Lecture 14 Boolean Expressions

Lecture 14 Boolean Expressions

Topics Topics Numeric Implementation of Booleans Positional Encoding of Booleans Short Circuit Evaluation If Then else (incorrect version)

Readings: 8.4Readings: 8.4

March 1, 2006

CSCE 531 Compiler Construction

– 2 – CSCE 531 Spring 2006

OverviewOverviewLast TimeLast Time

LALR(1) Parse Table construction Handling Ambiguous Programming Language Constructs

An Expression Interpreter Generating Postfix code

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

References: Sections 8.4References: Sections 8.4

Homework:Homework:

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Intermediate CodeIntermediate Code

Forms of Intermediate RepresentationsForms of Intermediate Representations

Code-like: quadruples (like postfix.y)Code-like: quadruples (like postfix.y)

Parse-tree likeParse-tree like

Lexicalanalyzer Parser Code

GeneratorSource

programObjectModule

Streamof tokens

Intermediateform

SymbolTable

Management

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Arithmetic ExpressionsArithmetic Expressions

E : E '+' EE : E '+' E

| E '*' E| E '*' E

| E ‘-' E| E ‘-' E

| E ‘/' E| E ‘/' E

| ‘(’ E ‘)’| ‘(’ E ‘)’

| - E| - E

| id| id

;;

Other expressionsOther expressions

E : E ‘^' EE : E ‘^' E//exponentiation//exponentiation

| id '(' Plist ')'| id '(' Plist ')' // ??// ??

| id '[' Elist ']'| id '[' Elist ']' // ??// ??

| * id| * id ////

| & id| & id ////

| id . id| id . id ////

| id | id id id ////

;;

Others ? - Bitwise and or xor, shifts, Others ? - Bitwise and or xor, shifts,

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Arithmetic Expressions: AttributesArithmetic Expressions: AttributesPossibilitiesPossibilities

E.place – pointer to the symbol tableE.place – pointer to the symbol table Type information Offset information for structures Scope information global/local/nested

contexts

E.code – pointer to code that will E.code – pointer to code that will evaluate the expressionevaluate the expression

E.typeE.type

We will generate “postfix” code and We will generate “postfix” code and assume E.place and install every assume E.place and install every temporary (as in Examples/PostfixCode)temporary (as in Examples/PostfixCode)

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Boolean Expression GrammarBoolean Expression Grammar

BoolExprBoolExpr not OrTerm | OrTerm not OrTerm | OrTerm

OrTerm OrTerm OrTerm OR AndTerm | AndTerm OrTerm OR AndTerm | AndTerm

AndTerm AndTerm AndTerm AND Bool | Bool AndTerm AND Bool | Bool

Bool Bool RelExpr | true | false RelExpr | true | false

RelExpr RelExpr E RelOp E E RelOp E

E E E + E | E * E | ( E ) | ID | NUM | … E + E | E * E | ( E ) | ID | NUM | …

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Numeric EncodingNumeric Encoding

True = non-zeroTrue = non-zero

False = zeroFalse = zero

ExampleExampleB OR C AND NOT DB OR C AND NOT D

QuadruplesQuadruples

NOTNOT rDrD __ T1T1

AND rCAND rC T1T1 T2T2

OROR rBrB T2T2 T3T3

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Numeric Encod. Extended to ComparisonsNumeric Encod. Extended to Comparisons

Comparison operations in HardwareComparison operations in Hardware IA32 -

CC register set as result of arithmetic operations

» Add, Subtract, … CMP = subtract without saving resultJumps then test certain bits in the CC register

» JLT, JLE, JEQ, JGE, JGT, JNEQSo encoding includes a lot of Jumps (x < y) AND (y < z) OR (y=x)

» Label Opcode LeftOp RightOp Target

» L0 cmp x y _

» _ JLT L2

» L1 LoadI 0 T1

» _ JMP L3

» L2 LoadI 1 _ T1

» L3 NOP

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Example: (x < y) AND (y < z) Example: (x < y) AND (y < z)

LabelLabel OpOp A1A1 A2A2 TargetTarget

CMPCMP XX YY __

JLTJLT L2L2

LOADILOADI 00 T1T1

JMPJMP L3L3

L2L2 LOADILOADI 11 T1T1

L3L3 CMPCMP YY ZZ __

JLTJLT L4L4

LOADILOADI 00 T2T2

JMPJMP L3L3

L4L4 LOADILOADI 11 T2T2

L5L5 ANDAND T1T1 T2T2 T3T3

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Positional EncodingPositional EncodingIn Positional Encoding we represent the value of an expression In Positional Encoding we represent the value of an expression

by where (the position) you end up in the codeby where (the position) you end up in the code

ExampleExample

while(k < 20) {while(k < 20) {

sum = sum + k*k;sum = sum + k*k;

k = k + 1;k = k + 1;

}}

Note in the code on the right the value Note in the code on the right the value of the boolean expr k<20 is never of the boolean expr k<20 is never explicitly represented other than in explicitly represented other than in the Condition Code Register (CC)the Condition Code Register (CC)

The value is represented by whether The value is represented by whether you end up at quad 7 or quad 3you end up at quad 7 or quad 3

QuadQuad OpOp S1S1 S2S2 TT

11 CMPICMPI kk 2020 __

22 JGEJGE ?7?7

33 MULTMULT kk kk T1T1

44 ADDADD sumsum T1T1 sumsum

55 ADDIADDI kk 11 kk

66 JMPJMP 11

77 ……

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Attributes for BooleansAttributes for Booleans

Consider the example on the next slideConsider the example on the next slide

As we generate the code we don’t know what the target As we generate the code we don’t know what the target branches should be.branches should be.

