Re: RFC: Reducing the number of non volatile GPRs in the ppc64 kernel

Hi Anton,

On Wed, Aug 05, 2015 at 02:03:00PM +1000, Anton Blanchard wrote:

While looking at traces of kernel workloads, I noticed places where gcc
used a large number of non volatiles. Some of these functions
did very little work, and we spent most of our time saving the
non volatiles to the stack and reading them back.

That is something that should be fixed in GCC -- do you have an example
of such a function?

It made me wonder if we have the right ratio of volatile to non
volatile GPRs. Since the kernel is completely self contained, we could
potentially change that ratio.

Attached is a quick hack to gcc and the kernel to decrease the number
of non volatile GPRs to 8. I'm not sure if this is a good idea (and if
the volatile to non volatile ratio is right), but this gives us
something to play with.

Instead of the GCC hack you can add a bunch of -fcall-used-r14 etc.
options; does that not work for you?


Segher