {

backpatch($2.true, $4);backpatch($2.true, $4);

backpatch($2.false, $8);backpatch($2.false, $8);

tmplist = merge($5, $6);tmplist = merge($5, $6);

$$ = merge(tmplist, $9);$$ = merge(tmplist, $9);

}}

;;

Why almost? Why almost?

N N εε { N.next = { N.next = makelist(makelist(nextquadnextquad));; gen(goto, _, _, _, void); } gen(goto, _, _, _, void); }

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Semantic Actions for Assignments Semantic Actions for Assignments

S: ID ASSIGNOP expr {S: ID ASSIGNOP expr {

gen(ASSIGNOP, $3, NULL, $1, VOID);gen(ASSIGNOP, $3, NULL, $1, VOID);

$$ = NULL;$$ = NULL;

}}

||

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Debugging Parsers written with YaccDebugging Parsers written with Yacc

1.1. Debug the grammarDebug the grammar1. Rewrite grammar to eliminate reduce/reduce and as many

shift/reduce as you can.

2. Tracing parses using –t option to bison or yacc -DYYDEBUG compile option int yydebug=1; in bison specification (C definitions section %{ ..%} extern int yydebug; in lex specification

2.2. Debug the semantic actionsDebug the semantic actions Compile with –g option; set CFLAGS=-g in Makefile and use

gcc $(CFLAGS) … as the compile (or rely on the builtin rules)

Use gdb (Gnu debugger) to debug the program

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Common MistakesCommon Mistakes

Segmentation fault - This means you have Segmentation fault - This means you have referenced a memory location that is outside of the referenced a memory location that is outside of the memory segment of your program.memory segment of your program. You have a pointer that has a bad value! First make sure everytime you copy a string value you use

strdup. Several people have had errors with strcat(s,t) where they did not allocate space for the string “s”.

Use gdb and bt (backtrace) to trace down the pointer with the bad value

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GDB - Essential CommandsGDB - Essential Commands

gdb program [core] - debug program gdb program [core] - debug program [using coredump core][using coredump core]

b [file:] function b [file:] function set breakpoint at function [in file]set breakpoint at function [in file]

run [arglist] run [arglist] start your program [with arglist]start your program [with arglist]

bt backtrace: bt backtrace: display program stackdisplay program stack

p expr p expr display the value of an expressiondisplay the value of an expression

c c continue running your programcontinue running your program

n n next line, stepping over function callsnext line, stepping over function calls

s s next line, stepping into function callsnext line, stepping into function calls

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Example using gdbExample using gdb

deneb> makedeneb> make

bison -d decaf.ybison -d decaf.y

decaf.y contains 51 shift/reduce conflicts.decaf.y contains 51 shift/reduce conflicts.

gcc -c -g decaf.tab.cgcc -c -g decaf.tab.c

flex decaf.lflex decaf.l

gcc -DYYDEBUG -g -c lex.yy.cgcc -DYYDEBUG -g -c lex.yy.c

gcc -c -g tree.cgcc -c -g tree.c

gcc -DYYDEBUG -g decaf.tab.o lex.yy.o tree.o -ly -o decafgcc -DYYDEBUG -g decaf.tab.o lex.yy.o tree.o -ly -o decaf

deneb> ./decaf < t1deneb> ./decaf < t1

Keyword intKeyword int

Segmentation Fault (core dumped) Segmentation Fault (core dumped) !!! !!!

