Chapter Eight(1)

COMPILER CONSTRUCTION

Principles and Practice

Kenneth C. Louden

8. Code Generation

PART ONE

• 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

Contents

Part One8.1 Intermediate Code and Data Structure for code Generation8.2 Basic Code Generation Techniques

Other Parts8.3 Code Generation of Data Structure Reference8.4 Code Generation of Control Statements and Logical Expression8.5 Code Generation of Procedure and Function calls8.6 Code Generation on Commercial Compilers: Two Case Studies8.7 TM: A Simple Target Machine8.8 A Code Generator for the TINY Language8.9 A Survey of Code Optimization Techniques8.10 Simple Optimizations for TINY Code Generator

8.1 Intermediate Code and Data Structures for Code Generation

8.1.1 Three-Address Code

• 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

2*a+(b-3) with syntax tree + * - 2 a b 3 The corresponding three-address code is T1 = 2 * a T2 = b – 3 T3 = t1 + t2

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-1until x=0;write fact { output factorial of x } ends

• The Three-address codes for above TINY program read x

t1=x>0if_false t1 goto L1fact=1label L2t2=fact*xfact=t2t3=x-1x=t3t4= x= =0if_false t4 goto L2

write factlabel L1halt

8.1.2 Data Structures for the Implementation of Three-Address Code

• 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.

• 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, _, _, _ )

• 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 Typedef struct

{ OpKind op; Address addr1, addr2, addr3;} Quad

• 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, _, _)

8.1.3 P-Code

• 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.

• 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 2lod a ; load value of variable ampi ; integer multiplicationlod b ; load value of variable bldc 3 ; load constant 3sbi ; integer subtractionadi ; integer addition

1) 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.

8.2 Basic Code Generation Techniques

8.2.1 Intermediate Code or Target Code as a Synthesized Attribute

• 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

Grammar Rule Semantic Rules

Exp1 id = exp2 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

1) P-Code

The expression (x=x+3)+4 has the following P-Code attribute:Lda xLod xLdc 3AdiStnLdc 4Adi

Grammar Rule Semantic Rules

Exp1 id = exp2 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

2) Three-Address Code

8.2.2 Practical Code Generation

The basic algorithm can be described as the following recursive procedure

Procedure gencode (T: treenode);BeginIf 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;

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;

Expression (x=x+3) + 4 +

x= 4

+

x 3

• 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;

case Assign: sprintf(codestr, “%s %s”, “lda”, t->strval);

emitcode(codestr);genCode(t->lchild);emitCode(“stn”);break;

}break;

case ConstKind:sprintf(codestr,”%s %s”,”ldc”,t->strval);emitCode(codestr);break;

case IdKind:

sprintf(codestr,”%s %s”,”lod”,t->strval);

emitCode(codestr);

break;

default:

emitCode(“error”);

break;

}

}

}

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”); }

| 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);

emitCode(codestr);};

%%/*utility functions… */

8.2.3 Generation of Target Code from Intermediate Code

• 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.

• Consider the expression (x=x+3) +4, translate the P-code into three-address code:

Lad xLod xLdc 3Adi t1=x+3Stn x=t1Ldc 4Adi t2=t1+4

• We perform a static simulation of the P-machine stack to find three-address equivalence for the given code

< -- top of stack 3

X Address of x

T1=x+3

< -- top of stack T1

Addrss of x

X=t1 < -- top of stack

T1 < -- top of stack

4 T1

T2=t1+4

< -- top of stack T2

• 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

• Then, the three-address code for the expression (x=x+3)+4:T1 = x + 3X = t1T2 = t1 + 4

• Can be translated into the following P-code:Lda t1Lod xLdc 3AdiStoLad xLod t1StoLda t2Lod t1Ldc 4AdiSto

If we want to eliminate the extra temporaries, then a more sophisticated scheme than pure macro expansion must be used.

T2 + X, t1 + 4 X 3

End of Part One

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