Extended Pascal to C++ converter
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1. 2 2 char c 4 1 J sizeof T6 6 bytes Any field that requires more than 2 bytes is always located at an offset which is a multiple to four from the start of the record Example 1 4 struct T4 byte offset byte size char a 0 1 int b 4 char c 8 1 sizeof T7 12 bytes Distinction 3 packed data types M Pascal No field will be allocated starting in other than bit 0 of a byte if this allocation would cause the field to extend into the next byte Example 1 5 T5 packed record a b c d e 0 7 end 19 bits sizeof T5 3 bytes The field c e is entirely contained in the second third byte GCC Bit fields of some types e g short can be allocated in two bytes in the same word Example 1 6 struct T6 short a 3 short b 3 short c 3 short d 3 short e 3 15 bits sizeof T6 2 bytes Extended Pascal to C converter 119 Two bits of c are allocated in the first byte the third bit in the second one Distinction 4 packed data types M Pascal A packed record or array of the size less than 8 bits can be allocated with other fields in the same byte Example 1 7 TO packed array 0 3 of boolean 4 bits T7 packed record bit offset bit size 32 0 7 0 3 b TO 3 4 end sizeof T7 1 byte GCC C always allocates the class array in a separate byte starting in no other bit than 0 it does not allow specifying any bi2. bit offset bit size a boolean 0 1 CASE b boolean OF 1 1 TRUE c 0 9 2 6 d a z 8 7 FALSE e A z 1 8 7 end sizeof T9 2 bytes can be converted into the C class class T9 bit offset bit size boolean a 1 0 1 boolean b 1 1 1 union struct unsigned char c 6 8 6 unsigned char d 7 16 7 14 unsigned char e 7 8 T A access methods sizeof T9 3 bytes The necessity to preserve data layout and initialization features essen tially influences the natural translation scheme shown above 3 3 1 Data Layout Preservation for records without variant part The size of M Pascal type T1 is equal to 7 bytes the size of T1 in C is 12 bytes With the use of GCC compiler extension __attribute_ packed attached to an enum struct or union type definition it specifies that the minimum required memory will be used to represent the type it is possible to get the following T1 representation Extended Pascal to C converter 129 class T1 byte offset byte size unsigned char a 0 1 int b 1 unsigned char c 5 1 access methods __attribute packed sizeof T1 6 bytes It is just enough to add unnamed field to preserve data layout class T1 byte offset byte size unsigned char a 0 1 char 8 1 1 int b 2 5 unsigned char c 6 1 access methods _
3. a b short outer ta b a 1 b 2 return inner a b This translation scheme apparently is the most obvious conversion re sulting in a desirable functionality A little less obvious are the consequences of such a conversion e The type declared in the scope of an upper level function requires moving it to the global level if it is used at least in one nested function or it is used as a template parameter in some type declarations e g the type of an array element see Arrays This can easily result in a name conflict at the global level if two or more nested types were of the same name and were both brought to the global level Conflict is avoided by the use of automatic unique names for types moved or by manual modification of the source Pascal code The latter appears to be more acceptable with the restrictions on readability of the target C code e Nested constants moved out as in the case of types can result in the name conflict which is avoided by the same methods as above An alternative would be passing the constants as additional parameters of constant reference type as compared to variable passing Extended Pascal to C converter 135 5 Conditional compilation 5 1 Translation scheme M Pascal provides several language features not to be found in the stan dard Pascal One of them which is reguired to be kept at translation is conditional compilation To arrange Conditional Compilation M Pascal
4. bad because parser does not know if the second CCC is alternative to the first one or it is a new CCC declaration successive CCCs It may be solved for some cases but in the general case especially in expressions it is impossible grammatical ambiguity Let us consider the following grammatical rules variable declaration identifier list type denoter initial value list identifier list identifier identifier 138 V A Markin S V Maslov A A Sulimov initial value list constant initial value initial value initial value expression index expression expression conditional statement if statement case statement if statement IF Boolean expression THEN statement ORIF Boolean expression THEN statement else part else part ELSE statement case statement CASE case index OF case list element case list element OTHERWISE statement sequence END 2 In each rule all non terminals and grammar seguences can be divided into iterative and non iterative Iterative non terminals grammatical sequences are enclosed in curly braces Examples of iterative non terminals grammar sequences are identifier in identifier list ORIF Boolean expression THEN statement grammar sequence case list element Assertion e If two or more Conditional Compilation Clauses are alternative and contain program fragme
