Decaf Programming Language Specification Summer 2019 Introduction
This is the reference manual for the Decaf programming language which is the programming language specifically for the CMPT 379: Compilers (http://anoopsarkar.github.io/compilers-class) course taught by Anoop Sarkar (http://www.cs.sfu.ca/~anoop/) at the SFU Computing Science School (Burnaby campus). For more information and other documentation, see the course web page.
Decaf is a strongly typed C-like language. The feature set is trimmed down considerably from what is usually part of a full-fledged programming language. This is done to keep the programming assignments manageable. Despite these limitations, Decaf will be able to handle interesting and non-trivial programs.
Here is an example Decaf program:
extern func print_int(int) void;
package GreatestCommonDivisor {
var a int = 10;
var b int = 20;
func main() int {
var x, y, z int;
x = a;
y = b;
z = gcd(x, y);
// print_int is part of the standard input-output library
print_int(z);
}
// function that computes the greatest common divisor
func gcd(a int, b int) int {
if (b == 0) { return(a); }
else { return( gcd(b, a % b) ); }
}
}
Notation
The syntax is specified using Extended Backus-Naur Form (http://en.wikipedia.org/wiki/Extended_Backus-Naur_Form) (EBNF):
Productions are expressions constructed from terms and the following operators, in increasing precedence:
Lower-case production names are used to identify lexical tokens. Non-terminals are in CamelCase. Lexical tokens are enclosed in double quotes or back quotes .
The form a b represents the set of characters from a through b as alternatives. The horizontal ellipsis is also used elsewhere in the spec to informally denote various enumerations or code snippets that are not further specified. The character is not a token of the Decaf language.
Source code representation
Decaf source code is encoded as ASCII text. Upper and lower case characters are considered different characters. For example, if is defined as a keyword, but IF would be considered an identifier.
ASCII table
The ASCII table and decimal equivalent for each character is shown below:
Production= production_name = [ Expression ] . .
Expression= Alternative { | Alternative } .
Alternative = Term { Term } .
Term
Group
Option
Repetition= { Expression } .
Repetition+ = { Expression }+ .
CommaList = { Expression }+, .
= production_name | token [ token ] | Group | Option | Repetition | Repetition+ | CommaList .
= ( Expression ) .
= [ Expression ] .
|alternation
() grouping
[] option (0 or 1 Expression)
{} repetition (0 to n Expressions)
{}+repetition (1 to n Expressions)
{}+, comma list (1 to n Expressions comma separated, e.g. x, y, z)
The set of valid characters in Decaf is all the ASCII characters:
Implementation restriction: For compatibility with other tools, a compiler should always disallow the nul character (decimal: 0) in the source text. Letters and Digits
The underscore character _ is considered a letter.
Lexical Elements Comments
Decaf only has line comments that start with the character sequence // and stop at the newline character. The newline character is assumed to be part of the comment. The comment representation is as follows:
Whitespace
Whitespace is used to separate tokens, and is defined as follows:
The following are special characters that are not part of white space:
Tokens
Tokens are the vocabulary of the Decaf language. There are four classes: identifiers, keywords, operators and literals. White space is ignored except as it separates tokens that would otherwise combine into a single token. For example, int3 is a single token but int 3 is two tokens, a keyword int and integer 3; and int(3) is a sequence of four tokens: int, (, 3 and ).
While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token.
Semicolons
The Decaf language uses semicolons ; as a terminator in a number of productions.
all_char = /* all ASCII characters from 7 13 and 32 126 */ .
char = /* all ASCII characters from 7 13 and 32 126 except char 10
, char 92 and char 34 */
char_lit_chars = /* all ASCII characters from 7 13 and 32 126 except char 39 and char 92 */ .
char_no_nl = /* all ASCII characters from 7 13 and 32 126 except char 10
*/ .
letter= A Z | a z | _ .
decimal_digit = 0 9 .
hex_digit = 0 9 | A F | a f .
digit = 0 9 .
// this is a line comment and it includes the newline at the end of the line
comment = // { char_no_nl }
newline = /* ASCII character nl :
*/ .
carriage_return = /* ASCII character cr : r */ .
horizontal_tab= /* ASCII character ht : t */ .
vertical_tab
form_feed
space
whitespace
= /* ASCII character vt : v */ .
