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

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

On Wed, 2005-03-16 at 14:42, Daniel Jacobowitz wrote:
> 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.
> 

I'll have to look at that bit in further detail.

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

Possibly.  Tail call?  Doesn't happen in GCC, but another compiler might
do it.

> 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.

We shouldn't be testing SymbianOS here.  We should be testing whether
blx is available.  If it is, then use the blx sequence; if not, then use
a further trampoline that switches to ARM state and then jumps to the
PLT.  Other code in the initialization sequence can then mark that
symbianos always has blx available.

R.