[RFC] ARM multilib changes and switch to EABI version 5

[Sebastian Huber]

Hello,

there were several requests for ARM Cortex-M support on RTEMS recently.  The
first step towards this is a suitable ARM tool chain.  I want to use this event
to clean up the multilibs and switch to the EABI version 5.  The benefit of
EABI version 5 is that this brings us more in line with the primary GCC
platform arm-linux-gnueabi.  For standards related to this please have a look
at the following documents.

Base Platform ABI for the ARM Architecture (BPABI):

http://infocenter.arm.com/help/topic/com.arm.doc.ihi0037b/IHI0037B_bpabi.pdf

ARM Architecture Procedure Call Standard (AAPCS):

http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042d/IHI0042D_aapcs.pdf

ELF for the ARM Architecture:

http://infocenter.arm.com/help/topic/com.arm.doc.ihi0044d/IHI0044D_aaelf.pdf

The EABI makes the VFP floating point architecture mandatory and enables us to
use hardware floating point support in the future.  RTEMS has currently no
support for hardware floating point units (the context switch part is missing).

I propose to change the ARM multilibs as follows:

 1. Default: armv4, ARM
 2. thumb: armv4t, Thumb
 3. armv6-m: armv6-m, subset of Thumb 2
 4. armv7: armv7, Thumb 2
 5. armv7-m: armv7-m, Thumb 2, hardware integer division (SDIV/UDIV)

Multilib 1. and 2. support the standard ARM7TDMI and ARM926EJ-S targets.

Multilib 3. supports the Cortex-M0 and Cortex-M1 cores.

Multilib 5. supports the Cortex-M3 and Cortex-M4 cores, which have a special
hardware integer division instruction (this is not present in the A and R
profiles).

Multilib 4. supports Cortex-A and Cortex-R variants.

The ABI change will break the BSPs.  We have to adjust the linker command files
to support the following sections:

  .ARM.extab   : { *(.ARM.extab* .gnu.linkonce.armextab.*) }

   __exidx_start = .;
  .ARM.exidx   : { *(.ARM.exidx* .gnu.linkonce.armexidx.*) }
   __exidx_end = .;

  .preinit_array     :
  {
    PROVIDE_HIDDEN (__preinit_array_start = .);
    KEEP (*(.preinit_array))
    PROVIDE_HIDDEN (__preinit_array_end = .);
  }

  .init_array     :
  {
     PROVIDE_HIDDEN (__init_array_start = .);
     KEEP (*(SORT(.init_array.*)))
     KEEP (*(.init_array))
     PROVIDE_HIDDEN (__init_array_end = .);
  }

  .fini_array     :
  {
    PROVIDE_HIDDEN (__fini_array_start = .);
    KEEP (*(.fini_array))
    KEEP (*(SORT(.fini_array.*)))
    PROVIDE_HIDDEN (__fini_array_end = .);
  }

We have to figure out how this affects the .init and .fini sections and there
usage.  There is also a disadvantage with the new .init_arry and .fini_array
sections.  These sections are in principle writeable to allow a loader of a
shared library to fix up the pointers.  Since we do not support shared
libraries we do not need this actually.

GCC patches are attached.

It would be nice if all ARM users may have a look at this and provide comments.

-- 
Sebastian Huber, embedded brains GmbH

Address : Obere Lagerstr. 30, D-82178 Puchheim, Germany
Phone   : +49 89 18 90 80 79-6
Fax     : +49 89 18 90 80 79-9
E-Mail  : sebastian.huber at embedded-brains.de
PGP     : Public key available on request.

Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.
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