Re: [PATCH] Fix thumb calls via PLT on ARM/SymbianOS

[Julian Brown <julian@codesourcery.com>]

Richard Earnshaw wrote:
> On Wed, 2005-03-16 at 17:33, Ian Lance Taylor wrote:
> 
>>Richard Earnshaw <rearnsha@gcc.gnu.org> writes:
>>
>>
>>>On Wed, 2005-03-16 at 16:39, Ian Lance Taylor wrote:
>>>
>>>
>>>>OUTPUT_ARCH lets you specify an architecture.  For ARM, this will be
>>>>one of the entries found in arch_info_struct in bfd/cpu-arm.c.  Of
>>>>course that list can be adjusted.
>>>>
>>>
>>>The current attributes proposal for the ABI lists 27 separate
>>>attributes, giving rise to a theoretical 10^14 possible variants.  In
>>>practice the number of realistic variants is much smaller than this, but
>>>it's still much larger than can be listed explicitly in an arch_info
>>>struct.
>>
>>I guess I misunderstood the issue.  I didn't realize that it was
>>necessary to support a set of different attributes.  I guess I don't
>>understand why -march was brought up---march in gcc doesn't take a set
>>of attributes.
>>
> 
> -march was brought up because *in this particular case* it's the
> attribute that we are interested in: does the target architecture have
> the blx instruction?  So it's true that *in this particular case* we
> could solve the problem in the info_arch vector: but we can't solve the
> general problem that way -- it requires more a complex solution.
> 
> 
>>Of course a set of attributes could be done in BFD by using the
>>machine number as a bitfield, but that is probably not the way to go.
> 
> 
> You'd need something like 68 bits to fully describe the 27 attributes.

Implementing the whole of the build attributes machinery is going to be 
a lot of work. Richard, do you have a suggestion for a short-term plan 
for fixing the problem in hand which would be acceptible for you?

After experimenting with the OUTPUT_ARCH machinery, I've decided it 
probably isn't the right way of solving this (the treatment of the 
R_ARM_THM_CALL relocation at link-time). There are just too many fiddly 
corners involved - e.g. bfd_arm_merge_machines will happily override the 
OUTPUT_ARCH set in a linker script (or from the command-line, after 
patching).

Cheers,

Julian