= /* ASCII character np : f */ .
= /* ASCII character sp : */ .
= { newline | carriage_return | horizontal_tab | vertical_tab | form_feed | space }+ .
bell = /* ASCII character bel : a */ .
backspace= /* ASCII character bs : b */ .
0nul 1soh 2stx 3etx 4eot 5enq 6ack 7bel
8bs 9ht 10nl 11vt 12np 13cr 14so 15si 16dle 17dc1 18dc2 19dc3 20dc4 21nak 22syn 23etb 24can 25em 26sub 27esc 28fs 29gs 30rs 31us 32sp 33! 34 35# 36$ 37% 38& 39 40( 41) 42* 43+ 44, 45- 46. 47/ 480 491 502 513 524 535 546 557 568 579 58: 59; 60< 61= 62> 63? 64@ 65A 66B 67C 68D 69E 70F 71G 72H 73I 74J 75K 76L 77M 78N 79O 80P 81Q 82R 83S 84T 85U 86V 87W 88X 89Y 90Z 91[ 92 93] 94^ 95_ 96` 97a 98b 99c 100d 101e 102f 103g
104h 105i 106j 107k 108l 109m 110n 111o 112p 113q 114r 115s 116t 117u 118v 119w 120x 121y 122z 123{ 124| 125} 126~ 127del
Identifiers
Identifiers name program entities such as variables and types. An identifier is a sequence of one or more letters and digits. The first character in an identifier must be a letter.
For example:
Type and constant identifiers are predeclared.
Keywords
The following keywords are reserved and may not be used as identifiers.
Operators and Delimiters
The following character sequences represent operators (see Operators section below) and delimiters.
Integer literals
An integer literal is a sequence of digits representing an integer constant. An optional prefix sets a non-decimal base: 0x or 0X for hexadecimal. In hexadecimal literals, letters a-f and A-F represent values 10 through 15.
For example, the following are integer literals:
For integer literals, the semantics of range checking occurs later, so that a long sequence of digits such as the last example above which is clearly out of range is still scanned as a single token. The semantic analyzer will come in later and reject this lexeme value as a valid integer constant.
Character literals
A character literal represents a character constant (see Constants section), which is an integer value that identifies an ASCII eqivalent. A character literal is expressed as one or more characters enclosed in single quotes. Within the quotes, any character may appear except single quote. A single quoted character represents the ASCII value of the character itself, while multi-character sequences beginning with a backslash encode special ASCII values as escaped characters.
The simplest form represents the single character within the quotes which is equal to the integer ASCII value. For example:
After a backslash, certain single-character escapes represent special values:
identifier = letter { letter | digit } .
a
x9
_x9
ThisVariableIsInCamelCase
boolbreak continueelse externfalse
for funcifintnullpackage
returnstringtruevarvoidwhile
{}[],;()= ! + * / << >> < > % <= >= == != && || .
int_lit = decimal_lit | hex_lit .
decimal_lit = { decimal_digit }+ .
hex_lit = 0 ( x | X ) { hex_digit }+ .
42
0xBadFace
170141183460469231731687303715884105727
a // equal to ASCII value 97 stored as an integer type
a ASCII 7:
b ASCII 8:
t ASCII 9:
ASCII 10:line feed or newline
v ASCII 11:vertical tab
f ASCII 12:form feed
r ASCII 13:carriage return
\ ASCII 92:backslash
single quote
double quote
alert or bell
backspace
horizontal tab
The following is a list of escaped character codes that refer to the equivalent ASCII codes above.
The following are some legal and illegal examples of character literals:
a
t
aa // illegal: too many characters
// illegal: no characters
// illegal: invalid closing delimiter
Unterminated character literals must be reported as errors.
String literals
A string literal represents a string constant obtained from concatenating a sequence of characters (also see the Constants section below).
A string literal must start and end on a single line, it cannot be split over multiple lines. It can include escape sequences like
and this is distinct from a newline character inside the string constant.
For example, the following is legal:
But the following is not legal:
Escaped characters have to used correctly. The following string has an invalid escaped character:
You can have an single quote or escaped single quote in a string. So both of the following are valid.
Empty strings are allowed.