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Example using gdbExample using gdb

deneb> makedeneb> make

bison -d decaf.ybison -d decaf.y

decaf.y contains 51 shift/reduce conflicts.decaf.y contains 51 shift/reduce conflicts.

gcc -c -g decaf.tab.cgcc -c -g decaf.tab.c

flex decaf.lflex decaf.l

gcc -DYYDEBUG -g -c lex.yy.cgcc -DYYDEBUG -g -c lex.yy.c

gcc -c -g tree.cgcc -c -g tree.c

gcc -DYYDEBUG -g decaf.tab.o lex.yy.o tree.o -ly -o decafgcc -DYYDEBUG -g decaf.tab.o lex.yy.o tree.o -ly -o decaf

deneb> ./decaf < t1deneb> ./decaf < t1

Keyword intKeyword int

Segmentation Fault (core dumped)Segmentation Fault (core dumped)

Note the use of the –g option (CFLAGS=-g in Makefile

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Run GDBRun GDB

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Attributes for Booleans and Control Flow*Attributes for Booleans and Control Flow*

*Assuming positional encoding.*Assuming positional encoding.

B (for boolean expression)B (for boolean expression) B.true – A list of …… that need to … B.false

Functions for Boolean Attributes Functions for Boolean Attributes int nextquad variable – Makelist (quad) Merge(l1, l2) Backpatch(List, target)

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MarkersMarkers

Markers – Grammar symbol that only derive Markers – Grammar symbol that only derive εε which are which are used perform some semantic action and usually to used perform some semantic action and usually to save some attribute valuesave some attribute value

Example S Example S A B M C D A B M C D In this case we want to perform some action when the

production M ε is reduced, like saving the starting place of the code the evaluates C

Markers for handling Booleans using the positional Markers for handling Booleans using the positional encodingencoding M ε (M.quad – marks the start of a piece of code) N ε (N.next – a list consisting of a single quad that

needs to have its target field filled in later.)

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Semantic Actions Control FlowSemantic Actions Control Flow

SS LHS ASSIGNOP E ‘;’LHS ASSIGNOP E ‘;’

|| if-statementif-statement

|| if-else-statementif-else-statement

|| while-statementwhile-statement

|| for-statementfor-statement

|| BEGIN L END BEGIN L END

|| function-calls ???function-calls ??? /* this is not really what /* this is not really what we mean by control flow we mean by control flow we will do this later */we will do this later */

;;

LL L SL S

|| SS

;;

Now what attributes do we need Now what attributes do we need for these nonterminals? (S, L)for these nonterminals? (S, L)

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Semantic Actions Statement ListsSemantic Actions Statement Lists

LL L SL S

|| SS

;;

As an example considerAs an example consider

while (B-expr){while (B-expr){

S1S1

S2S2

S3S3

}}

Parse TreeParse Tree

SS

while ‘(‘ B ‘)’ DO S ENDwhile ‘(‘ B ‘)’ DO S END

Now what attributes do we need Now what attributes do we need for these nonterminals? (S, L)for these nonterminals? (S, L)

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Semantic Actions for S if B then M S N else M S Semantic Actions for S if B then M S N else M S

S: IF B THEN M S S: IF B THEN M S NN ELSE M S { ELSE M S {

backpatch($2.true, $4);backpatch($2.true, $4);

backpatch($2.false, $8);backpatch($2.false, $8);

tmplist = merge($5, $6);tmplist = merge($5, $6);

$$ = merge(tmplist, $9);$$ = merge(tmplist, $9);

}}

;;

N N εε { N.next = nextquad;{ N.next = nextquad; gen(goto, _, _, _, void); } gen(goto, _, _, _, void); }

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Semantic Actions for S if B then M S Semantic Actions for S if B then M S

S: IF B THEN M S {S: IF B THEN M S {

backpatch($2.true, $4);backpatch($2.true, $4);

$$ = merge($2.false, $5);$$ = merge($2.false, $5);

}}

;;

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While StatementWhile Statement

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C For statementC For statement

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Project 4 – Generating Postfix Code for Expressions and Control FlowProject 4 – Generating Postfix Code for Expressions and Control Flow

BooleansBooleans

If B then assign else assignIf B then assign else assign

Undeclared variables print error message including line Undeclared variables print error message including line numbernumber

Write up in the email soonWrite up in the email soon

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Procedure CallsProcedure Calls