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 t he operating system running on the target machine

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

8.1 Intermediate Code and Data Structures for Code Generation

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

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

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

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, _, _)

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 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

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

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

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

8.2.2 Practical Code Generation

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;

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;

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 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

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 + 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 Sto Lda t2 Lod t1 Ldc 4 Adi Sto

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Chapter Eight(1)