Unterminated string literals must be reported as errors.
Type literals
The following are the keywords used to specify Decaf types.
Boolean constant literals
The following keywords are used as constants for boolean types.
List of Tokens
The following is an alphabetically sorted list of tokens for Decaf with the token names used in the homework.
string_lit = “ { char | escaped_char } “ .
‘
int bool void string
true false
char_lit = ( char_lit_chars | escaped_char ) .
escaped_char = ( n | r | t | v | f | a | b | “ | | “ ) .
Types
Decaf has four types: void, booleans, integers and strings. String types, however, can only be used with extern functions. Void types are for return types of functions only (called MethodType below) and not used in variable declarations.
Decaf also has a limited array type for arrays of integers and booleans. Boolean types
A boolean type represents the set of Boolean truth values denoted by the predeclared constants true and false . The predeclared boolean type is bool . This is represented as the LLVM type Int1 .
Integer types
A integer type refers to the set of all signed 32 bit integers (-2147483648 to 2147483647) corresponding to the LLVM type Int32 . The predeclared integer type is int .
String types
A string type represents the set of string values. A string value is a (possibly empty) sequence of bytes. Strings are immutable: once created, it is impossible to change the contents of a string. The LLVM type that corresponds to a Decaf string type is Int8Ptr . The predeclared string type is string .
ExternType = ( string | Type ) .
Type = ( int | bool ) .
MethodType = ( void | Type ) .
BoolConstant = ( true | false ) .
T_AND
T_ASSIGN
T_BOOLTYPE
T_BREAK
T_CHARCONSTANT
T_COMMA
T_COMMENT
T_CONTINUE
T_DIV
T_DOT
T_ELSE
T_EQ
T_EXTERN
T_FALSE
T_FOR
T_FUNC
T_GEQ
T_GT
T_ID
T_IF
T_INTCONSTANT
T_INTTYPE
T_LCB
T_LEFTSHIFT
T_LEQ
T_LPAREN
T_LSB
&&
=
bool
break
char_lit (see section on Character literals)
,
comment
continue
/
.
else
==
extern
false
for
func
>=
>
identifier (see section on Identifiers)
if
int_lit (see section on Integer literals)
int
{
<<<=([<-%*!=!null||package+}returnT_LTT_MINUST_MODT_MULTT_NEQT_NOTT_NULLT_ORT_PACKAGET_PLUST_RCBT_RETURNT_RIGHTSHIFT >>
T_RPAREN )
T_RSB]
T_SEMICOLON;
T_STRINGCONSTANT string_lit (see section on String literals)
T_STRINGTYPE string
T_TRUE
T_VAR
T_VOID
T_WHILE
T_WHITESPACE
true
var
void
while
whitespace (see section on Whitespace)
Array types
Decaf has integer and boolean arrays. However, arrays are declared only in the global (package declaration) scope as part of the field declarations (see FieldDecl).
All arrays are one-dimensional and have a size that is fixed at compile-time. Arrays are indexed from 0 to n 1, where n > 0 is the size of the array. The usual bracket notation is used to index arrays. Since arrays have a compile-time fixed size and cannot be declared as method parameters (or local variables), there is no facility to query the length of an array variable in Decaf. Arrays must be initialized to all zeroes at declaration time.
Constants
Decaf has boolean constants, integer constants, and string constants. Integer constants can be created using character literals and integer literals. BoolConstant is defined (in the Types section) as either true or false .
String constants can only be used with extern functions. See the Types section for more details.
Decaf program structure Program
A Decaf program starts with optional external function declarations followed by the package definition (a Decaf package is like a module or namespace). A package has optional global variables (called field variables) followed by method (function) definitions.
External Functions
A Decaf program can access external function that are linked, such as the Decaf standard library functions which are implemented in C, and accessed from within the Decaf program as external functions. For now, only external functions are allowed. External data cannot be declared.
Global variables
Decaf has global variables with scope limited to their package that appear before any method declarations. Global variables in Decaf are called field declarations. They can be simple declarations without initialization (assumed to be zero initialized by the compiler) or non-array variables can be declared with an assignment to a constant (see Constants section). Variables are always defined using the var reserved word.
