Re: arm-pe: Do not generate type 2 LDM instructions with writeback

[Nick Clifton <nickc at redhat dot com>]

Hi Richard,

> >   Whilst looking over the arm-pe port I found that it was generating
> >   LDM instructions using writeback "sp!" and CPSR update "^" which is
> >   UNPREDICTABLE if the PC is not one of the registers being loaded.
> >   The following patch takes care of this problem and fixes quite a few
> >   unexpected failures in the gcc testsuite.  (There are still lots
> >   more that need to be looked at though - maybe one day).
> 
> GCC should never be generating a forced user-bank accessing
> instruction (^ on non-pc loading) -- at least, it won't ever be
> doing so via these functions -- so I think the test for write-back
> is redundant.  That is, emit ^ if PC is in the list and !apcs-32,
> but not otherwise.

That seems reasonable.  Updated patch appended.


> PS. Is the pe port using apcs-26?

It would appear so.

> If so, why?

Because it is so old ?

It is a simple matter to fix it by changing the definition of
TARGET_DEFAULT in pe.h to:

  #define TARGET_DEFAULT  (ARM_FLAG_SOFT_FLOAT | ARM_FLAG_APCS_32 | ARM_FLAG_APCS_FRAME | TARGET_FLAG_NOP_FUN)

Although it would be simpler to make apcs-32 the default and require
that apcs-26 be deliberately selected by the appropriate header file.

I am not sure if this is a good idea though, since presumably any
arm-pe tools out there are currently using apcs-26.  Mind you, given
the terrible state of the arm-pe port at the moment, it is unlikely
that anyone is actually using it.

Not sure what to do really.  I do not have any real arm-pe
environments in which I can test the tools, so I do not know what
would be for the best.  Is there anybody out there who uses arm-pe ?


Cheers
        Nick


Index: gcc/config/arm/arm.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/config/arm/arm.c,v
retrieving revision 1.266
diff -c -3 -p -w -r1.266 arm.c
*** gcc/config/arm/arm.c	13 Mar 2003 12:03:36 -0000	1.266
--- gcc/config/arm/arm.c	13 Mar 2003 13:23:48 -0000
*************** print_multi_reg (stream, instr, reg, mas
*** 7207,7217 ****
    fprintf (stream, "}");
  
    /* Add a ^ character for the 26-bit ABI, but only if we were loading
!      the PC or not updating the stack pointer.  Otherwise we generate
!      an UNPREDICTABLE instruction.  */
    if (! TARGET_APCS_32
!       && (((mask & (1 << PC_REGNUM)) != 0)
! 	  || strchr (instr, '!') == NULL))
      fprintf (stream, "^");
    
    fprintf (stream, "\n");
--- 7207,7220 ----
    fprintf (stream, "}");
  
    /* Add a ^ character for the 26-bit ABI, but only if we were loading
!      the PC.  Otherwise we would generate an UNPREDICTABLE instruction.
!      Strictly speaking the instruction would be unpredicatble only if
!      we were writing back the base register as well, but since we never
!      want to generate an LDM type 2 instruction (register bank switching)
!      which is what you get if the PC is not being loaded, we do not need
!      to check for writeback.  */
    if (! TARGET_APCS_32
!       && ((mask & (1 << PC_REGNUM)) != 0))
      fprintf (stream, "^");
    
    fprintf (stream, "\n");