5. class Set with the third template parameter equal to the real packed set size preserves data layout for this type template lt size_t LOW size_t HIGH gt class PackedBitSet public Set lt LOW HIGH HIGH 1 gt public typedef Set lt LOW HIGH HIGH 1 gt Base BitSet Js Bound checking is carried out just as in the case of unpacked sets Since the packed and unpacked sets are inherited from the same base class Set they can be mixed in any set operations assignment relations difference union intersection Initialization is handled similarly in both packed and unpacked sets Let us consider initialization of unpacked sets for simplicity 3 2 3 Set initialization Any M Pascal set can be initialized by a set of values or by an interval range of values Correspondingly two auxiliary initializing classes are in troduced define ENDSET 1 class SetValues public Set lt 0 MAX_SET_SIZE MAX_SET_SIZE gt public SetValues int first class SetRange public Set lt 0 MAX_SET_SIZE MAX_SET_SIZE gt public SetRange size_t from size_t to F The class SetValues is used to create an object of the type Set and has a constructor with a variable number of parameters this is possible because the Pascal sets cannot contain negative elements and the value ENDSET 1 Extended Pascal to C converter 127 can be used as the end of an initializing list The class SetRange likewise is used to create t
6. generation A so called global CCC stack is introduced A pointer to the parse tree of a conditional expression is pushed onto the stack when encountering an if directive The pointer is being popped out as soon as an end directive is encountered During construction of a semantic named object corresponding to some named object of the source program variable constant etc not IF node the current state of the CCC stack is being copied to a special storage inside of this named object Thus for every named object we obtain a set of conditions CCC Set under which this object was declared In the above example two objects of the variable A are created The CCC Set of the first object contains expr1 and of the second one expr2 All the objects with the same name A in our example declared on the same scope are collected into one list A scope descriptor contains a pointer to the head of this list Now to set a correspondence between a name and an existing object in the current scope the converter searches for the pointer to the list of objects with the name specified and then if the list is found searches for the object with a CCC Set corresponding to the set of conditions in the global CCC stack its current state It is theoretically possible that an object has several declarations under different CCC directives but is used outside of any CCC statement an empty CCC stack In this case the converter issues a warning and
7. new_type value get T10 var rec T10 true INIT short 1 INIT char 0 INIT boolean true 4 Nested functions and procedures In contrast to C M Pascal allows us to declare nested functions and procedures Though some C compilers permit nested functions GCC extension it is really hasty to rely on extensions of clever implementa tions For simplicity only nested functions will be considered all further discourses are correct for procedures as a special case of functions Let us consider the following example FUNCTION outer INTEGER TYPE t INTEGER VAR a b t FUNCTION inner t BEGIN inner inner atb ENDFUNC end inner BEGIN outer a 1 b 2 outer inner ENDFUNC end outer Three translation schemes of conversion are possible 134 V A Markin S V Maslov A A Sulimov e de nesting moving the nested functions to the global level e usage of C namespaces for preserving the nested structure e simulation of nested functions by nested data structures In this paper we will consider only the first schema the others are described in 1 4 1 De nesting functions All nested functions are moved to the global level file scope as required by the C standard All the variables used in nested functions and not declared at the file scope are passed to these functions by reference as addi tional input parameters typedef short t t inner t amp a t Gb return
8. the general case a record can contain a field of type SET which has a constructor it comes out that this record cannot be initialized by a list of initializers being of a non aggregate type The following example illustrates this struct Set Set class Record Set field J r Set error non aggregates cannot be initialized with initializer list Since we are initializing C classes which correspond to the M Pascal records by constructors all the fields of these classes must have a construc tor at least a default one That is for every structured type in M Pascal a C class is introduced All such classes have the following properties e Internal representation of data is closed e A convenient and similar to the original interface is provided i e the assignment operators for all types arithmetic operations for sets and so on e Special constructors are defined to handle the initialization This approach to initialization gives us a guarantee that local variables not initialized explicitly would be implicitly initialized by zeroes by call of a default constructor which is a subject to be defined in every such class 3 Translation schemes 3 1 Arrays Arrays of M Pascal are not different from those of the standard Pascal neither syntax nor semantics is changed It was decided to translate M Pascal arrays to C classes on account of the following see Section 2 as well e the
