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flat assembler > Main > [fasmg] 1f/1b type discardable labels?

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Akeo



Joined: 19 Aug 2016
Posts: 8
Location: Ireland
Maybe this is something that has been answered before (couldn't seem to find it when performing a search), but with some assemblers (e.g. gas), you can use short discardable labels within branches, of the form "<number>[f|b]" such as "1f" or "2b", that work as follows:

1. When the assembler encounters a '#f' label in a branch instruction, with # being a numeric value, it automatically looks forward (hence the 'f') for the next '#' numeric label in the assembly, and generate a branch to it
2. Likewise, when the assembler encounters a '#b' label in a branch instruction, it automatically looks backward for the last '#' numeric label and branch to it.

This means you can do stuff like the following, instead of having to come with more innovative ways to individually label stuff, which can be super convenient:
Code:
1: ... ; <section 0> 2: ... ; <section 1> tst <some condition1> bge 1f ; -> section 2 bra 1b ; -> section 0 1: ... ; <section 2> tst <some condition2> beq 1f ; -> section 3 bgt 2f ; -> section 4 bra 2b ; -> section 1 1: ... ; <section 3> 2: ... ; <section 4>
So I'm wondering how the same could be achieved in fasmg. I guess one could create a whole set of elements '1f', '2f', '3f', ..., '1b', 2b', '3b', ... with metadata that could resolve to a label (though not very convenient - what if someone wants to use '1234f' as a discardable branch label?), but then handling the labels themselves seems a bit tricky... So I'm wondering if someone has already looked into doing something like this.

Part of me also thinks that, since it's really super convenient, that it would be nice to have such a feature as some kind of native macro of fasmg (like 'sizeof' or 'scale'), that would resolve to the relevant address and that we could just call on when processing branch instructions...
Post 22 Sep 2016, 14:01
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15870
Location: 162173 Ryugu
Use the dot (.):
Code:
function: ;..; .loop: ;<--- make a label named function.loop ;... jnz .loop ;branch to label named function.loop
Post 22 Sep 2016, 14:05
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15870
Location: 162173 Ryugu
Oh, I missed the second question. for temporary forward and backward labels use @@, @f and @b:
Code:
@@: jmp @f jmp @b @@:
Post 22 Sep 2016, 14:08
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6873
Location: Kraków, Poland
A simple macro that implements @@/@f/@b into fasmg is in the "Basic macros for fasmg" thread.

The same mechanic can be used to implement more levels (like @f1, @f2, etc.), or a separate sets of symbols could be created in parallel.

PS The name '1f' cannot be used for a symbol name, because it is a number, a floating-point one, to be exact.
Post 22 Sep 2016, 14:39
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Akeo



Joined: 19 Aug 2016
Posts: 8
Location: Ireland
Aha, so that's called an "anonymous label". I guess I should have checked the basic macros thread first.

Thanks for the replies!
Post 22 Sep 2016, 15:15
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6873
Location: Kraków, Poland
I'm going to present two possible ways of extending these basic macros. The fact that there are many "competing standards" (and that over the years people used to ask for various variants for fasm) was the main reason why I decided to leave it up to macros. And for non-tool-generated source codes the namespaces (like what revolution proposed in his first post) are in my opinion superior.

First method is to keep anonymous labels always named @@, but to allow jumping not only to the nearest ones with @b and @f, but also a bit further - for example @b2 and @f2 would to a second one in given direction. My sample defines it only up to @b3/@f3, and these definitions could be automated with "repeat" loop, but I kept it this simple so it is easier to visually parse what it does:
Code:
define @f @f1 define @ff @f2 define @b @b1 define @bb @b2 macro @@ tail match label, @f1? label tail end match local anonymous @b3? reequ @b2 @b2? reequ @b1 @b1? reequ @f1 @f1? reequ @f2 @f2? reequ @f3 @f3? reequ anonymous end macro struc dummy end struc repeat 3 @@ dummy end repeat purge dummy
This variant is used like this:
Code:
jmp @f2 ; -> 2 @@: ; 1 jmp @b ; -> 1 jmp @f2 ; -> 3 @@: ; 2 jmp @b2 ; -> 1 @@: ; 3 jmp @b3 ; -> 1


The second variant allows to define multiple labels called @1, multiple ones called @2, etc. This time it @1f to jump to the nearest @1 forward, @1b to jump to the nearest @2 backward, and so on. The number of available label names can be tweaked by modifying the first line only:
Code:
repeat 10 macro @#% tail match label, @#%#f? label tail @#%#b? equ @#%#f? end match local anonymous @#%#f? equ anonymous end macro define @#%#f? @#% end repeat
And here's an example usage for the second variant:
Code:
@1: ; a jmp @1f ; -> b jmp @2f ; -> c jmp @1b ; -> a @1: ; b jmp @1b ; -> b @2: ; c jmp @2b ; -> c @1: ; d jmp @1b ; -> d
Post 22 Sep 2016, 15:22
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Akeo



Joined: 19 Aug 2016
Posts: 8
Location: Ireland
That second variant is pretty much exactly what I was looking for, so that's what I went with.

Thanks, as usual, for the awesome help! Wink
Post 23 Sep 2016, 10:41
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VEG



Joined: 06 Feb 2013
Posts: 77
Location: Minsk, Belarus
I have another idea of anonymous labels. We have numbered labels from @0 to @9. When we at @0, it means that we can jump only forward to @1 .. @9. When we at @5, we can jump backward to @0 .. @5, or forward to @6 .. @9. When we at @9, we can jump backward only (because there is no @10).

