readme
author Mario de Sousa <msousa@fe.up.pt>
Tue, 14 Jun 2011 14:53:37 +0100
changeset 321 a96399ab57c2
parent 265 4d222f46f8cc
child 444 92d40d2a7adc
permissions -rwxr-xr-x
Add better comments, and change name of functions to clarify what they do.
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  MATIEC - IEC 61131-3 compiler
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  The following compiler has been based on the
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  FINAL DRAFT - IEC 61131-3, 2nd Ed. (2001-12-10)
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  Copyright (C) 2003-2011  Mario de Sousa (msousa@fe.up.pt)
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****************************************************************
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****************************************************************
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*********   O V E R A L L    A R C H I T E C T U R E   *********
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****************************************************************
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****************************************************************
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****************************************************************
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 The compiler works in 4(+1) stages:
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 Stage 1   - Lexical analyser      - implemented with flex (iec.flex)
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 Stage 2   - Syntax parser         - implemented with bison (iec.y)
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 Stage 3   - Semantics analyser    - currently in its early stages
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 Stage 4   - Code generator        - implemented in C++
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 Stage 4+1 - Binary code generator - gcc, javac, etc...
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 Data structures passed between stages, in global variables:
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 1->2   : tokens (int), and token values (char *)
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 2->1   : symbol tables (defined in symtable.hh)
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 2->3   : abstract syntax tree (tree of C++ classes, in absyntax.hh file)
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 3->4   : Same as 2->3
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 4->4+1 : file with program in c, java, etc...
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 The compiler works in several passes:
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 Pass 1: executes stages 1 and 2 simultaneously
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 Pass 2: executes stage 3
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 Pass 3: executes stage 4
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 Pass 4: executes stage 4+1
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 NOTE 1
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 ======
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 Note that stage 2 passes data back to stage 1. This is only
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possible because both stages are executed in the same pass.
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 NOTE 2
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 It would be nice to get this parser integrated into the gcc
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group of compilers. We would then be able to compile our st/il
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programs directly into executable binaries, for all the processor
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architectures gcc currently supports.
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 The gcc compilers are divided into a frontend and backend. The
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data structure between these two stages is called the syntax
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tree. In essence, we would need to create a new frontend that
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would parse the st/il program and build the syntax tree.
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Unfortunately the gcc syntax tree is not very well documented,
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and doing semantic checking on this tree would probably be a
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nightmare.
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 We therefore chose to follow the same route as the gnat (ada 95)
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and cobol compilers, i.e. generate our own abstract syntax tree,
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do semantic checking on our tree, do whatever optimisation
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we can at this level on our own tree, and only then build
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the gcc syntax tree from our abstract syntax tree.
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 All this may still be integrated with the gcc backend to generate
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a new gnu compiler for the st and il programming languages.
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Since generating the gcc syntax tree will probably envolve some
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trial and error effort due to the sparseness of documentation,
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we chose to start off by coding a C++ code generator for
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our stage 4. We may later implement a gcc syntax tree generator
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as an alternative stage 4 process, and then integrate it with
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the gcc toplevel.c file (command line parsing, etc...).
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****************************************************************
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****************************************************************
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*********               S T A G E      1               *********
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****************************************************************
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****************************************************************
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Issue 1
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=======
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 The syntax defines the common_character_representation as:
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<any printable character except '$', '"' or "'"> | <escape sequences>
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 Flex includes the function print_char() that defines
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all printable characters portably (i.e. whatever character
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encoding is currently being used , ASCII, EBCDIC, etc...)
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Unfortunately, we cannot generate the definition of
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common_character_representation portably, since flex
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does not allow definition of sets by subtracting
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elements in one set from another set (Note how
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common_character_representation could be defined by
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subtracting '$' '"' and "'" from print_char() ).
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This means we must build up the defintion of
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common_character_representation using only set addition,
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which leaves us with the only choice of defining the
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characters non-portably...
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 In short, the definition we use for common_character_representation
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only works for ASCII character encoding!
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Issue 2
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=======
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We extend the IEC 61131-3 standard syntax to allow inclusion of 
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other files. The accepted syntax is:
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   {#include "<filename>" }
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We use a pragma directive for this (allowed by the standard itself),
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 since it is an extension of the standard. In principle, this would 
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be ignored by other standard complient compilers!
