joey paquet, 2000, 20021 lecture 10 introduction to code generation and intermediate representations
TRANSCRIPT
Joey Paquet, 2000, 2002 1
Lecture 10
Introduction to Code Generationand
Intermediate Representations
Joey Paquet, 2000, 2002 2
Introduction to Code Generation
• Front end: – Lexical Analysis– Syntactic Analysis– Intermediate Code Generation
• Back end: – Intermediate Code Optimization– Object Code Generation
• The front end is machine-independent, i.e. it can be reused to build compilers for different architectures
• The back end is machine-dependent, i.e. these steps are related to the nature of the assembly or machine language of the target architecture
Joey Paquet, 2000, 2002 3
Introduction to Code Generation• After syntactic analysis, we have a number of options to
choose from:– generate object code directly from the parse– generate intermediate code, and then generate object
code from it– generate an intermediate abstract representation, and
then generate code directly from it– generate an intermediate abstract representation,
generate intermediate code, and then the object code
• All these options have one thing in common: they are all based on syntactic information gathered in the semantic analysis
Joey Paquet, 2000, 2002 4
Introduction to Code Generation
SyntacticAnalyzer
ObjectCode
SyntacticAnalyzer
IntermediateRepresentation
ObjectCode
Lexical Analyzer
Lexical Analyzer
Lexical Analyzer
SyntacticAnalyzer
IntermediateRepresentation
IntermediateCode
ObjectCode
SyntacticAnalyzer
IntermediateCode
ObjectCode
Lexical Analyzer
Front End Back End
Joey Paquet, 2000, 2002 5
Interm. Representations & Code• Intermediate representations synthetize
the syntactic information gathered during the parse, generally in the form of a tree or directed graph.
• Intermediate representations enable high-level code optimization.
• Intermediate code is a low-level coded (text) representation of the program, directly translatable to object code.
• Intermediate code enables low-level, architecture-dependent optimizations.
Joey Paquet, 2000, 2002 6
Part I
Intermediate Representations
Joey Paquet, 2000, 2002 7
Abstract Syntax Trees• Each node represents the application of a rule in the grammar• A subtree is created only after the complete parsing of a right
hand side• Pointers to subtrees are sent up and grafted as upper subtrees
are completed• Parse trees (concrete syntax trees) emphasize the
grammatical structure of the program• Abstract syntax trees emphasize the actual computations
to be performed. They do not refer to the actual non-terminals defined in the grammar, hence their name.
Joey Paquet, 2000, 2002 8
Parse vs Abstract Syntax Trees
x = a*b+a*b
x
+
=
a b* a b*
E
A
E
E
Parse Tree
x
a b a b
*
=
*
+
x = a*b+a*b
Abstract Syntax Tree
Joey Paquet, 2000, 2002 9
Directed Acyclic Graphs (DAG)
• Directed acyclic graphs (DAG) are a relative of syntax trees: they are used to show the syntactic structure of valid programs in the form of a “tree”.
• In DAGs, the nodes for repeated variables and expressions are merged into a single node.
• DAGs are more complicated to build than syntax trees, but directly implements lots of code optimization by avoiding redundant operations.
Joey Paquet, 2000, 2002 10
AST vs DAG
x
a b a b
*
=
*
+
x = a*b+a*b
Abstract Syntax Tree
x
=
a b
*
+
x = a*b+a*b
Directed Acyclic Graph
Joey Paquet, 2000, 2002 11
Postfix Notation• Every expression is rewritten with its operators at the
end, e.g.:
• Easy to generate from a bottom-up parse• Can be generated from a syntax tree using postorder
traversal
a+b ab+a+b*c abc*+if A then B else C ABC?If A then if B then C else D else E ABCD?E?x=a*b+a*b ab*ab*+x=
Joey Paquet, 2000, 2002 12
Postfix Notation• Its nature allows it to be naturally evaluated with the
use of a stack• Operands are pushed onto the stack; operators pop the
right amount of operands from the stack, do the operation, then push the result back onto the stack.