We need to build lists of quads whose target fields We need to build lists of quads whose target fields need to be filled in laterneed to be filled in later

B.True – list of quadruples that need to be filled in B.True – list of quadruples that need to be filled in later(backpatched) to the location that we should later(backpatched) to the location that we should branch to if this Boolean is true.branch to if this Boolean is true.

B.False - …B.False - …

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Example: (x < y) AND (y < z) Example: (x < y) AND (y < z)

QuadNumQuadNum OpOp A1A1 A2A2 TargetTarget

00 IF < XX YY 22

11 GOTO __ __ ??

22 IF < YY ZZ ??

33 GOTO __ __ ??

B.true

B.false 1 3

2

λ

λ

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Now a Boolean in an IF-ELSENow a Boolean in an IF-ELSE

programprogram

beginbegin

if x <if x < y and y < z y and y < z

then then

mid = bmid = b

else else

mid = cmid = c

endend

0 if < 0 if < x yx y 22

1 GOTO 1 GOTO _ _ __ 66

2 if < 2 if < y z y z 44

3 GOTO 3 GOTO _ _ _ _ 66

4 ASSIGN 4 ASSIGN b b _ _ midmid

5 GOTO 5 GOTO __ _ VOID_ VOID

6 ASSIGN 6 ASSIGN c c _ _ midmid

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Functions for Boolean AttributesFunctions for Boolean Attributes

int nextquad variable – a variable which maintain the int nextquad variable – a variable which maintain the quad number of the next quad that will be generatedquad number of the next quad that will be generated

Makelist (quad) – build a list with a single quad # on itMakelist (quad) – build a list with a single quad # on it

Merge(list1, list2) – merge two lists of quadsMerge(list1, list2) – merge two lists of quads

Backpatch(list, target) – for every quad on “list” fill in Backpatch(list, target) – for every quad on “list” fill in the branch target to “target”the branch target to “target”

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Quadlists Quadlists

A quadlist is just a list of intsA quadlist is just a list of ints

typedef struct node{typedef struct node{

int quadnum;int quadnum;

struct node *link;struct node *link;

} *QuadList, QuadListNode;} *QuadList, QuadListNode;

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Quadlists Quadlists

QuadListNode * QuadListNode *

makelist(int q)makelist(int q)

{{

QuadListNode *tmp;QuadListNode *tmp;

tmp = (QuadListNode *) malloc(sizeof (QuadListNode));tmp = (QuadListNode *) malloc(sizeof (QuadListNode));

tmp -> quadnum = q;tmp -> quadnum = q;

return(tmp);return(tmp);

}}

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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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Dumplist – A Debugging Support Routine Dumplist – A Debugging Support Routine

voidvoid

dumplist(char *label, LIST p)dumplist(char *label, LIST p)

{{

printf("\nDUMPLIST %s", label);printf("\nDUMPLIST %s", label);

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

printf(" %d ",p->quadnum);printf(" %d ",p->quadnum);

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

}}

}}

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Stack types – Multiple Type Attributes Stack types – Multiple Type Attributes

%union{%union{

struct nlist *place;struct nlist *place;

struct {struct {

QuadListNode *true;QuadListNode *true;

QuadListNode *false;QuadListNode *false;

} quadlist;} quadlist;

int quad;int quad;

int type;int type;

LIST next;LIST next;

}}

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Stack types – Multiple Type Attributes II Stack types – Multiple Type Attributes II

%type <place> expr%type <place> expr

%type <list> B%type <list> B

%type <quad> M%type <quad> M

%type <next> N%type <next> N

%type <next> L%type <next> L

%type <next> S%type <next> S

%token <place> ID%token <place> ID

%token <type> RELOP%token <type> RELOP

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Intermediate Code Generation Intermediate Code Generation

QuadruplesQuadruples OPCODE LeftOperand RightOperand Result ADD “x” “y” “z” GOTO quadnumber What are the type of these?

Quadruples – implemented as 5 parallel arraysQuadruples – implemented as 5 parallel arrays int opcode[CODESIZE]; struct nlist *left[CODESIZE]; struct nlist *right[CODESIZE]; struct nlist *result[CODESIZE]; int branchTarget[CODESIZE];

We could use a union and merge result and We could use a union and merge result and branchTarget, but this just over complicates issues.branchTarget, but this just over complicates issues.

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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 N ELSE M S {S: IF B THEN M S N 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?

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Semantic Actions for Assignments Semantic Actions for Assignments

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

gen(ASSIGNOP, $<place>3, NULL, $1, VOID);gen(ASSIGNOP, $<place>3, NULL, $1, VOID);

$$ = NULL;$$ = NULL;

}}

||

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Semantic Actions for Markers Semantic Actions for Markers

M: {$$ = nextquad;}M: {$$ = nextquad;}

;;

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Project 3 – Generating Postfix Code for Expressions Project 3 – Generating Postfix Code for Expressions

ExpressionsExpressions

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

Mixed mode expressions (Graduate students only) Mixed mode expressions (Graduate students only) Extra credit for ugradsExtra credit for ugrads int k; float f; k + f (float)k + f Code toFloat k _ t1 addf t1 f t2

Write up in the email soonWrite up in the email soon

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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