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

[Daniel Jacobowitz <drow at false dot org>]

On Wed, Mar 16, 2005 at 02:23:12PM +0000, Richard Earnshaw wrote:
> On Wed, 2005-03-16 at 14:17, Julian Brown wrote:
> > Hi,
> > 
> > This patch fixes calls made from thumb mode via the PLT on SymbianOS. 
> > PLT entries are written in ARM mode, but previously they were being 
> > called in thumb mode, with their address incorrectly offset by -4 
> > (PLT_THUMB_STUB_SIZE).
> > 
> > Now, the target address has been corrected and the thumb BL instruction 
> > is rewritten as BLX to perform the mode switch before attempting to 
> > execute the PLT entry. (BLX is an armv5t instruction, but as SymbianOS 
> > is only targeted at armv5t+, this should always be OK.)
> > 
> > The hardwired "4" for the thumb stub size on non-SymbianOS targets has 
> > also been rewritten as PLT_THUMB_STUB_SIZE.
> > 
> > OK to apply?
> 
> This sounds like a hack.  I'd much rather a generic solution were found
> (and which permitted the blx optimization if available).

The SymbianOS bits sound generally right to me.  The fact that the -4
bias is currently included is just a bug; that's from the code to
generate a Thumb header on PLT entries, which is already disabled
for SymbianOS.

OTOH, it may be missing some error checks - what if it's _not_ a BL? 
Could we reach here?

Ideally, yes, I want to do this transformation on non-SymbianOS too.
But, we don't have a clear indicator saying "yes it is save to generate
blx" yet.

-- 
Daniel Jacobowitz
CodeSourcery, LLC