[Thumb] Don't materialize a new base register when CPSR is live

[Jonathan Roelofs]

On 9/16/14 4:29 AM, Moritz Roth wrote:
> Hi Renato,
>
> unfortunately there is only ADDS (immediate) in Thumb1, so we have no choice :(
>
> I realised this morning that for this to work properly, the ADDS inserted to
> materialize the new base needs a <def,dead> of the CPSR (instead of just a def),
> so I added that to the patch. I also increased the neighbourhood size for
> computeRegisterLiveness slightly (10->15), as it turned out that quite often,
> CPSR liveness was undecidable in the default neighbourhood.
This doesn't sound correct to me. I think you need to calculate CPSR's liveness 
by looking at the whole basic block, not just a 15-instruction window.


Jon
>
> And of course good point about the redundant comparison, I've removed that.
>
> Regarding the tests, you're right - I found this while working on making the
> load/store merging more aggressive (currently, we only do it if it's obviously
> safe) - the register allocator inserted some spills between a CMP and Bcc which
> could be merged, but it isn't possible to safely materialize the base+offset
> there. Unfortunately, with the algorithm that's currently checked in, the STRs
> in that case aren't merged at all, and it's somewhat dependent on register
> allocation...
>
> Cheers
> Moritz
>

-- 
Jon Roelofs
jonathan at codesourcery.com
CodeSourcery / Mentor Embedded