9. Joint NCC amp IIS Bull Comp Science 18 2002 115 142 2002 NCC Publisher Extended Pascal to C converter V A Markin S V Maslov A A Sulimov Introduction The methods of compiler and converter as a special case development are sufficiently investigated and described 2 3 There are a number of con verters 4 6 from the standard Pascal language 7 to C C languages 8 In this work some translation schemes that are of theoretical interest in the authors opinion elaborated for a converter from an essential Pascal extension later M Pascal to C are described The converter has been developed by order of a large telecommunication company and it must not only translate input M Pascal code to functionally equivalent C code but also meet some requirements M Pascal language extensions and re quirements for the converter have essentially influenced the development of translation schemes In this work the following extensions of M Pascal are considered Variable initialization which is syntactically close to the initialization in the C language but has slightly different semantics e Conditional Compilation which differs from the conditional compila tion in C C in the program constants usage and absence of the preprocessor Statements or declarations can be copied from a library by using the include option e The strict ordering of definitions in the standard Pascal has been weak ened e I
10. RC station 1 Data layout preservation One of the main requirements to the converter is data layout preservation DLP This term means the following e preservation of the size of each data item including every component of its decomposition e preservation of data items locations up to bits in the memory this includes alignment issues and overlapping in variant records Alignment is an extension of the memory allocated for all values of a data type up to the boundary of some unit of memory byte word 2 bytes or double word 4 bytes Depending on the processor data layout can be such that e the most significant bit can be either the highest or the lowest bit in a byte e the most significant byte can be either at the highest or at the lowest address in a word Extended Pascal to C converter 117 The implementation of DLP allows ensured compatibility of modules of composite software hardware systems when some components are being modified re engineered for example when some modules written in one program language are translated into another language or one part of hard ware is replaced by another Further in this part we describe the main distinctions in data layout and alignment between the M Pascal compiler on M68000 platform and the GCC GNU compiler on SPARC Station Distinction 1 unpacked data types M Pascal the boolean type is allocated in a byte GCC the standard bool type is allocated in 4 by
11. _arg Marker boolean break default break case false f va_arg Marker uchar break default va_end Marker Within this approach the beginning of the parameter list relates to the fix part fields and to the tag field Further all parameters are treated by C mechanism of working with the variable number of parameters The use of this constructor requires that initial values for all elements of a record Extended Pascal to C converter 133 are to be specified explicitly in the constructor call For this purpose each incomplete initial list in the source code is extended by the default values of corresponding elements during translation to C The approach described above just as in the case of an array initial ization uses the mechanism of passing the variable number of parameters va_arg macro whose behavior is undefined when passing objects of non POD types according to the C language standard In the general case the record can contain any number of fields of such types In order to elim inate the dependence on a C compiler implementation the scheme of translation can be slightly changed It is proposed to pass not the initializ ing objects themselves but the pointers to them As the pointer type is the POD type and has a fixed size 4 byte such changes can resolve the problem with macro va_arg In this case the constructor call looks like follows define INIT type value auto_ptr lt type gt
12. _attribute__ packed sizeof T1 7 bytes The packed records without variant parts are similarly translated 3 3 2 Data Layout Preservation for the records with variant part From Example 3 3 2 one can see that the sizes of the type T9 in M Pascal and in C are different In this case __attribute__ packed does not help us since the fields of non integral types in C are allocated in separate bytes see DLP Section One of the possible solutions is to move the UNION construction to the external level class T9 union bit offset bit size struct boolean a 1 0 1 boolean b 1 1 1 x unsigned char c 6 2 6 unsigned char d 7 8 7 __attribute__ packed struct boolean 1 0 1 boolean 1 1 1 unsigned char 6 2 6 unsigned char e 7 8 7 J attribute packed access methods sizeof T9 2 bytes 130 V A Markin S V Maslov A A Sulimov Some remarks __attribute__ packed should be used exactly as shown the number of unnamed fields in structures can be reduced by increas ing their size e g in the second structure three unnamed fields can be replaced by one char e there are no nested unions all enclosed variant parts are transferred to the top level If in a target hardware the most significant bit has a number different from that in a source one the fields contained in each byte should be enu merated in inverse order to pres
