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Re: [avr-gcc-list] Using current avr-gcc for target ATmega2561

From: Martijn van Balen
Subject: Re: [avr-gcc-list] Using current avr-gcc for target ATmega2561
Date: Tue, 11 Oct 2005 13:10:32 +0200

On 2005-10-03 17:26:10 Martijn van Balen wrote:

> "What needs to be done (tasks)
>    (1) Align functions (and labels?) to 4-byte boundaries so that gcc can continue to use 16-bit values to represent function pointers.
>    (2) Take into account, that these devices store three bytes on the stack for each function call"

> I can easily avoid (1) by using <128Kbyte flash from C. But I cannot figure out what problem is solved by (2).

> Indirect jumps can be done by IJMP/ICALL, no need to push addresses yourself. Before diving into avr.md

> and other files, could anybody point my in the right direction ?

To answer my own question, at least one problem is the addressing of function parameters that were put on

the stack. I spend some time browsing the gcc source code and finally found at least one
function in .../gcc/config/avr/avr.c that needs knowledge of the size of the program counter:

initial_elimination_offset (int from, int to)
    return 0;
      int offset = frame_pointer_needed ? 2 : 0;

      offset += avr_regs_to_save (NULL);
      return get_frame_size () + 2 + 1 + offset;

After a small experiment, I concluded that replacing the last 2 with a 3 indeed solves
the problem with the function parameters. So for now, I can use my patched compiler
to generate mega2561 code.

What I want to ask to the guru's is: do you agree ? Or do I need to patch more to
get a useful compiler ?

Kind regards,

 * Test program, generate assembly with 'cc1 tst.c'
 * p6 is passed through the stack
char f(long p1, long p2, long p3, long p4, char p5, char p6)
  return p6;

  f(42, 42, 42, 42, 42, 42);

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