Re: symbol size as function size

[Alan Modra <amodra at gmail dot com>]

On Thu, Jun 23, 2011 at 05:16:16PM +0100, Paulo J. Matos wrote:
> sum:
>         ... // CODE FOR SUM
>         .global __size_of_sum
>         .equ __size_of_sum, .-sum
> 
> __size_of_sum would then be used as a symbol containing the size in
> words of sum.
> 
> The problem with the current setup is that as sets the value of
> __size_of_sum and when it is relaxed at link time, the size tends to
> decrease so the final value of __size_of_sum is wrong.

There isn't any provision for this sort of caper in current binutils
as far as I'm aware.  You're really asking for a special class of
symbol, which the linker relaxation functions must adjust.  If your
__size_of_sum were local and not emitted to the object symbol table
then you possibly could emit a special relocation on uses of
__size_of_sum that used the size of the function after relaxing.

> sum:
>         ... // CODE FOR SUM
>         .global __size_of_sum
>         .size sum, .-sum
>         .equ __size_of_sum, <reference to size of symbol sum>
> 
> Then in the linker relaxation function for my backend, modify the
> size of symbol sum according to the relaxation. I guess the missing
> piece in this is a way in assembler to refer to the size of symbol
> sum (this would be code generated by gcc).

Again, this boils down to the same thing, needed a special class of
symbol.  If I were implementing this feature I would not use global
symbols containing size info.  Instead

a) Invent an assembler syntax that allows access to ELF symbol size,
   "sum@size" perhaps.  (I'm assuming this is ELF only.)  "sum" would
   not even need to be a defined symbol, it could be external.
b) Arrange for the assembler to emit a relocation on uses of
   "sum@size", R_XXX_SIZE against symbol "sum".
c) Teach the linker relaxation functions to update ELF symbol size,
   and add processing of the new relocation.

-- 
Alan Modra
Australia Development Lab, IBM