13. _n lt 1 1 __n __BITS_PER_WORDT 1 __BITS_PER_WORDT template lt size_t LOW size_t HIGH size_t SIZE MAX_SET_SIZE gt class Set private unsigned char storage __BITSET_WORDS SIZE public Interface F 3 2 1 Unpacked sets The size of any unpacked M Pascal set is equal to 32 bytes as it contains 256 elements The class BitSet represents all unpacked sets and is inherited from the base class Set with the third template parameter instantiated by 255 0 is included in the set template lt size_t LOW size_t HIGH gt class BitSet public Set lt LOW HIGH MAX_SET_SIZE gt public typedef Set lt LOW HIGH MAX_SET_SIZE gt Base BitSet 3 In the general case only a part of memory reserved for the BitSet object 32 bytes is used indeed The low and high bound checking is carried out dynamically using first two template parameters LOW and HIGH 3 2 2 Packed sets Unlike the case of unpacked sets the memory size necessary to keep all el ements of a packed set depends on the real set bounds or to be precise 126 V A Markin S V Maslov A A Sulimov on its upper bound HIGH The M Pascal compiler does not use the in formation about the lower bound when determining the size of a set That is in the general case similar to unpacked sets memory is used partially The size of an unpacked set varies from 1 to 32 bytes according to its upper bound The class PackedBitSet inherited from the
14. anslation schemes may be found in 1 References 1 Markin V A Maslov S V Novikov R M Sulimov A A Extended Pascal to C converter Novosibirsk 2001 Prep SB RAS IIS N 92 2 Aho A V Ullman J D The Theory of Parsing Translation and Compiling Vol 1 Parsing N J Prentice Hall Englewood Cliffs 1972 3 Aho A V Sethi R Ullman J D Compilers Principles Techniques and Tools Addison Wesley Reading Mass 1988 4 Kasyanov V N Pottosin I V Methods of Compiler Development Novosi birsk Nauka 1986 5 GNU Pascal To C converter http www ibiblio org pub Linux devel lang pascal ptoc_3 34 tar gz 6 FreeBSD Pascal To C converter ftp ftp freebsd org pub FreeBSD ports i386 packages 4 stable lang ptoc 3 50 tgz 7 MSC Pascal To C C converter http lib buryatia ru cgi bin win SOFTWARE ptoc README txt 8 Jensen K Wirth N Pascal User Manual and Reports Springer Verlag 1978 142 V A Markin S V Maslov A A Sulimov 9 Program language C standard ANSI ISO IEC 14882 http www techstreet com cgi bin detail product_id 49964
15. chooses the first found object In our example if there are no CCC directives around the call statement of the procedure proc the converter produces a warning on this line because it cannot find any object with the name A and empty CCC Set However it will continue assuming that nothing depends on the type of A In the general case this may lead to erroneous under some conditions set output code It should be recommended that a named object be used under the same set of conditions as it is declared In our example it must be something like the following Extended Pascal to C converter 141 if expri var A integer 1 hend if expr2 var A longint 2 hend if expri proc A end routine call with object A 1 if expr2 proc A end routine call with object A 2 The approach described allows us to realize all the theoretical issues of CCC translation and to translate the majority of the source M Pascal code containing CCC directives 6 Concluding remarks We have described some translation schemes from extended Pascal to C which are of the most interest in our opinion The schemes satisfy the requirements which are essential for preservation of functionality of a large software hardware system after replacing a program language and or a part of hardware We tried to show our approaches to the development of these schemes and we hope that this may be useful for other developers More details and other tr
16. erve the bit order in a byte So by using e __attribute__ packed when a record has a field whose alignment dif fers in M Pascal and C for example field b in the type T1 or e additional empty fields char n n lt 8 for byte alignment and char 8 for word alignment see types T6 and T9 it is possible to preserve data layout for almost all records except for ones considered in item 4 of Section 1 see examples 1 7 and 1 8 3 3 3 DLP for the records with fields of non integral types The translation scheme described does not preserve data layout for packed records similar to the type T7 i e for records containing a field of a packed record or array of size less than 8 bits allocated in the same byte with other fields see Example 1 7 To translate such records we shall treat the non integral types of fields as integral types For example the bit field b in the type T7 is defined as follows class T7 bit offset bit size char a 3 O bit 3 bit char b 4 3 bit 4 bit J5 sizeof R 1 bytes All we have to do now is to implement some interface inside the class T7 which will allow us to access the field b as it were of packed array type For this purpose we shall use an auxiliary class BitRef the same one we used to access an element of a packed array class see Arrays An object of the type BitRef is used to extract the value of the bit object being referenced at the Extended Pascal to C c
