Lexical analysis. Syntactical analysis. Semantical analysis. Intermediate code generation. Optimization. Code generation. Target specific optimization
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1 Second round: the scanner Lexical analysis Syntactical analysis Semantical analysis Intermediate code generation Optimization Code generation Target specific optimization
2 Lexical analysis (Chapter 3) Why separate lexical analysis and parsing? We aim for a modular design! Increased portability. Platform specific issues (like text file format) affects a minimal part of the code. The compiler might be more efficien due to specializing.
3 Some definitions A token is an abstraction of some lexical element. A lexeme is an actual sequence of one or more characters from the code. A pattern is a (possibly general) description of how the lexeme looks. Token NUMBER THEN RELOP ID Lexeme 42 then <= foo Pattern A sequence of digits then {<= < == > >=} A letter followed by a sequence of letters or digits
4 The task of the scanner Identify a lexeme in the input using the patterns Find the corresponding token If applicable: attach some attribute Return the token In the pattern rules are implied what is a delimiter Whitespace and comments are handled according to the language (usually ignored)
5 Languages may be more or less easy to scan Examples: In Fortran identifiers are allowed to include arbitrary blanks DO 5 I= 1.25 (means DO5I = 1.25; ) DO 5 I= 1,25 (means iterate line 5 25 times using 'I' as counter) In the first case DO 5 I is one token, in the second 3 tokens! In PL/1 keywords are allowed as identifiers: IF THEN THEN THEN=ELSE;ELSE ELSE= THEN; E.g. 'THEN' is sometimes a keyword token and sometimes an ID!
6 Attributes to token Some token types needs an attribute. Why? - Because the parser will (later) need details about the lexeme. There may be needed more than one attribute. Can be implemented in various ways in your language.
7 Error handling We want the parser to parse the entire file. This requirement propagates to the scanner. The prefix of the remaining input to the scanner may at some point not match any of the patterns. Some error strategies: Panic mode Delete one character from the remaining input Insert a missing character into the remaining input Replace a character by another character Transpose two adjacent characters
8 Using a DFA We can use a DFA to specify a pattern here relation operators < = = > > return(relop,le) other 3 return(relop,ne) 5 * return(relop,eq) 4 return(relop,lt) = 6 7 return(relop,ge) other * 8 return(relop,gt)
9 Encoding a DFA A DFA is may be encoded using a table Here the DFA for the relation operator: other 4 8 < > = 5 2 7
10 Regular expressions as pattern specification Notation: a b means { a, b } ab means { ab } a * means {Ɛ, a, aa, aaa,...} a + means {a, aa, aaa,...} (a b) * means {Ɛ, a, b, aa, bb, ab, ba,...} E.g. <(= >) = >=
11 Some exercises 1. Use natural language to describe the languages described by the following regular expressions. (a b)*ab (ab ba)* 2. Write a regular expression that accepts all strings with 3 consecutive a:s. Σ = {a, b} 3. Consider floating point number in C. Examples: 1.02E20 1e e+22 a) Draw a DFA for a floating point number b) Write a regular expression for a floating point number
12 Flex (Lex) Flex is a tool to generate a scanner automatically Basically: Specify your scanner by writing reg-exps in a file Feed flex with that file and you get a C-file as output Compile that file and you have a scanner!
13 The input file to Flex There are three sections in the specification file to flex: Definitions %% Rules (reg-exp) %% C-code %% is a marker to delimit the sections
14 The definition section In this section you can define macros These can be used later in the same section or in the rule section Examples: Specify a letter: letter [A-Za-z] Specify a delimiter: delimiter [,:;.] Specify a digit: digit [0-9] Specify an identifier: id letter (letter digit)*
15 The definition section (cont.) You can also add lines of C-code that will appear at the top of the generated C-file. Example: %{ #include <stdio.h> int a_nice_global_variable = 0; inte my_favourite_function(void) { return 42; } %}
16 The rules section In this section you specify the translation rules A number of patterns are specified using a reg-exp for each For each pattern specify an action to be taken when the pattern matches. The action is written in C Example: = >=? <(= >)? { return RELOP; } {id} { return ID; }
17 The language of flex Some more useful codes for the extended regexp of flex:? Previous regexp is optional {} Macro expansion. Matches any character that is not end of line $ Matches end of line ^ Matches the beginning of a line [] Matches any enclosed character
18 Similar to but more powerful. Example: digit [ ] digit Some special codes within the brackets and ^: digit [0-9] letter [A-Za-z] non_digit [^0-9] The [] specification
19 The last section in the flex file: Code As the name suggests: C-code Only C-code can be written here. No special syntax to enclose the C-code The code will be copied as is to the generated C-file
20 %% int yywrap(void) { printf("number of words %d \n",words)}; return 1; } A complete example %{ #include <stdio.h> int words=0; /* Global variable */ %} letter [A-Za-z] delimiter [^A-Za-z] %% {delimiter} {/* No action*/} {letter}+ {words++;}
21 Some predefined globals present in flex yytext yyleng yylineno This is where you find the matched lexeme (a string) The lenght of yytext. The line number.
22 Suggested solutions 1.The language with the strings ending with ab The language with the strings that have the same number of a:s as b:s and that have at most 2 adjacent of the same type except first and last where there is a single a or b. 2.(a b)* aaa (a b)* 3. b) (- \+)?(d+.d* d*.d+ d+)((e E)(- \+)d+)?
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