Q48089: Description of the /Or Switch

Article: Q48089
Product(s): See article
Version(s): 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 16-JAN-1990

Question:

When I run CL with the /HELP option to get command-line help, there is
listed an optimization flag, /Or, that is supposed to disable in-line
returns. I can't find information about this switch documented
anywhere else. What does this switch really do?

Response:

The term "in-line return" is misleading; there is not a call to the
return function that is eliminated. The /Or switch affects the code
that is produced for functions lacking local variables and/or formal
parameters.

If a function takes no formal parameters and has no local variables, a
stack frame is not necessary. /Or prevents stack frame creation and
clean-up code from being produced when possible. The code for
preservation and restoration of the SI and DI registers is also
eliminated.

The typical instructions that are eliminated are as follows:

   push bp     ; This code is used on entry
   mov  bp, sp ; to establish a stack frame.
   ...
   push di     ; This code is used on entry
   push si     ; to preserve SI and DI.
   ...
   ...         ; The code to perform the function
   ...         ; would be here.
   ...
   pop  si     ; This code is used on exit to
   pop  di     ; restore SI and DI, and to
   mov  sp, bp ; clean up the established
   pop  bp     ; stack frame.

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