1. COMPILER CONSTRUCTION
Principles and Practice
Kenneth C. Louden

2. 8. Code Generation
PART ONE

3. Generate executable code for a target machine that is a faithful representation of the semantics of the source code
Depends not only on the characteristics of the source language but also on detailed information about the target architecture, the structure of the runtime environment, and the operating system running on the target machine

4. Contents
Part One
8.1 Intermediate Code and Data Structure for code Generation
8.2 Basic Code Generation Techniques
Other Parts
8.3 Code Generation of Data Structure Reference
8.4 Code Generation of Control Statements and Logical Expression
8.5 Code Generation of Procedure and Function calls
8.6 Code Generation on Commercial Compilers: Two Case Studies
8.7 TM: A Simple Target Machine
8.8 A Code Generator for the TINY Language
8.9 A Survey of Code Optimization Techniques
8.10 Simple Optimizations for TINY Code Generator

5. 8.1 Intermediate Code and Data Structures for Code Generation

6. 8.1.1 Three-Address Code

7. A data structure that represents the source program during translation is called an intermediate representation, or IR, for short
Such an intermediate representation that resembles target code is called intermediate code
Intermediate code is particularly useful when the goal of the compiler is to produce extremely efficient code;
Intermediate code can also be useful in making a compiler more easily retarget-able.
Study two popular forms of intermediate code: Three -Address code and P-code
The most basic instruction of three-address code is designed to represent the evaluation of arithmetic expressions and has the following general form:
X=y op z

9. Figure 8.1 Sample TINY program:
{ sample program
in TINY language -- computes factorial }
read x ; { input an integer }
if 0 > x then { don’t compute if x <= 0 }
fact:=1;
repeat
fact:=fact*x;
x:=x-1
until x=0;
write fact { output factorial of x }
ends

10. The Three-address codes for above TINY program
read x t1=x>0
if_false t1 goto L1
fact=1
label L2
t2=fact*x
fact=t2
t3=x-1
x=t3
t4= x= =0
if_false t4 goto L2
write fact
label L1
halt

11. 8.1.2 Data Structures for the Implementation of Three-Address Code

12. A different implementation of three-address code is called a triple:
The most common implementation is to implement three-address code as quadruple, which means that four fields are necessary:
One for the operation and three for the addresses
A different implementation of three-address code is called a triple:
Use the instructions themselves to represent the temporaries.
It requires that each three-address instruction be reference-able, either as an index in an array or as a pointer in a linked list.

13. Quadruple implementation for the three-address code of the previous example
(rd, x , _ , _ )
(gt, x, 0, t1 )
(if_f, t1, L1, _ )
(asn, 1,fact, _ )
(lab, L2, _ , _ )
(mul, fact, x, t2 )
(asn, t2, fact, _ )
(sub, x, 1, t3 )
(asn, t3, x, _ )
(eq, x, 0, t4 )
(if_f, t4, L2, _)
(wri, fact, _, _ )
(lab, L1, _ , _ )
(halt, _, _, _ )

14. C code defining data structures for the quadruples
Typedef enum { rd, gt, if_f, asn, lab, mul, sub, eq, wri, halt, …}
OpKind;
Typedef enum { Empty, IntConst, String } AddrKind;
Typedef struct
{ AddrKind kind;
Union
{ int val;
char * name;
} contents;
} Address
{ OpKind op;
Address addr1, addr2, addr3;
} Quad

15. A representation of the three-address code of the previous example as triples
(0) (rd, x , _ )
(1) (gt, x, 0)
(2) (if_f, (1), (11) )
(3) (asn, 1,fact )
(4) (mul, fact, x)
(5) (asn, (4), fact )
(6) (sub, x, 1 )
(7) (asn, (6), x)
(8) (eq, x, 0 )
(9) (if_f, (8), (4))
(10) (wri, fact, _ )
(11) (halt, _, _)

16. 8.1.3 P-Code

17. It was designed to be the actual code for a hypothetical stack machine, called the P-machine, for which an interpreter was written on various actual machines
Since P-code was designed to be directly executable, it contains an implicit description of a particular runtime environment, including data sizes, as well as a great deal of information specific to the P-machines.

18. The P-machine consists of a code memory, an unspecified data memory for named variables, and a stack for temporary data, together with whatever registers are needed to maintain the stack and support execution
2*a+(b-3)
ldc 2 ; load constant 2
lod a ; load value of variable a
mpi ; integer multiplication
lod b ; load value of variable b
ldc 3 ; load constant 3
sbi ; integer subtraction
adi ; integer addition

19. Comparison of P-Code to Three-Address Code P-code is in many respects closer to actual machine code than three-address code.
P-code instructions also require fewer addresses:
One the other hand, P-code is less compact than three-address code in terms of numbers of instructions, and P-code is not “self-contained” in that the instructions operate implicitly on a stack.
2) Implementation of P-Code Historically, P-code has largely been generated as a text file, but the previous descriptions of internal data structure implementations for three-address code will also work with appropriate modification for P-code.

