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

Segher Boessenkool segher at kernel.crashing.org
Wed Aug 5 14:19:28 AEST 2015

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?


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