DL_compiler_frontend.md

# BNF of the DL programming language ``` ::= | ::= | ::= | ::= int ; ::= | , ::= | [ ] ::= (); | ( ); ::= | ::= | , ::= { } ::= | ; ::= | | | | | | | ::= = | [ ] = ::= if ( ) else | if ( ) ::= while ( ) ::= print ( ) ::= read ( ) ::= return ::= | | ::= ::= < | <= | == | >= | > | != ::= + | - ::= | ::= * | / ::= | , ::= | | [ ] | ( ) | ( ) | ( ) ::= | ::= | ::= | ::= a|b|c|d|e|f|g|h|i|j|k|l|m|n|o|p|q|r|s|t|u|v|w|x|y|z ::= 0|1|2|3|4|5|6|7|8|9 has the obvious meaning ``` Names have to be declared before they are used. This implies that mutually recursive functions are not allowed. All functions return a single integer result. Comments (zero or more characters enclosed between the standard C comment brackets /*...*/) can be inserted. DL has rudimentary support for one-dimensional arrays. The declaration int a[3] declares an array of three elements, referenced as a[0], a[1] and a[2]. Names are scoped in the traditional way. ## Lexical analyser for DL (`flex`) ```C %{ #include dlrecog_defs.h #include dlrecog_syn.h . . . int lexnumval; 66 3 Lexical Analysis char lexident[MAXIDLEN+1]; %} letter [a-z] digit [0-9] letter_or_digit [a-z0-9] white_space [ \t\n] other . %% "==" return EQSYM; "<=" return LESYM; ">=" return GESYM; "!=" return NESYM; else return ELSESYM; if return IFSYM; int return INTSYM; print return PRINTSYM; read return READSYM; return return RETURNSYM; while return WHILESYM; {letter}{letter_or_digit}* return IDENTIFIER; {digit}+ { lexnumval = atoi(yytext); return CONSTANT; } "/*" { comment(); } {white_space}+ ; {other} return yytext[0]; %% . . . int main() { int lextoken; while (lextoken = yylex()) printf("%d - %s\n",lextoken, yytext); } int yywrap() { return 1; } int comment(){ return 0; } ``` " - The two-character tokens are defined first, followed by the reserved words, followed by identifiers and numerical constants. - Reserved words have to be matched before the identifiers to prevent an input such as else being recognised as an identifier rather than a reserved word. " ## Syntax analyser for DL (`bison`) ```C %{ #include "dlrecog_defs.h" %} %token CONSTANT IDENTIFIER LESYM EQSYM GESYM NESYM ELSESYM IFSYM %token INTSYM PRINTSYM READSYM RETURNSYM WHILESYM %token CONSTANT %token IDENTIFIER %token ELSESYM %token IFSYM %token LESYM . . %type program %type declarations %type declaration . . %type relop %type identifier %type constant %% program: block { syntrace("program - without declarations"); } | declarations block { syntrace("program - with declarations"); } functiondeclaration: IDENTIFIER ’(’ ’)’ ’;’ functionbody { syntrace("functiondeclaration - no args"); } 126 5 Practicalities of Syntax Analysis | IDENTIFIER ’(’ arglist ’)’ ’;’ functionbody { syntrace("functiondeclaration - with args"); } ifstatement: IFSYM ’(’ bexpression ’)’ block ELSESYM block { syntrace("ifelsestatement"); $$ = newnode(N_IF, 0, $3, $5, $7); } | IFSYM ’(’ bexpression ’)’ block { syntrace("ifstatement"); $$ = newnode(N_IF, 0, $3, $5, NULL);} expression: expression addingop term { syntrace("expression"); $$ = newnode($2, 0, $1, $3, NULL); } | term {syntrace("expression - just term"); } | addingop term { syntrace("expression - unary op"); if ($1 == N_MINUS) $$ = newnode(N_MINUS, 0, $2, NULL, NULL); else $$ = $2; } statementlist: statement { syntrace("statemenlist"); $$ = newnode(N_SLIST, 0, $1, NULL, NULL); | statement ';' statementlist { syntrace("statemenlist"); $$ = newnode(N_SLIST, 0, $1, $3, NULL); } functiondeclaration: identifier ’(’ { /* Code placing identifier into symbol table. Save the state of the symbol table. Save and reset a pointer to the storage of local variables in the runtime stack. */ } arglist ’)’ ’;’ functionbody { syntrace("functiondeclaration"); $$ = newnode(N_FUNCTION, functioncount++, $4, $7, NULL); /* Save storage requirements of this function in the symbol table. Reset local variable pointer. */ } %% void syntrace(char *s) { if (TRACE) fprintf(stdout, "%s\n", s); } void yyerror(char *s) { printf("%s on line %d\n",s,yylineno); } int main(int argc, char *argv[]) { if (yyparse() == 0) syntrace("Recogniser succeeds"); return 0; } ``` Where `newnode()` is a function which generates new nodes and has the following definition. `newnode(type_of_node, node_value, node_pointer_1, node_pointer_2, node_pointer_3)` " - The header file dlrecog_defs.h defines any function prototypes required and can contain #include directives for other (system) header files. - The %token declarations correspond to the tokens used in the lexical analyser - Terminal tokens are in upper case and non-terminals are in lower case - The `syntrace() outputs it's argument` - The generatd parser/recogniser is called by `yyparse()` in the `main()`program. "