Re: [RFC PATCH 5/7] powerpc/64s: update generic cpu option name and compiler flags

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Tue, Sep 20, 2022 at 12:01:47AM +1000, Nicholas Piggin wrote:
> Update the 64s GENERIC_CPU option. POWER4 support has been dropped, so
> make that clear in the option name.

AFAIR the minimum now is POWER4+ (ISA 2.01), not POWER5 (ISA 2.02).

> -mtune= before power8 is dropped because the minimum gcc version
> supports power8, and tuning is made consistent between big and little
> endian.

Tuning for p8 on e.g. 970 gives quite bad results.  No idea if anyone
cares, but this is a serious regression if so.

> Big endian drops -mcpu=power4 in favour of power5. Effectively the
> minimum compiler version means power5 was always being selected here,
> so this should not change anything. 970 / G5 code generation does not
> seem to have been a problem with -mcpu=power5, but it's possible we
> should go back to power4 to be really safe.

Yes, -mcpu=power5 code does *not* run on 970, if you are unlucky enough
that the compiler does something smart with popcntb (the sole non-float
insn new on p5, not counting hrfid).

> +# -mcpu=power5 should generate 970 compatible kernel code

It doesn't.  Even if it did, it would need more explanation!


Segher