The assignment to an identifier has to be a constant:
The following is an example of an array field declaration. Notice the array type has the length before the type of the elements of the array.
Method declarations
Functions or methods in Decaf start with the reserved word func , then the name of the method and in parentheses is the argument list followed by the return type of the method.
The program must contain a declaration for a method called main that has no parameters. The return type of the method main has to be type int , however the compiler does not enforce a return statement within the main definition (just like ANSI C). Execution of a Decaf program starts at this method
main . Methods defined as part of a package can have zero or more parameters and must have a return type of type MethodType explicitly defined.
Blocks
Decaf blocks have a section for local variable definitions first followed by statements.
ArrayType = [ int_lit ] Type .
Constant = ( int_lit | char_lit | BoolConstant ) .
Program = Externs package identifier { FieldDecls MethodDecls } .
Externs= { ExternDefn } .
ExternDefn = extern func identifier ( [ { ExternType }+, ] ) MethodType ; .
FieldDecls = { FieldDecl } .
FieldDecl= var { identifier }+, Type ; .
FieldDecl= var { identifier }+, ArrayType ; .
FieldDecl= var identifier Type = Constant ; .
package foo { var a int; var b int = a; } // Invalid!
package foo { var list [100]int; } // Array declaration
MethodDecls = { MethodDecl } .
MethodDecl= func identifier ( [ { identifier Type }+, ] ) MethodType Block .
Block = { VarDecls Statements } .
Variable Declarations
Local variables are declared using the reserved word var followed by a comma separated list of variables for each type and followed by the type of the variable(s). They cannot be assigned a value when they are defined.
There is no assignment allowed for local variables:
Statements
Statements in Decaf consist of variable assignment, method calls, syntax for various kinds of control flow, special statements for breaking out of or continuing to the top of the block.
Blocks statement
Statements can also be Blocks (see section on Blocks).
Assign statement
Assignment to an Lvalue is a statement in Decaf . The location for the Lvalue can be either a scalar variable or an array location.
Method calls
External functions are declared using the extern keyword. These functions are provided at using a separate library which is linked with your Decaf program at runtime. Some minimal type checking is done using the declaration. The most useful library functions that you will use are the print_string , print_int and read_int functions.
The return value can be a type that can be assigned to an Lvalue :
In this case, the integer variable z receives the result of calling the read_int library function. The return value can also be declared to be void in which case assigning the output of a library function to an Lvalue will result in a semantic error.
If statement
While statement
For statement
The for loop in Decaf has the usual structure for ( init ; check ; post ) followed by the Block of the for loop.
The init, check and post parts of the for loop cannot be empty:
Return statement
The following are all valid return statements:
VarDecls = { VarDecl } .
VarDecl= var { identifier }+, Type ; .
func foo() int { var a int = 10; } // Invalid!
Statements = { Statement } .
Statement = Block .
Statement = Assign ; .
Assign= Lvalue = Expr .
Lvalue= identifier | identifier [ Expr ] .
Statement= MethodCall ; .
MethodCall = identifier ( [ { MethodArg }+, ] ) .
MethodArg= Expr | string_lit .
z = read_int();
Statement = if ( Expr ) Block [ else Block ] .
Statement =while ( Expr ) Block .
Statement = for ( { Assign }+, ; Expr ; { Assign }+, ) Block .
