Re: ARM thumb interworking hole?

[Richard Earnshaw <rearnsha at gcc dot gnu dot org>]

On Tue, 2005-01-11 at 16:12, Ian Lance Taylor wrote:
> Charles Manning <manningc2@actrix.gen.nz> writes:
> 
> > 2) Why do we need the call_via_rx stub anyway? It seems to me this is only 
> > required if the target address is ARM code that does not return via a "bx 
> > lr". Surely we should be able to assume that if the code is marked as 
> > interworking code, then straght interworking will do the job.
> 
> The problem is that ARMv4 Thumb code did not support blx.  The only
> way to jump to an address held in a register was bx.  It would be
> possible to use a two-instruction sequence (mov lr, pc; bx reg)
> instead.  
No, because it doesn't work:

	mov lr, pc 

strips the Thumb bit.  That is, it really does give you '.+4' rather
than the '.+5' that's needed for a return value.

R.