17. esulting parse tree of the expression is then being attached to the definite lexeme and after that to the object of an Intermediate Repre sentation IR of a source program Later at the code generation stage for every object in the IR the code generator checks if there were any com ments or CCC comments attached to the object and output them properly into the target code The other task arising during construction of the IR of the source pro gram is to set up a correct correspondence between named objects in the IR variable constants etc Let us consider the example if expri var A integer 1 hend if expr2 var A longint 2 hend proc A any routine call During construction of IR for this source program the following should be emphasized e Two named objects variables with the same name A are being created Having the same name they differ in their types as a matter 140 V A Markin S V Maslov A A Sulimov of fact Each of the two objects is represented by a node IR Variable A declaration node To preserve the semantics of the routine s input parameters the only one A should be considered out of two existing A s For example in the call of proc it is necessary to know the exact type of A to generate an appropriate typecasting operator in the target C code The following technical solutions are proposed to achieve the proper con struction of IR and successful code
18. expression any text endif 136 V A Markin S V Maslov A A Sulimov 5 2 Translated and not translated CCC Directives of the conditional compilation are external constructions of the source language but not members of the input grammar However for preser vation of CCCs in the target code it is necessary to parse all CCCs and to build their internal representation The extension of the source language grammar considering an arbitrary arrangement of CCCs is impossible inside CCCs there can be enclosed not only a complete syntactical con struction derived from a non terminal but also any sequence of terminals It is possible to extend grammar if we agree to restrict the form of construc tions enclosed in CCCs but in our case the source code would need a lot of changes In our approach we slightly restrict the use of CCCs but the grammar is left unchanged All CCCs are divided into two types translated and not translated One can manually change tt not translated CCCs so that they become translated Definition 5 2 1 Let us call a simple CCC the one of the kind if boolean_expression any text end 1 program fragment without CCC Definition 5 2 2 In CCCs of the kind if boolean_expressioni any text1 hend 2 if boolean expression2 any text2 hend program fragment without CCC any_texti and any_text2 are alternative to each other if they are derived from the same non terminal or grammatica
19. has directives if and end hif boolean_expression any text end Any program code and other directives may be enclosed within these two directives Let us call the pair of conditional compilation directives and a program code between them Conditional Compilation Clauses CCC In addition there is a set directive set identifier expression which is used to assign a constant value to a compile time identifier The differ ence between M Pascal conditional compilation and C C one is in the following e there is no preprocessor in the compiler of M Pascal the compiler calculates a boolean expression and compiles or not the code inside CCC e as a result in expressions of CCC both the CCC variables and pro gram constants including constants of enumerated types can be used For example const a 1 hif a a is a program constant set b 10 b is a CCC variable set a true a is a new CCC variable hend The M Pascal conditional compilation directives are translated directly into the C ones The set directive is translated into define directive set debug TRUE gt define debug_ccc TRUE The postfix _ccc will be used to distinguish program constants from CCC variables since the preprocessor having executed define directives would otherwise replace the program constants The directives if and Zend are translated accordingly into if and endif directives if C _boolean_
20. he temporary set which contains the elements of the range Thus during initialization of a set temporary objects of the type Set Values or SetRange are created first After that a temporary set object is being constructed as a union of the constructed temporary set objects and passed as a parameter to the BitSet constructor template lt size_t LOW size_t HIGH gt class BitSet public Set lt LOW HIGH MAX SET SIZE gt public typedef Set lt LOW HIGH MAX SET SIZE gt Base BitSet BitSet const Set lt 0 MAX SET SIZE MAX SET SIZE gt amp other 3 To correctly initialize M Pascal sets to empty the set the default con structor of the BitSet class is redefined Let us consider an example of set initialization TYPE T SET OF 1 10 VAR V T 1 3 5 VAR Vi PACKED SET OF char translated to typedef BitSet lt 1 10 gt T T V SetRange 1 3 SetValues 5 ENDSET PackedBitSet lt 0 255 gt V1 default constructor is called 3 3 Records Taking into account the encapsulation request M Pascal records are natu rally translated into C classes with using UNION construction for variant parts Example 3 3 1 see Example 1 5 M Pascal record type T1 T1 record a char b longint c char end can be converted into the C class 128 V A Markin S V Maslov A A Sulimov class T1 unsigned char a int b unsigned char c access methods Example 3 3 2 Type T9 T9 packed record
