This document discusses representing scope information in compilers. It involves storing variable names along with an indicator of the block in which they appear in the symbol table. This allows the same variable name to be used in different blocks. For FORTRAN, each name has a scope limited to the end of the routine. The symbol table stores only names external to the routine and of common blocks. A hashing scheme is proposed to store external names at the beginning of chains and append internal names to the end, reusing storage when a routine ends.

1. NADAR SARASWATHI COLLEGE OF ARTS&SCIENCE,THENI
DEPARTMENT OF COMPUTER SCIENCE&INFORMATION TECHNOLOGY
R.RAMYA DEVI
I-MSC(CS)

2. COMPILER DESIGN
TOPIC: REPRESENTING SCOPE
INFORMATION,FORTRAN

3. REPRESENTING SCOPE INFORMATION
Representing scope information is a concept
in the scope of each variable name is
preserved in the symbol table so That we can
use the same name in different blocks and
different locations.

4. Representing Scope Information involves:
 A lifetime of a variable in a particular block.
 Representing name in symbol table along
with an indicator of the block in which it
appears.

5. Representing Scope Information involves:
 Suppose we have a variable name 'a' in block A and
the same variable in block B. Suppose we store 'a' in
symbol table without block information. In that se, it
will only keep the first instance of 'a' which it
encounters, hence to overcome this problem names
are stored in the form of pair (variable name, block
name) so that the same name can be used in
different blocks and procedures.
 Scope Representation reflects the visibility of the
variable name in the source program.

6. Representing Scope Information in
FORTRAN:
 FORTRAN program consists of the main
program, subroutines, and functions.
 Each name has a scope consisting of one
routine, i.e., the variable scope is limited to
the end of the routine.

7. Representing Scope Information in
FORTRAN:
 It can generate object code for each
routine upon reaching the end of the routine
and hence the names stored in the symbol
table for variables belonging to that particular
routine need to be eliminated.
 Hence we need to store only names that
are external to routine and also of the
common block in the symbol table.

8. CONSIDER A HASHING SCHEME, AS SHOWN BELOW

9. CONSIDER A HASHING SCHEME, AS SHOWN BELOW
 In each chain, names that are external to the
current routine will appear first.
 It can append new internal names to the end
and new external names to the beginning.

10. CONSIDER A HASHING SCHEME, AS SHOWN BELOW
 It is clear from the above diagram hash value
has selected name 2, and hence 2 iscpart of
the current routine, and Name 1 is external to
it, that’s why Name 1 is written in the
beginning and Name 2 is appended at the
end.
 When we reach the end of the routine and
generate object code for it, we reset the
pointer to available storage in the reusable
table but not a permanent one.

11. CONSIDER A HASHING SCHEME, AS SHOWN BELOW
 All pointer that is from the permanent table to
the reusable table is set to nill, and the hash
table will directly point to the reusable table.
 We can also reuse the storage space used
for names in the following way. We will use
location in name link storage areas to
represent the name, and such a location can
describe different routines since that area is
reused.