RE: [Patch, MIPS] Change mips4 default processor to r10K

[Matthew Fortune <Matthew.Fortune@imgtec.com>]

Steve Ellcey  <sellcey@imgtec.com> writes:
> This patch changes the default processor for mips4 from the r8000 to
> the
> r10000.  There are several reasons for this change, the main one
> being the difference in the r8000 madd instruction and the rest of the
> mips4
> family.  The r8000 has a fused madd instruction (no truncation between
> the
> multiply and add) but the rest of the mips4 CPUs have unfused madd
> instructions where truncation is done and the result is identical to
> using
> separate multiply and add instructions.
> 
> I plan to submit a patch later to clean up MIPS handling of madd and
> other
> fma style instructions but to do that it is necessary to do different
> things
> for r8000 vs the other mips4 targets and that is easier if r8000 is not
> the
> default processor selected when -mips4 is used.
> 
> I have also been told that very few r8000 machines were ever released
> and
> that the r10k/r12k/etc. series is probably the most common mips4
> machines
> still out there and so would be a better default.
> 
> This patch will have no affect on compiles where a specific processor
> is
> specified (i.e. -march=r80000, -march=r10000), but only when using the
> more generic -mips4 or -march=mips4 flags.  The only visible difference
> in that case is that GCC will use the r10k instruction scheduler
> instead
> of the generic MIPS scheduler (there is no r8000 specific scheduler)
> and
> that should be a good thing for most mips4 machines.
> 
> Tested with the mips-linux-gnu toolchain.
> 
> OK for checkin?
> 
> Steve Ellcey
> sellcey@imgtec.com
> 
> 
> 2015-04-28  Steve Ellcey  <sellcey@imgtec.com>
> 
> 	* config/mips/mips-cpus.def: (mips4): Change default processor
> 	from PROCESSOR_R8000 to PROCESSOR_R10000.

Ok with me. I'd like Catherine to have the chance to raise any concerns
though.

Thanks,
Matthew