Efficient use of symbolic constants

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DimonSoft

Joined: 03 Mar 2010
Posts: 1197
Location: Belarus

DimonSoft 12 Dec 2013, 21:14

I have a symbolic constant, its value is affected by several different macros. Generally they use the constant to store temporary list. Thus its value changes this way:

Code:

Item1
Item1 Item2
Item1 Item2 Item3
<Empty>
Item4
Item4 Item5
<Empty>
etc…

The use of the symbolic constant expands to sth like this (pseudocode):

Code:

MyConst equ
MyConst equ Item1
MyConst equ MyConst Item2
MyConst equ MyConst Item3
MyConst equ
MyConst equ Item4
MyConst equ MyConst Item5
MyConst equ
etc…

Since you can use restore, I guess, some kind of stack is maintained by FASM, which can grow quite large. In my case I don't need the previous values of the constant and I'd better decrease the amount of memory used by the preprocessor since the lists can become pretty large.

What is the preferred way of symbolic constant usage for such cases?

12 Dec 2013, 21:14

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 13 Dec 2013, 03:17

DimonSoft,

That depends. If you simply need to redefine symbolic constant, you may use somethine like this:

Code:

struc reequ [val] {
common
  restore .
  . equ val
}

When val can refer to symbol being redefined, match should be used to expand it beforehand:

Code:

struc reEqu [val] {
common
  match _val, val \{ . reequ _val \}
  match , val \{ . reequ \}
}

These macros work well for building intra-macros symbolics; to maintain lists you may find several implementations around here.

Another way is to use symbolics' stack directly: put a sentinel value in advance, then simply use equ to push values; pop them with restore.

13 Dec 2013, 03:17

l_inc

Joined: 23 Oct 2009
Posts: 881

l_inc 13 Dec 2013, 09:44

DimonSoft

Quote:

In my case I don't need the previous values of the constant and I'd better decrease the amount of memory used by the preprocessor

I completely understand your need to clean up, and I also try to clean up after myself, but it empirically turns out, that you cannot decrease the memory consumption by restoring the values of symbolic constants. I'm not aware of whether fasm frees any stack memory, but even if it does the very usage of any additional directive in the source increases the memory consumption, just because fasm does a straightforward textual expansion of macros.

Thus to sum up reequ provided by baldr is a nice macro and I use it extensively, but every time you invoke an additional restore you make fasm consume more memory than you could free from the symbolic constant stack.

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Faith is a superposition of knowledge and fallacy

13 Dec 2013, 09:44

l_inc

Joined: 23 Oct 2009
Posts: 881

l_inc 13 Dec 2013, 09:55

baldr

Quote:

When val can refer to symbol being redefined, match should be used to expand it beforehand

This one is a bit unfortunate because of the constant expanding behaviour of match. In order to have a no-strings-attached-additional-variable, you could either use irp {common } or replace your reequ with an analogous redefine instead. I prefer to use the following way:

Code:

struc reequ [val]
{
        common
                tmp equ val
                        restore .
                        . equ tmp
                restore tmp
}

It does not look that nice and cannot be rewritten into a single line, but it also consumes a bit less memory.

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Faith is a superposition of knowledge and fallacy

13 Dec 2013, 09:55

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