Re: ARM EABI port: minimum CPU choice
On Monday 12 June 2006 17:51, Martin Guy wrote:
> 2006/6/12, Martin Guy <martinwguy@gmail.com>:
> > recompiling their biggest apps using Thumb until they get within their
> > permitted code size. This is important for v4t because it is currently
> > one of the bulk-market flavours
>
> Mind u, even on v4t they will probably lose out worse in code size by
> having 2 extra instructions in every function than they would ever
> gain by interworking with thumb...
> so i guess LDM it is. That *does* exist and work on v4[t], true?
False.
The only way to do interworking on v4t is to use bx, which implies the three
instruction sequence. Yes, this really sucks, which is why it was fixed in
v5 :-)
Annother consideration is that arm-linux PLT sequence is only interworking
safe on v5 hardware. ie. on v4t hardware all functions exported from shared
libraries must be entered in Arm mode. It's possible to make the linker
create export stubs to achieve this, but noone's done that yet.
These two factors mean that supporting Thumb on v5 hardware is relatively
low-impact. Supporting Thumb on v4t hardware can introduce significant
overhead, and is even worse if you also want the Arm portion of that code to
run on v4.
Paul
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