Re: [PATCH 1/3] powerpc64: Replace some PPC_FEATURE_HAS_VSX with PPC_FEATURE_ARCH_2_06

[Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>]

Anton Blanchard via Libc-alpha <libc-alpha@sourceware.org> writes:

> We use PPC_FEATURE_HAS_VSX to select a number of POWER7 optimised
> functions. These functions don't use any VSX instructions, so
> PPC_FEATURE_ARCH_2_06 seems like a better fit.

I don't think we can replace PPC_FEATURE_HAS_VSX with PPC_FEATURE_ARCH_2_06
because it would cause issues similar to the ones fixed in patch 3/3, i.e.
this function would run on a processor that implements the Power ISA 2.06
but does not implement Altivec.

So, if testing for PPC_FEATURE_ARCH_2_06 is important, I think it has to test
for:

    (hwcap & PPC_FEATURE_ARCH_2_06 && hwcap & PPC_FEATURE_HAS_ALTIVEC)

However, VSX was introduced by Power ISA 2.06. So, PPC_FEATURE_HAS_VSX
implies PPC_FEATURE_ARCH_2_06.  PPC_FEATURE_HAS_VSX also implies
PPC_FEATURE_HAS_ALTIVEC.
The change would only make a difference if a processor implements both Power
ISA 2.06 and Altivec, but does not implement VSX.

Am I missing anything?

Another important point is to keep the tests in the IFUNC resolvers in sync
with sysdeps/powerpc/powerpc64/multiarch/ifunc-impl-list.c
Otherwise, glibc tests and benchtests may not execute correctly.

-- 
Tulio Magno