• However, this notation is restricted to simple expressions such as in arithmetics where every rule conveys an operation
• It cannot be used for the expression of most programming languages constructs
Joey Paquet, 2000, 2002 13
Three-Address Code
• Three-address codes (3AC) is an intermediate language that maps directly to assembly code, but that is not architecture-dependent
• It breaks the program into short statements requiring no more than three variables and no more than one operator, e.g:
x = a+b*c t := b*cx := a+t
source 3AC
Joey Paquet, 2000, 2002 14
Three-Address Code• The temporary variables are
generated at compile time and added to the symbol table
• In the generated code, the variables will refer to actual memory cells. Their address is also stored in the symbol table
• 3AC can also be represented as quadruples, which are even more related to assembly languages
t := b*c L 3,bM 3,cST 3,t
x := a+t L 3,aA 3,tST 3,x
3AC ASM
t := b*c MULT t,b,c x := a+t ADD x,a,t
3AC Quadruples
Joey Paquet, 2000, 2002 15
Intermediate Languages• In this case, we generate code in a language for which
we already have a compiler or interpreter• Such languages are generally very low-level and
dedicated to the compiler construction task• It provides the compiler writer with a “virtual machine”• Various compilers can be built using the same virtual
machine• The virtual machine compiler can be compiled on
different machines to provide a translator to various architectures.
• For the project, we have the moon compiler, which
provides a virtual assembly language and a compiler.
Joey Paquet, 2000, 2002 16
Project Overview
SyntacticAnalyzer
Lexical Analyzer
MoonCode
TokenStream
MoonCompiler
ObjectCode
SourceCode
• Your compiler generates Moon code• The Moon compiler (virtual machine) is used to
generate an exectuable for your program• Your compiler is thus retargetable by
recompilation of the moon compiler on a new processor
Joey Paquet, 2000, 2002 17
Part II
Semantic Actions and
Code Generation
Joey Paquet, 2000, 2002 18
Semantic Actions• Semantics is about giving a meaning to the compiled
program.• Semantic actions have two parts:
– Semantic checking: check if the compiled program has a meaning, e.g variables are declared, operator and function have the right parameter types and number of parameters upon calling
– Semantic translation: translate declarations, statements and expressions to machine code
• Semantic translation is conditional to semantic checking
Joey Paquet, 2000, 2002 19
Semantic Actions• Semantic actions are inserted in the grammar (thus
transforming it in an attribute grammar)– In recursive descent parsers, they are represented by
function calls imbedded in the parsing functions– In table-driven top-down parsers, they are represented by
functions pushed on the stack along with the right hand sides they belong to
• Most semantic actions use attributes for their resolution:– In recursive descent parsers, they are migrated using
reference parameter passing– In table-driven top-down parsers, they are migrated using
a semantic stack
Joey Paquet, 2000, 2002 20
Semantic Actions• There are semantic actions associated with:
– Declarations: • variable declarations• type declarations• function declarations
– Control structures: • conditional statements• loop statements
– Expressions: • assignment operations• arithmetic and logical expressions
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Processing Declarations• In processing declarations, the only semantic checking
there is to do is to ensure that every object (e.g. variable, type, class, function, etc.) is declared once and only once
• This restriction is tested using the symbol table entries• Symbol table entries are generated as declarations are
encountered• Afterwards, every time an identifier is encountered, a
check is made in the symbol table to ensure that it has been properly defined
Joey Paquet, 2000, 2002 22
Processing Declarations• Code generation in declarations comes in the form of
memory allocation for the objects defined• Every object defined, no matter its type, will eventually
have to be stored in the computer’s memory• Memory allocation must be done according to the size
of the objects defined, which depends on the target machine
• For each identifier declared, you must generate a label that will be used to refer to that variable in the ASM code and store it in the location field of its entry in the symbol table
• See the Moon machine description documentation for more explanations specific to the project
Joey Paquet, 2000, 2002 23
Processing Variable Declarations• <varDecl> <type><id>; {varDeclSem}
– An entry is created in the corresponding symbol table. Memory space is reserved for the variable according to the size of the type of the variable and linked to a label in the ASM code
– The starting address (or its label) is stored in the symbol table entry of the variable. In the case of arrays, the offset of (size of the elements) is often stored in the symbol table record
