Re: powerpc-eabi subroutine calls

[Joćo Cadamuro Junior <cadamuro at lit dot cpdtt dot cefetpr dot br>]

> Hi,
>
> I have observed a gcc problem for an embedded mpc8xx target (powerpc-eabi):
>
> I have 2 memory banks, in which .text has to be linked into. These memory
> banks are more than 16MB apart (0x40000000 and 0x00000000). A custom linker
> script tells which source module goes into which bank.
>
> GCC generates a "bl" (Opcode 0x48000000) for procedure
> calls. Unfortunately this PPC instruction allows only 24bit (signed)
> relative offset, i.e. the branch destination must be within +-8MByte
> distance from the caller.
>

This is not true... gcc is generating absolutely correct code in this case!

The "bl" instruction uses the link register (LR) as a target address, and with
link register the core can achieve every position within 4Gbytes of the PowerPC
(32-bit implementation) address space...

Please, take a look into "PowerPC Microprocessor Family: The Programming
Environments for 32-Bit Microprocessor". It's free on the Motorola web site.



>
> For cross bank subroutine calls different banks the linker gives a
> relocation error:
>
>       relocation truncated to fit: R_PPC_REL24 foo
>
> The source of trouble seems, that gcc does generate an offset limited
> relative jump instruction, where an absolute jump would be more adequate.
>
> I know, PPC does not have absolute jumps, but that may be mimiced by
> a bctrl or a blrl, after LR or CTR has been loaded with the absolute
> destination address.
>
> Is there a way to tell gcc how to generate those absolute call methods?
> Handcrafted inline assembler seems messy to me because of parameter
> handling.
>
> gcc-2.95.2
> GNU ld 2.10.1
> gcc command line parameters:
> /opt/gnu/powerpc-eabi/bin/gcc  -c -g -Wall  -fno-inline  -O3  -mcpu=860 \
> -msoft-float  -o foo.o ../src/foo.c
>
> P.S.
>
> Of course, there is the (obvious) solution for my immediate problem to
> remap one memory bank into the 24bit scope of the other.
>
> However, the problem still remains if the .text segment would grow beyond
> 8 MB.
>
> gm
> --
> Gunter Magin                                         magin[AT]skil.camelot.de
>

Joćo Cadamuro Junior
LIT / CPDTT / CEFET-PR


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