21. l sequence in the same grammatical construction or any_text1 and any_text2 are declarations and they declare the same identifier Definition 5 2 3 Two CCCs of kind 2 where any_text1 and any_text2 are alternative are called CCCs with alternative CCCsA Two CCCs in which any_text1 and any_text2 are not alternative are called successive simple CCCs Example 5 2 1 IF MODE DEBUG table_infor ARRAY debug_files OF select_record ZEND Extended Pascal to C converter 137 IF MODE RELEASE table infor ARRAY release files OF select record END These CCCs are CCCsA because they declare the same identifier table_infor Example 5 2 2 table_infor IF MODE DEBUG ist CCC ARRAY debug_files OF select_record END IF MODE RELEASE 2nd CCC ARRAY debug_files OF select_record END These CCCs are CCCsA because they are derived from the same non terminal type denoter in the same grammatical construction variable declaration see below Example 5 2 3 IF MODE DEBUG wait_time EXTERNAL DEFINE relative_time 0 0 6 0 END IF MODE MAIN help_pid process_id END These CCCs are two successive simple CCCs because they declare dif ferent identifiers The main problem is to recognize if any_text1 and any_text2 in 2 are alternative to each other or not It is not a problem to translate the CCCs similar to Example 5 2 1 and Example 5 2 3 Example 5 2 3 is
22. ntegers are represented by two different predefined types integer and longint The most important requirements to the converter are the following e Data Layout Preservation binary correspondence between M Pascal and C data types e preservation of Conditional Compilation directives 116 V A Markin S V Maslov A A Sulimov e textual similarity of M Pascal and C codes e g textual representa tion of array index expressions the ranges and index types in M Pascal differ from those in C and preservation of copy and comparison operations of structured types Moreover the following is desirable e encapsulation capability of hiding some aspects of data representa tion For example the M Pascal record fields become private members in C classes and one has access to them through methods In Section 1 a conception of Data Layout Preservation DLP is pre sented Section 2 describes the general approaches to initialization Section 3 is an overview of the translation schemes of the most interesting aggre gate M Pascal types records arrays and sets In Section 4 the translation schemes of nested procedures in M Pascal are given In Section 5 the trans lation scheme of conditional compilation directives is described the sets of translated and non translated Conditional Compilation Clauses CCC are defined All the examples are concerned with the M Pascal compiler on M68000 and the GCC GNU compiler on SPA
23. nts derived from iterative non terminals gram matical sequences then they can be translated into C CCCs suc cessfully e Any simple CCC can be translated into C CCC successfully e CCCs meeting the syntactic property or property and only them are valid for translation Example 5 2 4 The CCCs with alternative CCSsA if cond_expr1 IF expression1 THEN END if cond_expr2 IF expression2 THEN END statement sequence ENDIF do not meet property by its definition and because they contain program fragments derived from non iterative grammatical seguences Extended Pascal to C converter 139 Example 5 2 5 The CCCs with alternative hif DEBUG mls key hend if not DEBUG mls key hend meet properties because they contain program fragments derived from iterative non terminal initial value 5 3 CCC implementation In the approach proposed the initial M Pascal grammar is not being ex tended to provide the analysis of the source M Pascal code with arbitrary insertion of CCCs Instead to preserve the layout of the source code con ditional compilation directives are treated as comments and attached to definite lexemes of the source program While an ordinary comment does not need any parsing to be performed on it at the time of analysis of a CCC comment an additional parser run This parser creates an internal form of an expression contained in the CCC The r
24. onverter 131 time of its BitRef construction All the modifications are carried out over the temporary BitRef object which keeps a copy of the actual bit object After that at the time of destruction the actual bit object is being updated by value from BitRef s copy In this particular case we will access the field of type PackedArray lt boolean 1 1 3 gt which starts in the first byte at the offset of 3 bits class T7 bit offset bit size char a 3 O bit 3 bit char b 4 3 bit 4 bit public BitRef lt TO 3 gt BQ return BitRef lt TO 3 gt char this 3 F sizeof R 1 bytes Now the class T7 preserves the data layout of the original record of the type T7 The method B is used to access field b of type char as it were of the packed array T0 This technique may be either automated or performed manually depending on the number of occurrences of these bad types in the source code 3 3 4 Record initialization As discussed in Section 2 the initialization of variables of class types cor responding to records is carried out through the constructor calls For this purpose in a C class two constructors are introduced e The default constructor for the case without an initializing list It initializes all data by zero just as in M Pascal The initializing constructor with a variable number of parameters It receives the number of parameters known for each particular record and related