Basically, the idea is:
Code:
; it is possible to jump (only): @0: ; backward to @0, forward to @1 .. @9 @1: ; backward to @0 .. @1, forward to @2 .. @9 @2: ; backward to @0 .. @2, forward to @3 .. @9 @3: ; backward to @0 .. @3, forward to @4 .. @9 @4: ; backward to @0 .. @4, forward to @5 .. @9 @5: ; backward to @0 .. @5, forward to @6 .. @9 @6: ; backward to @0 .. @6, forward to @7 .. @9 @7: ; backward to @0 .. @7, forward to @8 .. @9 @8: ; backward to @0 .. @8, forward to @9 @9: ; backward to @0 .. @9, no forward labels @0: ; backward to @0, forward to @1 .. @9 @1: ; backward to @0 .. @1, forward to @2 .. @9 @2: ; backward to @0 .. @2, forward to @3 .. @9 @3: ; backward to @0 .. @3, forward to @4 .. @9 ...
How it looks like:
Code:
@0: ; @0 removes old labels @1 .. @9, so we can jump only forward here test eax, eax jz @5 @1: ; at this point we can jump backward to @0 or @1, or forward to @5 or @9 mov eax, 500 jmp @9 @5: ; at this point we can jump backward to @0, @1 or @5, or forward to @9 mov eax, 100 @9: ; here we can jump only backward to @0, @1, @5 or @9 @0: ; here we can jump only forward to following @0 .. @9 ...
So, we can jump backward only to current or to labels with lower number, and we can jump forward only to labels with higher number. So, it is not required to write direction, because it is clear from a number.

The problem is here that it requires using macros @0 .. @9, and the same name required for "equ"s.

I've tried to write an experimental macro which will use the same name for a macro and for a "label".

It seems that it works nice with "define":
Code:
macro @0 tail if defined @0 redefine @0 2 else define @0 1 end if end macro @0: db @0 @0: db @0
Outputs: 0x01, 0x02, as expected. But I need to set @0 to an anonymous label, and I have to use "equ" for it, but I can't:
Code:
macro @0 tail @0 equ 1 end macro @0: db @0 @0: db @0
It seems, that "@0 equ 1" is treated as a call to the @0 macro, because @0 is on the first place of the line. So, there is an endless recursion.

Maybe it is better to allow to use "equ" in the same way as "define", like:
Code:
define @0 1 equ @0 1

It seems that this difference in syntax was dictated by the FASM1. IMHO, it is better to use similar syntax for similar tasks. It seems that it is possible to extend syntax of the define itself for supporting of the equ behavior. For example, "define!" may work like the equ Smile

And yes, do you think that this idea is ok, or maybe it is better to refuse it, because it is not ok to use the same name for a macro and for an equ?


Last edited by VEG on 07 May 2017, 19:10; edited 1 time in total
Post 07 May 2017, 18:52
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6873
Location: Kraków, Poland
VEG wrote:
But I need to set @0 to an anonymous label, and I have to use "equ" for it, but I can't
The only difference between EQU and DEFINE, just like in fasm 1, is that EQU does a single pass of replacement of symbolic variables in its value. You can do the same thing with MATCH+DEFINE.

VEG wrote:
Maybe it is better to allow to use "equ" in the same way as "define", like:
Code:
define @0 1 equ @0 1
You can try it like this:
Code:
macro equ? statement& rawmatch name= raw_value, statement match value, raw_value define name value else define name end match else define statement end rawmatch end macro
Post 07 May 2017, 19:09
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VEG



Joined: 06 Feb 2013
Posts: 77
Location: Minsk, Belarus
Tomasz Grysztar, it works, thank you Smile
Post 07 May 2017, 19:21
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VEG



Joined: 06 Feb 2013
Posts: 77
Location: Minsk, Belarus
I've modified my first idea a bit. Every @@ resets @0..@9. So, there are own @0..@9 between @@ and @@, like local anonymous sublabels. @b and @f are still available, and they point to the nearest @@ labels as always before.
Code:
macro @@ tail match label, @f? label tail @b? equ @f? end match local anonymous @f? equ anonymous repeat 10, i:0 local anonymous#i redefine @#i anonymous#i end repeat end macro define @f? @@ repeat 10, i:0 macro @#i tail match label, @#i label tail end match end macro end repeat
Code:
; This example outputs these bytes, as expected: 1,2,3,4,4,6,7,8,9. @@: db @1 @1: db @2 @2: db @f @@: db @b @1: db @2 @2: db @f @@: db @1 @1: db @2 @2: db @3 @3:

I've written it (based on the example by Tomasz Grysztar), it works, but I don't know how Smile This part of FASM macros is still pure magic for me. For example, I thought that equ will be required here, but it works only with redefine (redefine @#i anonymous#i) =) I'll try to read the FASM Manual again. Maybe I'll understand it better.


Last edited by VEG on 07 May 2017, 21:44; edited 1 time in total
Post 07 May 2017, 21:27
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6873
Location: Kraków, Poland
VEG wrote:
I've written it (based on the example by Tomasz Grysztar), it works, but I don't know how Smile This part of FASM macros is still pure magic for me. For example, I thought that equ will be required here, but it works only with redefine (redefine @#i anonymous#i) =) I'll try to read the FASM Manual again. Maybe I'll understand it better.
DEFINE creates symbolic links just as well as EQU does. The only real difference between them is if the value given to EQU contains some symbols that are symbolic replacements themselves. And even then in many cases you may not notice the difference, because nested symbolic replacements happen anyway if you use such symbol in a final expression.
Post 07 May 2017, 21:38
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