• <varDecl> <type><idList>; {varDeclSem}– To generate each entry, (one for each element in the list),
the compiler must keep track of the type of the declaration. This is an attribute that is migrated using a technique appropriate to the parsing method used
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Processing Type Declarations• Most programming languages allow the definition of
types that aggregates of the basic types defined in the language
• There are typically arrays or record types, or even abstract data types (or classes) in object-oriented programming languages
• <typeDecl> <type><id> is <typeDef>; {typeDeclSem}– An entry is created in the symbol table for the new type
defined. It contains a definition (e.g. size) of all the elements of the new type
– This information is used when new objects of that type are declared in the program, and to compute the offset when arrays of elements of that type are created
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Type Compatibility• In Pascal:
– A,B: array (1..10) of integer;– C,D: array (1..10) of integer;
• this defines two data types: – type Type1 is array (1..10) of integer;– A,B: Type1– type Typ2 is array (1..10) of integer;– C,D: Type2
• Here, Type1 and Type2 are clearly two distinct data types, e.g. A := D is not permitted in the program
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Type Compatibility• Some compilers use some rules to define type
equivalence. One of the first compilers to implement this was the Algol68 compiler
• Advantage: – gives more flexibility to the language
• Drawbacks: – compiler is much more complicated to implement– compiler will hardly distinguish between equivalent types
Joey Paquet, 2000, 2002 27
Processing Arrays• Static arrays are arrays with static size defined at
compile time• Most programming languages allow only integer litterals
for the initialization of array size, or constant integer variables when available in the language
• Pascal: A: array (1..10) if integer• C: int A[10];
orconst size=10;
int A[size];
Joey Paquet, 2000, 2002 28
Processing Arrays• This restriction comes from the fact that the memory
allocated to the array has to be set at compile time, and is fixed throughout the execution of the program
• When processing an array declaration, a sufficient amount of memory is allocated to the variable depending on the size of the elements and the cardinality of the array
• Only the starting address (or a label) is stored in the symbol table. The offset (the size of the array in memory) is also sometimes stored in the symbol table record to avoid referring of elements outside the bounds
• Dynamic arrays are generally implemented using pointers, dynamic memory allocation functions and an execution stack or heap
Joey Paquet, 2000, 2002 29
Processing Expressions• Semantic records contain the type and location for
variables (normally labels in the ASM code) or the type and value for constant factors
• Semantic records are created at the leafs of the tree when factors (F) are recognized, and then passed upwards in the tree
• These semantic records contain the attributes that are migrated within the tree to find a global result for the symbol on top of the tree for that expression
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Processing Expressions• As new nodes (or subtrees) are created going up in the
tree, intermediate results are stored in temporary semantic records containing subresults for subexpressions
• Each time an operator node is resolved, its corresponding semantic checking and translation is done and its subresult is stored in a temporary variable for which you have to allocate some memory and generate a label
• You can even put an entry in the symbol table for each intermediate result you generated. You can then use these for further reference, e.g. for debugging
Joey Paquet, 2000, 2002 31
Processing Expressions• Doing so, the code is generated sequentially as the tree
is traversed:
a
b c
+
*
x
=t1 = b*c L 3,b
M 3,cST 3,t1
t2 = a+t1 L 3,aA 3,t1ST 3,t2
x = t2 L 3,t2ST 3,x
subtree ASM
Joey Paquet, 2000, 2002 32
Conclusions
• Most compilers build an intermediate representation of the parsed program, normally as an abstract syntax tree.
• These will allow high-level optimizations to occur before the code is generated.
• In the project, we are outputting MOON code, which is an intermediate language.
• MOON code could be the subject of low-level optimizations.
Joey Paquet, 2000, 2002 33
Conclusions
• Semantic actions are composed of a semantic checking, and a semantic translation part.
• Semantic actions are inserted at appropriate places in the grammar to achieve the semantic checking and transaltion phase.
• Semantic translation is conditional to semantic checking.
Joey Paquet, 2000, 2002 34
Conclusions
• There are semantic actions for: – Declarations (variables, functions, types,
etc)– Expressions (arithmetic, logic, etc)– Control structures (loops, conditions, etc)