Re: [patches] Possible PowerPC LIBC optimization

[Mark Mitchell <mark@xxxxxxxxxxxxxxxx>]

Steven Munroe wrote:

> My concern is that what works for 750 may not be best for 8540 or 970.
> The powerpc-cpu add-on handles this by adding additional <cpu-type>
> specific directory layer to the make system search order (i.e.
> ./sysdeps/powerpc/powerpc32/750) that can override the base. This is
> selected at configure time via --with-cpu=750 or your favorite <cpu-type>

As an aside, do you think that any of these kinds of optimizations are
worth doing dynamically, based on the CPU type that we actually have?
So that, for example, "strcpy" can be optimized for your CPU,
independently of what CPU was used when configuring GLIBC?

In some situations, you may know for sure what CPU you're targeting, and
you want to build everything for that.  But, it might also be useful to
be able to dynamically adjust; I've wondered whether we might be able to
get 80% of the bang of a multilib by dynamically choosing a few
performance-critical routines (like memcpy, etc.).  My guess was that
I/O-bound routines (like printf) would be only slightly affected by the
particular CPU for which they were built, and so dynamically selecting a
few CPU-bound routines could make a big difference.  I don't know enough
about GLIBC implementation details to know how possible it would be to
efficiently implement that dynamic selection, though.

-- 
Mark Mitchell
CodeSourcery
mark@xxxxxxxxxxxxxxxx
(650) 331-3385 x713