Re: [RFC/PATCH] Make powerpc64 use __thread for per-cpu variables

From: Paul Mackerras
Date: Thu May 18 2006 - 19:49:48 EST

David S. Miller writes:

> If you have to hide the operation so deeply like this, maybe you can
> do something similar to sparc64, by explicitly doing the per-cpu fixed
> register and offsets, and still get the single instruction relocs that
> powerpc can do for up to 64K by doing something like:
> &per_cpu_blah - &per_cpu_base
> to calculate the offset.

I don't know how to tell gcc that (&per_cpu_blah - &per_cpu_base) is a
quantity that the linker can compute and that will fit into a 16-bit
offset. If I use an inline asm, then I have to generate the address
and let gcc dereference it, because __get_cpu_var is used both as an
lvalue and an rvalue. That means two instructions where one would
suffice. So there doesn't seem to be a way to get the optimal code,
unless the gcc folks are willing to add a -fkernel or something for
us. :)

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