25. re are no Packed Arrays in C language e there is a requirement to preserve data layout in arrays see DLP Section e as distinct from C there is a bound checking mechanism for the M Pascal arrays 122 V A Markin S V Maslov A A Sulimov e the target C code would preserve the text notation of array index expressions e it is necessary to preserve the assignment operator and the comparison operation for M Pascal arrays The template C class which models M Pascal arrays is defined as follows template lt class T long L long H bool is word aligned gt class Array uchar storage H L 1 is_word_aligned 44 sizeof T 2 sizeof T 1 sizeof T public T amp operator long index const T amp operator long index const bool operator const Array amp other const bool operator const Array amp other const Array amp operator const Array amp other F Here the template parameters are e Class T is the array s element type e L and H are lower and upper array bounds correspondingly e is_word_aligned is set to TRUE if the array element is word aligned and the element type is of odd size see Example 1 2 3 1 1 Packed arrays An array with the element size more than 8 bits is not packed If the element size is smaller than one byte then it occupies the minimal amount of bits divisible by exponent of 2 required to contain values of the array element For example the 3 bi
26. se classes e T does not have virtual functions Therefore only classic C structures can be initialized by the list of ini tializers Moreover according to the standard of C 8 only global scope objects are initialized by zeros while an initial value of the nested scope objects is undefined if not specified explicitly The syntax of the list of initializers in M Pascal and C is much alike which is an essential argument for the constructor less approach However there are a number of notes which make such a scheme impracticable e The presence of the following structured types in M Pascal variant records sets arrays strings and sub strings and the demands to preserve Data Layout semantics and appearance of code force us to use C classes to simulate the types mentioned Furthermore it would be desirable to secure the internal data of these classes making it private this would conflict with the second re quirement on using the list of initializers e There is no possibility to specify an interval range in a list of ini tializers in C This means that initialization of sets i e VAR a SET of char a z will require a special constructor to be de fined this would conflict with the first requirement on using the list of initializers Extended Pascal to C converter 121 e M Pascal records are naturally converted into C classes structures Taking into account that in
27. t 256 Both approaches do not conflict with each other and are implemented within one template class Array 3 2 Sets A set type is by no means a new type in M Pascal but it is still worthy to describe the translation scheme for this type because of the global require ments Data Layout Preservation see Section 1 and initialization handling TYPE T SET OF char A set is a well known data container therefore there are many differ ent implementations of it Probably the most famous of them is the tem plate class std bitset from Standard Template Library STL As there is no proper mechanism to handle Pascal like initialization and STL library is highly system dependent some bitset implementations restrict the size of a set to be a multiple to four bytes a new class is proposed to simulate M Pascal sets Extended Pascal to C converter 125 The base class for all set implementations is the template class Set whose parameters are lower and upper set bounds and its size is determined by M Pascal data layout rules see below about packed and unpacked sets The set itself is represented by a byte array storage data of the corresponding size with bit addressing organized Besides all set operations union inter section difference set comparison and element membership checking are implemented within the Set class define MAX SET SIZE 255 define __BITS_PER_WORDT 8 sizeof char define __BITSET_WORDS __n _
28. t field for a field of type class array or their part Example 1 8 boolean T01 3 struct T8 bit offset bit size short a 3 0 3 TO1 b 8 4 sizeof T8 2 bytes Translation schemes have been developed taking into account these data layout differences 2 Initialization M Pascal In contrast to the standard Pascal variables in M Pascal can be initialized at the time of their declaration that is an initial value can be specified explicitly for variables of any type at any scope of declaration global scope or procedure function scope For example VAR A integer 1 built in data type B RECORD record with variant part ID one two three CASE Boolean OF True name STRING 10 False 120 V A Markin S V Maslov A A Sulimov ENDREC one true ONE C ARRAY 1 5 OF char a b c array D PACKED SET OF 1 10 1 3 5 set It should be noticed that the objects both global and local not initialized explicitly will be set at zero implicitly by the M Pascal compiler C There are two major possibilities of objects initialization in C using an object class constructor and using a list of initializers this feature is rather an inheritance of C Generally an object of class T may be initialized using the list of initializers if only e T does not define constructors e all members of T are public e T does not have ba
