Re: Gnu assembler question for ARM

[Ciaccia <ciacciax at yahoo dot com>]

> > Let's assume my function gets a pointer to a "C"-struct and wants to use its fields. Currently my code references the fields by hard-coding the
> > offsets in the code, which makes the code not very readable and error prone. What is the best way to use records in assembler? I don't want to
> > define them or allocate them in assembler, just access their fields when I receive a reference to them.
>
> Why not let gcc do this for you by using its support for extended inline 
> assembler ?  eg:
>
>   % cat struct.c
>   struct a {
>     int field1;
>     char field2;
>     int field3;
>   } A;
>
> [...]
> ie the compiler has worked out for you that "field3" of the "struct a" 
> structure is at an 8 byte offset from the start of the structure.

Hi Nick,
I think my question was not clear. My problem is not "how to find" the offsets, but rather "configure them once" so that I don't have to write the offsets in assembler. For example, let assume my function receives a pointer to a A struct as parameter and I want to write some data to it. The following C function

----------------------------------------
void fill(struct a *ptr)
{
    a->field1= 1;
    a->field2= (char)2;

    a->field3= 3;
}

----------------------------------------

could be rewritten in (arm) ASM as following:

----------------------------------------
fill:
   mov r1, #1
   str   r1, [r0, #0]      @ store 1 in field1
   mov r1, #2

   strb r1, [r0, #4]      @ store 2 in field2

   mov r1, #3

   str   r1, [r0, #8]      @ store 3 in field3
   mov pc, lr
----------------------------------------


What I don't like are the #0, #4 and #8 in the store instruction. I would like to have something like this:

----------------------------------------
@this is pseudocode, it's not expected to work
#define field1 0
#define field2 (field1 + 4)
#define field3 (field2 + 4)

fill:

   mov r1, #1

   str   r1, [r0, #field1]      @ store 1 in field1

   mov r1, #2


   strb r1, [r0, #field2]      @ store 2 in field2


   mov r1, #3


   str   r1, [r0, #field3]      @ store 3 in field3

   mov pc, lr

----------------------------------------

How could I get that?
Thanks again
Andrea





 
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