Static checking and symbol table

1

• Static checking and symbol table
• Chapter 6, Chapter 7.6 and Chapter 8.2
• Static checking check whether the program
  follows both the syntactic and semantic
  conventions at compile time (versus dynamic
  checking -- check at run time).
• Examples of static checking
• Type checks
• Flow of control checks

int a, b10, c a b c
main int I . I break
2

• Examples of static checks
• uniqueness check
• defined before use
• name related check
• Some checks can only be done at runtime
• Array-bound checking in java
• ai 0

main() int i, j double i, j .
main() int i i1 0 .
LOOPA LOOP EXIT WHEN IN
II1 END LOOP LOOPB
3

• To perform static checks, semantic information
  must be recorded in some place -- symbol table.
• Grammar specifies the syntax, additional
  (semantic) information, sometimes called
  attributes, must be recorded in symbol table for
  all identifiers.
• Typically attributes in a symbol table entry
  include type and offset (where in the memory can
  I find this variable?).
• Struct int id int type int offset stentry
• Organization of a symbol table
• basic requirement must be able to find the
  information associated with a symbol (identifier)
  quickly.
• Example array, link list, hash table.
• Provides two functions enter(table, name, type,
  offset) and lookup(name)

4

• Dealing with nested scope
• How to organize the symbol table?
• How to do lookup and enter?
• One symbol table for each scope (procedure,
  blocks)?
• Maintain a stack of symbol tables for lookup/enter

Program sort(input, output) var a array
0..10 of integers x integer
procedure readarray var x real
begin . x . End procedure quicksort(i, j)
begin x end begin x end
main() int a, b a 0 int
a a 1 printf(a d\n,
a)
5

• Symbol tables for sort
• nil header
• a ...
• x ...
• readarray
• quicksort

Symbol table for sort
header x .
header
Symbol table for quicksort
Symbol table for readarray
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• How does the compiler created the symbol table?
• First let us consider the simple case no nested
  scope, every thing entered into one symbol table
  table by using
• enter (table, id, type, offset)
• grammar
• P -gtD
• D -gtD D
• D -gtid T
• T -gt integer
• T -gtreal
• T -gtarray num of T
• T -gtT

I array 10 of integer j real k integer
I array(10, integer) 0 j real
40 k integer
48
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• P -gt offset 0 D
• D -gtD D
• D -gtid T enter(table, id.name, T.type,
  offset)
• offset offset T.width
• T -gt integer T.type integer T.width 4
• T -gtreal T.type real T.width 8
• T -gtarray num of T1 T.type array(num.val,
  T1.type)
• T.width
  num.val T1.width
• T -gtT1 T.type pointer(T1.type) T.width 4

8

• Now consider the case when you have nested
  procedures (blocks can be considered as special
  procedures)
• must maintain a stack of symbol tables, create
  new ones when entering new procedure
• must reset offset when entering new procedures (a
  stack of offsets)
• Let us also compute the total size of a table
• Grammar
• P-gtD
• D -gtD D
• D-gtid T
• D-gtproc id D S
• T -gtinteger real arraynum of T T

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• mktable(previous) make a new table, properly set
  all links and related information.
• Enter(table, name, type, offset).
• Addwidth(table, width) compute all memory needed
  by the symbol table.
• Enterproc(table, name, newtable) enter the
  procedure name with its symbol table into the old
  table.
• Grammar
• P-gttmktable(nil) push(t, tblptr)push(0,
  offset)D
• addwidth(top(tblptr), top(offset))
• D -gtD D
• D-gtid T enter(top(tblptr), id.name, T.type,
  top(offset))
• top(offset) top(offset)
  T.width
• D-gtproc id tmktable(top(tblptr))push(t,
  tblptr) push(0, offset)D S t
  top(tblptr)addwidth(t, top(offset))
  pop(tblptr) pop(offset)enterproc(top(tblptr),
  id.name, t)

10

• Dealing with structure (record)
• T -gtrecord D end
• Make a new symbol table for all the fields in the
  record.

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• T-gtrecord
• tmktable(nil)
• push(t, tblptr)
• push(0, offset)
• D end
• T.type record(top(tblptr))
• T.width top(offset)
• pop(tblptr)
• pop(offset)

12

• Question
• How does allowing variable declaration at
  anywhere in a program (like in C, java) affect
  the maintenance of the symbol tables?

13

• Type checking
• Make sure operations can be performed on
  operands.
• Make sure the types of actual arguments matches
  the types of formal arguments.
• Need a type system to do the job.
• A type system is a collection of rules for
  assigning type expression to the various parts of
  a program.
• The type system for a practical language can be
  complicated.
• Type checking of expressions
• P-gtDE
• D-gtDD id T
• T-gtchar integer arraynum of T T
• E-gtliteral num id E mod E EE E

14

• P-gtDE
• D-gtDD
• D-gtid T enter(id.val, T.type)
• T-gtchar T.type char integer T.type
  integer
• arraynum of T1 T.type array(num.val,
  T1.type)
• T1 T.type pointer(T1.type)
• E-gtliteral E.type char
• num E.type integer
• id E.type lookup(id.val)
• E1 mod E2
• if E1.type integer E2.type
  integer then E.type integer
• 
  else E.type
  error
• E1E2
• if E1.type array(s, t) E2.type
  integer then E.type t
• 
  else E.type
  error
• E1 if E1.type pointer(t) then E.type
  t else E.type error

15

• Type checking for statements
• S -gt id E
• S -gt if E then S1
• S -gtwhile E do S1
• S-gtS1S2

16

• Type checking for statements
• S -gt id E
• if id.type E.type then S.type void
• else S.type
  error
• S -gt if E then S1
• if E.type boolean then S.type
  S1.type
• else
  S.type error
• S -gtwhile E do S1
• if E.type boolean then S.type
  S1.type
• else
  S.type error
• S-gtS1S2
• if S1.type void and S2.type void
  then
• S.type void
• else S.type type_error

17

• Type checking for functions
• T-gtT1-gtT2 / function declaration /
• T.type T1.type -gtT2.type
• E-gtE1(E2) / function call /
• if E1.type t1.type-gtt2.type E2.type
  t1.type then
• T.type t2.type
• else T.type - error

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• Equivalence of type expressions
• Name equivalence - each type with different name
  is different
• structural equivalence - names are replaced by
  the type expressions they define
• Example
• type link cell
• var next link
• last link
• p cell
• Is structural equivalence good for C?

19

• Other things related to type.
• coercion implicit type conversion
• e.g. double x .x 1
• overloading
• a function or operator can represent different
  operations in different contexts.
• polymorphic functions
• the body of a polymorphic function can be
  executed with arguments of different types.