0
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****************************************************************
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****************************************************************
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****************************************************************
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*********                                              *********
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*********                                              *********
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*********               S T A G E      2               *********
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*********                                              *********
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*********                                              *********
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****************************************************************
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****************************************************************
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****************************************************************
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 Overall Comments
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 ================
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 Comment 1
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 ---------
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 We have augmented the syntax the specification defines to include
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restrictions defined in the semantics of the languages.
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 This is required because the syntax cannot be parsed by a LALR(1)
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parser as it is presented in the specification. Many reduce/reduce
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and shift/reduce conflicts arise. This is mainly because the parser
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cannot discern how to reduce an identifier. Identifiers show up in
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many places in the syntax, and it is not entirely possible to
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figure out if the identifier is a variable_name, enumeration
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value, function block name, etc... only from the context in
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which it appears.
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 A more detailed example of why we need symbol tables are
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the type definitions...  In definition of new types
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(section B 1.3.3) the parser needs to figure out the class of
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the new type being defined (enumerated, structure, array, etc...).
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This works well when the base classes are elementary types
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(or structures, enumeration, arrays, etc. thereof). It becomes
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confusing to the parser when the new_type is based on a previously
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user defined type.
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TYPE
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  new_type_1 : INT := 99;
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  new_type_2 : new_type_1 := 100;
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END_TYPE
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 When parsing new_type_1, the parser can figure out that the
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identifier new_type_1 is a simple_type_name, because it is
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based on a elementary type without structure, arrays, etc...
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 While parsing new_type_2, it becomes confused how to reduce
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the new_type_2 identifier, as it is based on the identifier
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new_type_1, of which it does not know the class (remember, at this
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stage new_type_1 is a simple identifier!).
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 We therefore need to keep track of the class of the user
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defined types as they are declared, so that the lexical analyser
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can tell the syntax parser what class the type belongs to. We
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cannot use the abstract syntax tree itself to search for the
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declaration of new_type_1 as we only get a handle to the root
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of the tree at the end of the parsing.
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 We therefore maintain an independent and parallel table of symbols,
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that is filled as we come across the type delcarations in the code.
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Actually, we ended up also needing to store variable names in
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the symbol table. Since variable names come and go out of scope
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depending on what portion of code we are parsing, we sometimes
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need to remove the variable names from the symbol table.
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Since the ST and IL languages only have a single level of scope,
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I (Mario) found it easier to simply use a second symbol table for
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the variable names that is completely cleared when the parser
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reaches the end of a function (function block or program).
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What I mean when I say that these languages have a single level
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of scope is that all variables used in a function (function block
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or program) must be declared inside that function (function block
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or program). Even global variables must be re-declared as EXTERN
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before a function may access them! This means that it is easy
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to simply load up the variable name symbol table when we start
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parsing a function (function block or program), and to clear it
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when we reach the end. Checking whether variables declared
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as EXTERN really exist inside a RESOURCE or a CONFIGURATION
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is left to stage 3 (semantic checking) where we can use the
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abstract tree itself to search for the variables (NOTE: semantic
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cheching at stage 3 has not yet been implemented, so we may yet
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end up using a symbol table too at that stage!).
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 Due to the use of the symbol tables, and special identifier
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tokens depending on the type of identifier it had previously
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been declared in the code being parsed, the syntax was slightly
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changed regarding the definition of variable names, derived
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function names, etc... FROM for e.g.:
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variable_name: identifier;
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TO
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variable_name: variable_name_token;
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 Flex first looks at the symbol tables when it finds an identifier,
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and returns the correct token corresponding to the identifier
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type in question. Only if the identifier is not currently stored
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in any symbol table, does flex return a simple identifier_token.
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 This means that the declarations of variables, functions etc...
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were changed FROM:
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function_declaration: FUNCTION derived_function_name ...
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TO
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function_declaration: FUNCTION identifier ...
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since the initial definition of derived_function_name had been
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changed FROM
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derived_function_name: identifier;
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TO
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derived_function_name: derived_function_name_token;
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 Comment 2
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 ---------
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 Since the ST and IL languages share a lot of common syntax,
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I have decided to write a single parser to handle both languages
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simultaneously. This approach has the advantage that the user
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may mix the language used in the same file, as long as each function
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is written in a single lanuage.
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 This approach also assumes that all the IL language operators are
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keywords, which means that it is not possible to define variables
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using names such as "LD", "ST", etc...