for(; a < b; ) // Invalid! Statement = return [ “(” [ Expr ] “)” ] “;” .Break statementA break statement terminates execution of the innermost for or while loop (branches to end of loop).Continue statementA continue statement begins the next iteration of the innermost for or while loop at its post statement (see For statement).Expressions OperandsOperands are the elementary values in an expression.Unary OperatorsThere are only two unary operators in Decaf. One for logical negation and the other for unary minus of arithmetic expressions. The result of UnaryNot is of type bool and the result of UnaryMinus is of type int .Binary OperatorsBinary operators are split into boolean binary operators and arithmetic binary operators. The result of using a boolean operator is the type bool and the result of using an arithmetic operator is the type int .The boolean connectives && and || are interpreted using short circuit evaluation (http://en.wikipedia.org/wiki/Short-circuit_evaluation). This means: the second operand is not evaluated if the result of the first operand determines the value of the whole expression. For example, if the result is false for && ortrue for || .Binary % computes the modulus of two numbers. Given two operands of type int , a and b : If b is positive, then a % b is a minus the largest multiple of b that is not greater than a . If b is negative, then a % b is a minus the smallest multiple of b that is not less than a (in this case the result will be lessthan or equal to zero).Operators and PrecedenceUnary operators have the highest precedence. For the other binary operators the precedence is defined as follows. All operators at the same precedence level get equal precedence. All operators with equal precedence associate left. The UnaryMinus operator associates to the right.Precedence Operator7 UnaryMinus 6 UnaryNot5 */%<<>> 4+-
3 ==!=<<=>>= 2 &&
1 ||
Primary expressions
Primary expressions build larger expressions from operands, operators and parentheses. The parentheses are used to group expressions to obtain different orders of evaluation. Parentheses can be omitted if the desired evaluation is consistent with the precedence rules (see the Operators and Precedence section). The type of the Expr on the left hand side is determined by the Expr on the right hand side and the operator used.
Statement = break ; .
Statement = continue ; .
Expr = identifier .
Expr = MethodCall .
Expr = Constant .
UnaryOperator = ( UnaryNot | UnaryMinus ) .
UnaryNot = ! .
UnaryMinus = – .
BinaryOperator = ( ArithmeticOperator | BooleanOperator ) .
ArithmeticOperator = ( + | – | * | / | <<” | “>> | % ) .
BooleanOperator = ( == | != | <” | “<=” | “> | >= | && | || ) .
return(3);
return(b); // where b was declared as var b bool;
return();
return;
Index expression
In this expression, the identifier must be an Array Type (see section on Array Types). The Expr is evaluated to give an array index and the result of the evaluation must be of type int . The integer value is then used to find the element of the array which is of type int or bool depending on the Array Type.
Decaf grammar
The entire set of rules that describe the Decaf grammar specification is collected in one place below. For explanation of each of the rules read the descriptions provided in the above sections.
Expr = identifier [ Expr ] .
Program = Externs package identifier { FieldDecls MethodDecls } .
Externs= { ExternDefn } .
ExternDefn = extern func identifier ( [ { ExternType }+, ] ) MethodType ; .
FieldDecls = { FieldDecl } .
FieldDecl= var { identifier }+, Type ; .
FieldDecl= var { identifier }+, ArrayType ; .
FieldDecl= var identifier Type = Constant ; .
MethodDecls = { MethodDecl } .
MethodDecl= func identifier ( [ { identifier Type }+, ] ) MethodType Block .
Block = { VarDecls Statements } .
VarDecls = { VarDecl } .
VarDecl= var { identifier }+, Type ; .
Statements = { Statement } .
Statement = Block .
Statement = Assign ; .
Assign= Lvalue = Expr .
Lvalue= identifier | identifier [ Expr ] .
Statement= MethodCall ; .
MethodCall = identifier ( [ { MethodArg }+, ] ) .
MethodArg= Expr | string_lit .
Statement = if ( Expr ) Block [ else Block ] .
Statement =while ( Expr ) Block .
Statement = for ( { Assign }+, ; Expr ; { Assign }+, ) Block .
Statement = return [ ( [ Expr ] ) ] ; .
Statement = break ; .
Statement = continue ; .
Expr = identifier .
Expr = MethodCall .
Expr = Constant .
UnaryOperator = ( UnaryNot | UnaryMinus ) .
UnaryNot = ! .
UnaryMinus = – .
BinaryOperator = ( ArithmeticOperator | BooleanOperator ) .
ArithmeticOperator = ( + | – | * | / | <<” | “>> | % ) .
BooleanOperator = ( == | != | <” | “<=” | “> | >= | && | || ) .
Expr = Expr BinaryOperator Expr .
Expr = UnaryOperator Expr .
Expr = ( Expr ) .
Expr = identifier [ Expr ] .
ExternType = ( string | Type ) .
Type = ( int | bool ) .
MethodType = ( void | Type ) .
BoolConstant = ( true | false ) .
ArrayType = [ int_lit ] Type .
Constant = ( int_lit | char_lit | BoolConstant ) .
Decaf Semantics Program Structure
A method called main has to exist in the Decaf program.
