Register allocation cost question

[Andrew Stubbs <ams@codesourcery.com>]

Hi all,

I'm trying to add a new register set to the GCN port, but I've hit a 
problem I don't understand.

There are 256 new registers (each 2048 bit vector register) but the 
register file has to be divided between all the running hardware 
threads; if you can use fewer registers you can get more parallelism, 
which means that it's important that they're allocated in order.

The problem is that they're not allocated in order. Somehow the IRA pass 
is calculating different costs for the registers within the class. It 
seems to prefer registers a32, a96, a160, and a224.

The internal regno are 448, 512, 576, 640. These are not random numbers! 
They all have zero for the 6 LSB.

What could cause this? Did I overrun some magic limit? What target hook 
might I have miscoded?

I'm also seeing wrong-code bugs when I allow more than 32 new registers, 
but that might be an unrelated problem. Or the allocation is broken? I'm 
still analyzing this.

If it matters, ... the new registers can't be used for general purposes, 
so I'm trying to set them up as a temporary spill destination. This 
means they're typically not busy. It feels like it shouldn't be this 
hard... :(

Thanks in advance.

Andrew