Re: [semi PATCH] ARM/Thumb branches out of range for MI thunks

[Daniel Jacobowitz <drow at mvista dot com>]

On Fri, Oct 24, 2003 at 09:55:51AM -0700, Zack Weinberg wrote:
> Richard Earnshaw <rearnsha@arm.com> writes:
> 
> > Yes.  Output the Thunk in ARM code (seriously, this is one of those cases 
> > where you get better code density in ARM state than you can in Thumb).  
> > But to do that, the C++ front end needs fixing so that we can get hold of 
> > the thunk properly in the back end and control how it outputs the thunk 
> > definition.
> >
> > I've posted on this before.
> 
> I've read old postings of yours that I can find -
> 
> http://gcc.gnu.org/ml/gcc/2002-06/msg01598.html
> http://gcc.gnu.org/ml/gcc-patches/2002-07/msg00769.html
> http://gcc.gnu.org/ml/gcc-patches/2003-07/msg01703.html
> 
> It sounds like the problem is that "this is a thunk" is a concept only
> visible within the C++ front end, whereas it needs to be language-
> independent.  That ought to be a feasible change to make, if we can
> agree on what "this is a thunk" means.
> 
> I am not sure if the present state of thunk generation is better or
> worse than it was at the time you posted these messages.
> 
> What is the cost of an arm/thumb mode switch, compared to the extra
> cycles required to execute a purely Thumb thunk?

The cost of a Thumb->arm switch is just the cost of a branch - none of
the ARM cores I'm familiar with take an extra hit on switching.  Three
cycles on arm9tdmi which is what I have the manual open for.  It's
just a "bx pc; nop" switch although it also means you need to make an
interworking branch to whatever you're thunking.

The same thing applied to PLT entries - you take more delays in trying
to make it work in Thumb than you do in switching.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer