1. Lecture 16 Boolean Expressions and Control Flow
CSCE 531 Compiler Construction
Lecture 16 Boolean Expressions and Control Flow
Topics
Numeric Implementation of Booleans
Positional Encoding of Booleans
Short Circuit Evaluation
Debugging
No class Thursday, we will reschedule!
Readings: 5.7, 6.3, 7.4
March 27, 2018

2. Overview Last Time Last Time Didn’t Finish Today’s Lecture
Attribute Grammars
Type Systems
ILOC
Symbol Table
Last Time Didn’t Finish
Pascal like declarations
Hierarchical Symbol Tables
Today’s Lecture
Numeric Implementation of Booleans
Positional Encoding of Booleans
Short Circuit Evaluation
Debugging YACC generated parsers
References: Sections 7.4
Homework:

3. Pascal like declarations
What if the language gets it wrong?
Pascal
x,y,z : integer;
Grammar?

4. Hierarchical Symbol Tables
From last time figure 5.12 page 246

5. Boolean Expression Grammar
BoolExpr  not OrTerm | OrTerm
OrTerm  OrTerm OR AndTerm | AndTerm
AndTerm  AndTerm AND Bool | Bool
Bool  RelExpr | true | false
RelExpr  Expr RelOp Expr
Expr  Expr + Expr | Expr * Expr | ( Expr ) | ID | NUM

6. Numeric Encoding True = non-zero False = zero Example B OR C AND NOT D
Quadruples
NOT rD _ rt1
AND rC rt1 rt2
OR rB rt2 rt3

7. Numeric Encod. Extended to Comparisons
Comparison operations in Hardware
IA32 -
CC register set as result of arithmetic operations
Add, Subtract, … CMP = subtract without saving result
Jumps then test certain bits in the CC register
JLT, JLE, JEQ, JGE, JGT, JNEQ
So encoding includes a lot of Jumps
(x < y) AND (y < z) OR (y=x)
L0 cmp x y _
_ JLT L2
L1 LoadI 0 T1
_ JMP L3
L2 LoadI 1 _ T1
L3 NOP

8. Example: (x < y) AND (y < z)
Label Op A1 A2
Target
CMP X Y _
JLT L2
LOADI T1
JMP L3 1 Z L4 T2 L5
AND T3

9. Positional Encoding
In Positional Encoding we represent the value of an expression by where (the position) you end up in the code
Example
while(k < 20) {
sum = sum + k*k;
k = k + 1; }
Note in the code on the right the value of the boolean expr k<20 is never explicitly represented other than in the Condition Code Register (CC)
The value is represented by whether you end up at quad 7 or quad 3
Quad Op S1 S2 T
CMPI k 20 _
JGE ?7
MULT k k T1
ADD sum T1 sum
ADDI k 1 k
JMP 1 …

10. Attributes for Booleans
Consider the example on the next slide
As we generate the code we don’t know what the target branches should be.
We need to build lists of quads whose target fields need to be filled in later
E.True
E.false

11. Example: (x < y) AND (y < z)
QuadNum Op A1 A2
Target 1
CMP X Y _ 2 3 4 5 6 7 8 9 10 11

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

13. /class/csce531-001/web/Examples/Booleans
Don’t use this make or lex file as example.
Focus on boolean.y

14. %{
#include <ctype.h>
#include <stdio.h>
#define YYDEBUG 1
#define ADDOP 401
#define MULTOP 402
#define GOTO
/* $Header: gram,v /12/07 12:01:01 matthews Exp $ */
extern char *yytext;
char *strsave();
char *newtemp();
typedef struct node{
int quadnum;
struct node *link;
} *LIST, LISTNODE;

15. #define CODESIZE 1000
int opcode[CODESIZE];
char *op1[CODESIZE], *op2[CODESIZE], *target[CODESIZE];
char *VOID = "VOID";
LIST tmplist;
int nextquad = 0; %}
%union{
char *place;
struct {
LIST *true;
LIST *false;
} list;
int quad;
int type;
LIST next; }

16. B:
ID RELOP ID {
gen($2, $1, $3, VOID);
gen(GOTO, VOID, VOID, VOID);
$$.true = makelist(nextquad -2);
$$.false = makelist(nextquad - 1); } ;

17. | B AND M B {
backpatch($1.true,$3);
$$.true = $4.true;
$$.false = merge($1.false, $4.false); }
| B OR M B {
backpatch($1.false,$3);
$$.false = $4.false;
$$.true = merge($1.true, $4.true);

18. S: ID ASSIGNOP expr {
gen(ASSIGNOP, $<place>3, VOID, $1);
$$ = NULL; }
| IF B THEN M S N ELSE M S {
backpatch($2.true, $4);
backpatch($2.false, $8);
tmplist = merge($5, $6);
$$ = merge(tmplist, $9); ;
M: {$$ = nextquad;}
N: {gen(GOTO, VOID, VOID, VOID);
$$ = makelist(nextquad - 1);