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Note that the spec does not consider these operators to be keywords,
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so it means that they should be available for variable names! On the
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other hand, all implementations of the ST and IL languages seems to
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treat them as keywords, so there is not much harm in doing the same.
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 If it ever becomes necessary to allow variables with names of IL 
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operators, either the syntax will have to be augmented, or we can 
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brake up the parser in two: one for ST and another for IL.
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/********************************/
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/* B 1.3.3 - Derived data types */
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/********************************/
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Issue 1
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=======
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According to the spec, the valid construct
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TYPE new_str_type : STRING := "hello!"; END_TYPE
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has two possible routes to type_declaration...
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Route 1:
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type_declaration: single_element_type_declaration
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single_element_type_declaration: simple_type_declaration
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simple_type_declaration: identifier ':' simple_spec_init
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simple_spec_init: simple_specification ASSIGN constant
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(shift:  identifier <- 'new_str_type')
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simple_specification: elementary_type_name
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elementary_type_name: STRING
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(shift: elementary_type_name <- STRING)
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(reduce: simple_specification <- elementary_type_name)
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(shift: constant <- "hello!")
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(reduce: simple_spec_init: simple_specification ASSIGN constant)
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(reduce: ...)
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Route 2:
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type_declaration: string_type_declaration
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string_type_declaration: identifier ':' elementary_string_type_name string_type_declaration_size string_type_declaration_init
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(shift:  identifier <- 'new_str_type')
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elementary_string_type_name: STRING
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(shift: elementary_string_type_name <- STRING)
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(shift: string_type_declaration_size <- /* empty */)
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string_type_declaration_init: ASSIGN character_string
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(shift: character_string <- "hello!")
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(reduce: string_type_declaration_init <- ASSIGN character_string)
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(reduce: string_type_declaration <- identifier ':' elementary_string_type_name string_type_declaration_size string_type_declaration_init )
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(reduce: type_declaration <- string_type_declaration)
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 At first glance it seems that removing route 1 would make
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the most sense. Unfortunately the construct 'simple_spec_init'
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shows up multiple times in other rules, so changing this construct
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would mean changing all the rules in which it appears.
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I (Mario) therefore chose to remove route 2 instead. This means
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that the above declaration gets stored in a
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simple_type_declaration_c, and not in a string_type_declaration_c
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as would be expected!
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/***********************/
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/* B 1.5.1 - Functions */
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/***********************/
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Issue 1
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=======
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 Due to reduce/reduce conflicts between identifiers
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being reduced to either a variable or an enumerator value,
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we were forced to keep a symbol table of the names
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of all declared variables. Variables are no longer
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created from simple identifier_token, but from
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variable_name_token.
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 BUT, in functions the function name may be used as
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a variable! In order to be able to parse this correctly,
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the token parser (flex) must return a variable_name_token
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when it comes across the function name, while parsing
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the function itself.
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We do this by inserting the function name into the variable
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symbol table, and having flex return a variable_name_token
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whenever it comes across it.
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When we finish parsing the function the variable name
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symbol table is cleared of all entries, and the function
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name is inserted into the library element symbol table. This
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means that from then onwards flex will return a
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derived_function_name_token whenever it comes across the
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function name.
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In order to insert the function name into the variable_name
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symbol table BEFORE the function body gets parsed, we
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need the parser to reduce a construct that contains the
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the function name. That is why we created the extra
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construct 'function_name_declaration', i.e. to force
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the parser to reduce it, before parsing the function body,
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and therefore get an oportunity to insert the function name
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into the variable name symbol table!
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/********************************/
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/* B 3.2.4 Iteration Statements */
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/********************************/
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Issue 1
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=======
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For the 'FOR' iteration loop
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  FOR control_variable ASSIGN expression TO expression BY expression DO statement_list END_FOR
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The spec declares the control variable in the syntax as
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  control_variable: identifier;
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but then defines the semantics of control_variable (Section 3.3.2.4)
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as being of an integer type (e.g., SINT, INT, or DINT).
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Obviously this presuposes that the control_variable must have been
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declared in some 'VAR .. VAR_END' construct, so I (Mario) changed
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the syntax to read
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  control_variable: variable_name;
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**************************************************************************
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265
4d222f46f8cc Updating license info (with Edouard's permission for relevant files).
Mario de Sousa <msousa@fe.up.pt>
parents: 67
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  Copyright (C) 2003-2011  Mario de Sousa (msousa@fe.up.pt)