Type Checking
Make sure the following type checks are implemented in the compiler.
Binary + * / % >> << < > <= >= and unary only work on integer expressions. Binary && || and unary ! only work on boolean expressions.
Binary == != work on any type, but both operands have to have the same type.
Assignment to a function parameter is valid and should change the value as for a local variable The && and || operators are short-circuiting (this is already specified in the spec)
Expr = Expr BinaryOperator Expr .
Expr = UnaryOperator Expr .
Expr = ( Expr ) .
If you have multiple return statements in one block then only the first is used, but the others should still be type checked.
Indexing a scalar is a semantic error. { var x int; x[0] = 1; } is a semantic error, and { var x,y int; y = x[0]; } is a semantic error, and the same if x is a field variable.
Indexing with a bool is a semantic error. { var xs[10] int; func main() int { var x int; x = xs[true]; } is a semantic error. Usinganon-boolexpressionforaloopconditionisasemanticerror. {while(1){}} and {varxint;for(x=0;1;x=x+1){}} are semantic errors.
Using a non-bool expression in an if statement condition is a semantic error. { if (0) {} } is a semantic error.
A return statement with an expression is not allowed in function with void return type. { func foo() void { return (1); } and
{ func bar() void {} func foo() void { return (bar()); } are both semantic errors.
A return statement with no expression in a non-void function produces an undefined return value.
{ func foo() int { return; } func main() int { var x int; x = foo(); } } is a not a semantic error, but the value of x may be anything (typically zero due to zero initialization).
Cannot use a void function in an expression. func foo() void {} func main() int { if (foo()) {} } is invalid.
Cannot call a method with the wrong number of arguments.
Find all cases where there is a type mismatch between the definition of the type of a variable and a value assigned to that variable. e.g. bool x; x = 10; is an example of a type mismatch.
Find all cases where an expression is well-formed, where binary and unary operators are distinguished from relational and equality operators. e.g. true + false is an example of a mismatch but true != true is not a mismatch.
Check that all variables are defined in the proper scope before they are used as an lvalue or rvalue in a Decaf program.
Check that the return statement in a method matches the return type in the method definition. e.g. func foo() bool { return(10); } is an example of a mismatch.
Scoping Rules
This section clarifies the behaviour with scoping.
Having two fields with the same name is a semantic error.
Having two methods with the same name is a semantic error.
Having a field and a method with the same name is a semantic error.
externs count as methods for scoping.
Having two local variables with the same name declared at the same block is a semantic error. { var x int; var x int; } is an error, but
{varxint;{varxint;}} isok.
Having a local variable in the outer block of a method that has a parameter with the same name is a semantic error.
funcfoo(xint)void{varxint;} isanerror,but funcfoo(xint)void{{varxint;}} isok. A function can be referred to anywhere in the program, including before its definition.
package C { func foo() void { bar() }; func bar() void {}; /* */ } is ok.
Functions, fields, arguments, and local variables all share the same namespace (symbol table) and can shadow each other except for the above rules. e.g. in package C { func foo() void {}; func bar() void { var foo int; foo(); } /* */ } the foo in foo(); refers to the local int variable, not the function resulting in an error.
break and continue only apply to the innermost containing loop. Using break or continue outside of a loop results in a semantic error.
Statements
These are semantic errors that can occur when using statements in Decaf.
There are no restrictions on the type of main but a return statement inside main must match the return type of main . Always emitting int as the return type for main is safe.
Assigning a scalar to an array is considered a type mismatch.
The following produce undefined behaviour, but must not produce compile time semantic errors:
Using the value of any uninitialized scalar variable or array element
A function with no return statement is equivalent to ending the function with a return statement that has no expression: return; Assigning to an array cell at an invalid index
Any bool argument to a integer parameter must be converted while keeping its value, not just for print_int .
Passing a argument to a function parameter with a different type is a semantic error except for the special case of passing a bool as an int . Declaring an array of size less than or equal to zero is a semantic error.
Assignment to a function parameter is valid and should change the value as for a local variable.
Last updated June 17, 2019.
Forked from the JHU MT class code on github (https://github.com/mt-class/jhu) by Matt Post (https://github.com/mjpost) and Adam Lopez (https://github.com/alopez).
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