3.22 Segment Attribute COMDAT

With segment attribute COMDAT one may define COMDAT sections. A COMDAT section is a section that can be defined by more than one object file. A COMDAT section must contain at least one symbol, the COMDAT symbol. The syntax to define the section is:

 
    segname SEGMENT COMDAT( selection [, assoc_segment ] ) ...

The selection argument tells the linker what to do if multiple definitions of a COMDAT symbol are found; the accepted values are:

Value		Description
1	no duplicates	If the symbol is already defined, the linker issues a "multiply defined symbol" error.
2	any	Any section that defines the same COMDAT symbol can be linked; the rest are removed.
3	same size	The linker chooses an arbitrary section among the definitions for this symbol. If all definitions are not the same size, a "multiply defined symbol" error is issued.
4	exact match	The linker chooses an arbitrary section among the definitions for this symbol. If all definitions do not match exactly, a "multiply defined symbol" error is issued.
5	associative	The section is linked if a certain other COMDAT section ( see the assoc_segment in the syntax description ) is linked.
6	largest	The linker chooses the largest definition from among all of the definitions for this symbol. If multiple definitions have this size, the choice between them is arbitrary.

Currently support for COMDAT is restricted to COFF.

JWasm won't do anything special with COMDAT sections; in future releases this may change: cmdline options similar to the Microsoft VC compiler options -Gf or -Gy may be added.

To create an object module that places each function in its own COMDAT section ( as it is done by MS VC if the -Gy option is given ), it is recommended to use COMDAT in conjunction with ALIAS:

 
    _TEXT_proc1 segment flat comdat(1) alias(".text")
    proc1 proc c
        ...
    proc1 endp
    _TEXT1_proc1 ends

    _TEXT_proc2 segment flat comdat(1) alias(".text")
    proc2 proc c private
        ...
    proc2 endp
    _TEXT2_proc2 ends

    _TEXT_proc3 segment flat comdat(1) alias(".text")
    proc3 proc c
        ...
    proc3 endp
    _TEXT_proc3 ends