29. t size element occupies 4 bits the 5 bit size element 8 bits The signature of the C class for packed arrays is the following template lt class T unsigned long B long L long H unsigned long W B 1 1 B 2 2 B lt 4 4 8 gt class PackedArray uchar storage H L 1 W48 1 H L 1 W 8 H L 1 W 8 public BitRef lt T B gt operator long index Extended Pascal to C converter 123 bool operator const PackedArray amp other const bool operator const PackedArray amp other const Because of impossibility to address a bit within a byte in a packed array an auxilary BitRef class was introduced template lt class T unsigned long W gt class BitRef protected unsigned char byte a pointer to the byte unsigned long bit bit position within the byte public BitRef unsigned char byte unsigned long bit BitRef amp operator const T amp value BitRef amp operator const BitRef amp value The BitRef class contains a pointer to the byte which contains the ele ment of the array and the bit offset of the element inside of this byte An access to the elements of packed arrays is implemented through the interface of this class 3 1 2 Array initialization There are two approaches to implementation of class constructors to initial ize the array e a constructor with a variable number of parameters e a constructor that accepts an object of an auxiliary ini
30. tes For DLP of the boolean type typedef unsigned char boolean n is used Distinction 2 unpacked data types M Pascal Any field that requires more than a byte integer 2 bytes longint 4 bytes pointer 4 bytes set 32 bytes the enumerated types with the number of members more than 256 is always allocated at an even offset in bytes from the start of the record Example 1 1 T1 record byte offset byte size a char 0 1 b longint 2 4 c char 6 1 end sizeof T1 7 bytes Array record is word aligned the word aligned means 2 byte aligned in this paper but the size of a record can have an odd number of bytes i e word alignment of a record type is executed only if this type is used in other record or array types and only if it is not the last field in record types Example 1 2 T2 record a integer b char end sizeof T2 3 bytes T21 record a b T2 end sizeof T21 7 bytes T22 array 1 2 of T1 sizeof T22 8 bytes C The fields of classes are allocated in successive bytes from low to high memory addresses and follow the same alignment and size rules as their corresponding types require 118 V A Markin S V Maslov A A Sulimov Any field that reguires more than one byte and less than or egual to 2 bytes e g short is always located at an even offset from the start of the class Example 1 3 struct T3 byte offset byte size char a 0 1 short b
31. tializing class Let us consider an example TYPE A array 0 5 of integer var V A 128 256 With the first approach the declaration of a constructor looks like Array int num_of_params where num_of_params is the number of elements in the initializing list which in the general case is not equal to the number of array elements In this case the remaining elements are initialized by zeros A V 2 128 256 124 V A Markin S V Maslov A A Sulimov This approach uses the mechanism of working with a variable number of parameters refer to macro va_arg According to the latest standard of C the behavior of the mechanism is undefined if complex types non POD types 8 are passed through ellipsis That is such a behavior of a constructor highly depends on a particular C compiler used The second approach removes this restriction by using a temporary ob ject for initialization This object simulates an input stream for a variable number of initial values of array elements Once the temporary object is filled with the initializers it is passed to a constructor of Array For these purposes a supplementary interface is added to the class Array class Array I typedef Init lt T 0 L H is_word_aligned gt Init public Init operator lt lt const T amp elem Array const Init amp ar MI teed F In this case the array initialization in C looks like AV A lt lt 128 lt l
32. to the fixed part fields in the record After that it receives all the other parameters through ellipsis to initialize the fields of variant parts of the record 3 3 5 Records without variant part M Pascal allows initialization of a part of a record an initial list can con tain a number of values less than it is necessary to initialize all the elements in the record For example in the following initialization of a variable of type T1 only the fields a and b can be initialized VAR var_rec T1 q 10 132 V A Markin S V Maslov A A Sulimov To properly handle initialization in the target C code it is necessary to initialize the rest fields by their default values in the constructor definition class T1 Ti unsigned char a 0 int b_ 0 unsigned char c_ 0 T1 var_rec q 10 3 3 6 Records with variant part As the amount of fields and their types in variant parts can differ the initializing constructor for variant part would receive the variable parameters number Let us consider the init constructor for the type T10 T10 boolean variant_a va_list Marker va start Marker variant a a variant a tag value receiving switch variant a according to the tag value the number and the order of receiving of the last parameters is determined in switch operator case true b va_arg Marker short c va_arg Marker uchar switch c case 0 d va
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