gas preprocessor

gas preprocessor

[Crni Gorac]

Is preprocessing support in gas working? Am trying to
utilize it with gas 2.11.90.0.8 in order to create
alias names for functions arguments instead of use
[ebp+8] etc. (am using Intel syntax). What I'm
actually looking for is gas equivalent of EQU
directive commonly found in other (well, at least
IA-32) assemblers. I know that I could accomplish same
using gcc preprocessor but I don't like this solution
(when debugging code using gdb I have to struggle
again with [ebp+8] and alike).

Thanks.


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Re: gas preprocessor

[Nick Clifton]

Hi Crni,

> Is preprocessing support in gas working? Am trying to utilize it
> with gas 2.11.90.0.8 in order to create alias names for functions
> arguments instead of use [ebp+8] etc. (am using Intel syntax). What
> I'm actually looking for is gas equivalent of EQU directive commonly
> found in other (well, at least IA-32) assemblers. I know that I
> could accomplish same using gcc preprocessor but I don't like this
> solution (when debugging code using gdb I have to struggle again
> with [ebp+8] and alike). 

I believe that the short answer to your question is "no".

You can use the .set directive to set up assemble time aliases for
expressions, so you could do:

        .set foo, [ebp+8]

This will not generate a debugging entry however, so you will not be
able to see 'foo' in your GDB sessions.

You might be able to use the register aliasing ability of gcc to solve
your problem however.  For example a declaration like this:

  register int ebp_reg asm ("ebp");

would create a variable called 'ebp_reg' which was mapped onto the ebp
register and which could be viewed inside gdb.

Cheers
        Nick

Re: gas preprocessor

[Crni Gorac]

Hi Nik, thank you for your reply.

I tried what you suggested but seems like it doesn't
work. Here is small assembly program:

.intel_syntax noprefix
	
.text
.global  _start

_start:
	call answer
	mov	ebx, eax
	
	mov	eax, 1
	int	0x80

answer:
.set result, [ebp-4]
        enter   4, 0

	mov	result, dword ptr 42
	mov	eax, result
        leave
        ret

I'm using following commands to compile it:
as -o foo.o foo.s
ld -o foo foo.o

However, linker reports errors:
foo.o: In function `answer':
foo.o(.text+0x14): undefined reference to `ebp'
foo.o(.text+0x1d): undefined reference to `ebp'

Thanks.

--- Nick Clifton <nickc@cambridge.redhat.com> wrote:
> Hi Crni,
> 
> > Is preprocessing support in gas working? Am trying
> to utilize it
> > with gas 2.11.90.0.8 in order to create alias
> names for functions
> > arguments instead of use [ebp+8] etc. (am using
> Intel syntax). What
> > I'm actually looking for is gas equivalent of EQU
> directive commonly
> > found in other (well, at least IA-32) assemblers.
> I know that I
> > could accomplish same using gcc preprocessor but I
> don't like this
> > solution (when debugging code using gdb I have to
> struggle again
> > with [ebp+8] and alike). 
> 
> I believe that the short answer to your question is
> "no".
> 
> You can use the .set directive to set up assemble
> time aliases for
> expressions, so you could do:
> 
>         .set foo, [ebp+8]
> 
> This will not generate a debugging entry however, so
> you will not be
> able to see 'foo' in your GDB sessions.
> 
> You might be able to use the register aliasing
> ability of gcc to solve
> your problem however.  For example a declaration
> like this:
> 
>   register int ebp_reg asm ("ebp");
> 
> would create a variable called 'ebp_reg' which was
> mapped onto the ebp
> register and which could be viewed inside gdb.
> 
> Cheers
>         Nick
> 


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Re: gas preprocessor

[Nick Clifton]

Hi Crni,

> I tried what you suggested but seems like it doesn't
> work. Here is small assembly program:
> 
> .set result, [ebp-4]
> 	mov	result, dword ptr 42
> 	mov	eax, result

> However, linker reports errors:
> foo.o: In function `answer':
> foo.o(.text+0x14): undefined reference to `ebp'
> foo.o(.text+0x1d): undefined reference to `ebp'

*sigh* Yes, it seems that the assembler does not recheck the
expression in an evaluated .set symbol to see if it is a register
name, so it creates a reference to a symbol called 'ebp'.  I thinkt
you are going to have to use the C preprocessor instead.  Sorry.

Cheers
        Nick