20. 8.2 Basic Code Generation Techniques

21. 8.2.1 Intermediate Code or Target Code as a Synthesized Attribute

22. For an example: a small subset of C expressions:
Intermediate code generation can be viewed as an attribute computation.
This code becomes a synthesized attribute that can be defined using an attribute grammar and generated either directly during parsing or by a post-order traversal of the syntax tree.
For an example: a small subset of C expressions:
Exp  id = exp | aexp
Aexp  aexp + factor | factor
Factor  (exp) | num | id

23. The expression (x=x+3)+4 has the following P-Code attribute:
Grammar Rule Semantic Rules
Exp1 id = exp exp1.pcode=”lda”|| id.strval++exp2.pcode++”stn”
Exp  aexp exp.pcode=aexp.pcode
Aexp1  aexp2 + factor aexp1.pcode=aexp2.pcode++factor.pcode++”adi”
Aexp  factor aexp.pcode=factor.pcode
Factor  (exp) factor.pcode=exp.pcode
Factor  num factor.pcode=”ldc”||num.strval
Factor  id factor.pcode=”lod”||id.strval
The expression (x=x+3)+4 has the following P-Code attribute:
Lda x
Lod x
Ldc 3
Adi
Stn
Ldc 4

24. 2) Three-Address Code T1=x+3 x=t1 t2=t1+4 Grammar Rule Semantic Rules
Exp1 id = exp exp1.name=exp2.name
Exp1.tacode=exp2.tacode++ id.strval||”=”||exp2.name
Exp  aexp exp.name=aexp.name; exp.tacode=aexp.tacode
Aexp1  aexp2 + factor aexp1.name=newtemp( )
aexp1.tacode=aexp2.tacode++factor.tacode++
aexp1.name||”=”||aexp2.name||”+”||factor.name
Aexp  factor aexp.name=factor.name; aexp.tacode=factor.tacode
Factor  (exp) factor.name=exp.name; factor.tacode=exp.tacode
Factor  num factor.namenum.strval; factor.tacode=” “
Factor  id factor.namenum.strval; factor.tacode=” “
T1=x+3 x=t1 t2=t1+4

25. 8.2.2 Practical Code Generation

26. The basic algorithm can be described as the following recursive procedure
Procedure gencode (T: treenode);
Begin
If T is not nil then
Generate code to prepare for code of left child of T;
Gencode(left child of T);
Generate code to prepare for code of right child of T;
Gencode(right child of T);
Generate code to implement the action of T;
End;

27. Typedef enum {plus, assign} optype;
Typedef enum {OpKind, ConstKind, IdKind} NodeKind;
Typedef struct streenode
{ NodeKind kind;
Optype op; /* used with OpKind */
Struct streenod *lchild, *rchild;
Int val; /* used with ConstKind */
Char * strval;
/* used for identifiers and numbers */
} streenode;
typedef streenode *syntaxtree;

29. A genCode procedure to generate P-code.
Void genCode (SyntaxTree t)
{ char codestr[CODESIZE];
/* CODESIZE = max length of 1 line of P-code */
if (t !=NULL)
{ switch (t->kind)
{ case opKind:
switch(t->op)
{ case Plus:
genCode(t->lchild);
genCode(t->rchild);
emitCode(“adi”);
break;

30. case Assign:
sprintf(codestr, “%s %s”, “lda”, t->strval);
emitcode(codestr);
genCode(t->lchild);
emitCode(“stn”);
break; }
case ConstKind:
sprintf(codestr,”%s %s”,”ldc”,t->strval);
emitCode(codestr);

31. case IdKind: sprintf(codestr,”%s %s”,”lod”,t->strval);
emitCode(codestr);
break;
default:
emitCode(“error”); }

32. Yacc specification for the generation P-code according to the attribute grammar of Table 8.1%{
#define YYSTYPE char *
/* make Yacc use strings as values */
/* other inclusion code … */ %}
%token NUM ID %%
exp : ID
{ sprintf (codestr, “%s %s”, “lda”, $1);
emitCode ( codestr); }
‘ = ‘ exp
{ emitCode (“stn”); }

33. | aexp ;
aexp : aexp ‘+’ factor {emitCode(“adi”);}
| factor
factor : ‘(‘ exp ‘)’
| NUM {sprintf(codestr, “%s %s”,”ldc”,$1);
emitCode(codestr);}
| ID {sprintf(codestr,”%s %s”, “lod”,$1); %%
/*utility functions… */

34. 8.2.3 Generation of Target Code from Intermediate Code

35. Code generation from intermediate code involves either or both of two standard techniques:
Macro expansion and Static simulation
Macro expansion involves replacing each kind of intermediate code instruction with an equivalent sequence of target code instructions.
Static simulation involves a straight-line simulation of the effects of the intermediate code and generating target code to match these effects.

36. Consider the expression (x=x+3) +4, translate the P-code into three-address code:
Lad x
Lod x
Ldc 3
Adi t1=x+3
Stn x=t1
Ldc 4
Adi t2=t1+4
We perform a static simulation of the P-machine stack to find three-address equivalence for the given code

39. We now consider the case of translating from three-address code to P-code, by simple macro expansion. A three-address instruction:
a = b + c
Can always be translated into the P-code sequence
lda a
lod b
lod c
adi
sto

40. Then, the three-address code for the expression (x=x+3)+4:
T1 = x + 3
X = t1
T2 = t1 + 4
Can be translated into the following P-code:
Lda t1
Lod x
Ldc 3
Adi
Sto
Lad x
Lod t1
Lda t2
Ldc 4

42. End of Part One
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