20. Debugging Parsers written with Yacc
Debug the grammar
Rewrite grammar to eliminate reduce/reduce and as many shift/reduce as you can.
Tracing parses using
–t option to bison or yacc
-DYYDEBUG compile option
Int yydebug=1; in lex specification (C definitions section %{ ..%}
Extern int yydebug; in yacc specification
Debug 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

21. Parsing Traces with yydebug=1
Setting up for traces
–t option to bison or yacc
-DYYDEBUG compile option
int yydebug=1; in lex specification
C definitions section %{ … %}
extern int yydebug; in yacc specification
Generate .output file and print out so you can follow along

22. deneb> ./expr
Starting parse
Entering state 0
Reading a token: 23+34*3;
Next token is 262 (INT)
Shifting token 262 (INT), Entering state 2
Reducing via rule 6 (line 28), INT -> expr
INT=23
state stack now 0
Entering state 4
Reading a token: Next token is 260 (PLUS)
Shifting token 260 (PLUS), Entering state 8
Reading a token: Next token is 262 (INT)
INT=34
state stack now 0 4 8
Entering state 13
Reading a token: Next token is 259 (TIMES)
Shifting token 259 (TIMES), Entering state 7

23. Common Mistakes
Segmentation fault - This means you have referenced a memory location that is outside of the 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

24. GDB - Essential Commands
gdb program [core] - debug program [using coredump core]
b [file:] function set breakpoint at function [in file]
run [arglist] start your program [with arglist]
bt backtrace: display program stack
p expr display the value of an expression
c continue running your program
n next line, stepping over function calls
s next line, stepping into function calls

25. Example using gdb deneb> make bison -d decaf.y
decaf.y contains 51 shift/reduce conflicts.
gcc -c -g decaf.tab.c
flex decaf.l
gcc -DYYDEBUG -g -c lex.yy.c
gcc -c -g tree.c
gcc -DYYDEBUG -g decaf.tab.o lex.yy.o tree.o -ly -o decaf
deneb> ./decaf < t1
Keyword int
Segmentation Fault (core dumped)  !!!

26. Note the use of the –g option (CFLAGS=-g in Makefile
Example using gdb
deneb> make
bison -d decaf.y
decaf.y contains 51 shift/reduce conflicts.
gcc -c -g decaf.tab.c
flex decaf.l
gcc -DYYDEBUG -g -c lex.yy.c
gcc -c -g tree.c
gcc -DYYDEBUG -g decaf.tab.o lex.yy.o tree.o -ly -o decaf
deneb> ./decaf < t1
Keyword int
Segmentation Fault (core dumped)
Note the use of the –g option (CFLAGS=-g in Makefile

27. Getting in and out of GDB
deneb> gdb decaf
GNU gdb 4.18
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, …
(gdb) run < t1
Starting program: /class/csce /submissions/huang27/review01/decaf < t1
before treeprint
demo=Program info=X
demo= info=
Program received signal SIGSEGV, Segmentation fault.
0xff2b3474 in strlen () from /usr/lib/libc.so.1
(gdb) quit
The program is running. Exit anyway? (y or n) y
deneb>

28. Backtrace(bt) To see the Activation Stack
(gdb) run < t1
Starting program: /class/csce /submissions/huang27/review01/decaf < t1
before treeprint
demo=Program info=X
demo= info=
Program received signal SIGSEGV, Segmentation fault.
0xff2b3474 in strlen () from /usr/lib/libc.so.1
(gdb) bt
#0 0xff2b3474 in strlen () from /usr/lib/libc.so.1
#1 0xff in _doprnt () from /usr/lib/libc.so.1
#2 0xff3072cc in printf () from /usr/lib/libc.so.1
#3 0x199e8 in treeprint (p=0x32718) at tree.c:9
#4 0x19a0c in treeprint (p=0x32738) at tree.c:11
#5 0x19a0c in treeprint (p=0x32748) at tree.c:11
#6 0x11414 in yyparse () at decaf.y:91
#7 0xff3804e0 in main () from /usr/lib/liby.so.1

29. Printing values (gdb) up
#1 0xff in _doprnt () from /usr/lib/libc.so.1
#2 0xff3072cc in printf () from /usr/lib/libc.so.1
#3 0x199e8 in treeprint (p=0x32718) at tree.c:9
printf("demo=%s info=%s\n", p->demo, p->info);
(gdb) print *p
$1 = {tag = VDECL, child = 0x326e8, next = 0x1, info = 0x1adf8 "",
tagname = 0x7 <Address 0x7 out of bounds>, demo = 0x0, value = {i = 1,
d = e-314, s = 0x1 <Address 0x1 out of bounds>}}
(gdb)

30. Project Two and Three Due Tonight – dropbox project two
Test cases using yourlogin name followed by a numeral as the names, e.g., matthews1, matthews2, matthews3
Your grammar file without necessarily having any semantic actions
Your Lex, Makefile (not necessarily any routines for semantic actions)
We will create a TestDir with all of these files that you can get to. Note I will not vouch for the validity of any test program.
Project Three (the old project two)
Due Sunday Night
Note I will be at an ABET meeting the rest of the week!
I will try to read my , but no promises!
No class Thursday, we will reschedule!

31. Test 2