Re: PPC64 Illegal Instruction

[Gabriel Paubert <paubert@iram.es>]

On Mon, Mar 04, 2019 at 11:44:37AM +0100, John Paul Adrian Glaubitz wrote:
> Hi!
> 
> On 3/4/19 10:24 AM, Gabriel Paubert wrote:
> > No, the difference is VSX. Altivec is the same instruction set, whether
> > on G4, G5, Power5 and later (IBM Power4 did not have Altivec, Power5 did,
> > VSX was added to Power7).
> 
> Okay, I had the assumption that AltiVec and VSX were the same. Thanks for
> the clarification. The amount of different POWER targets is really
> confusing.

It's a bit confusing, because IBM uses VMX to refer to Altivec, so you
really have two successive extensions to the instruction set, first VMX
(Vector Multimedia eXtension) then VSX (Vector Scalar eXtension).

> 
> > G5 or PPC970 is actually closer to Power5 from this point of view.
> 
> Yeah, if POWER4 was without AltiVec, that makes sense.

Actually, as Carlos pointed out, it was Power6 who added Altivec. 

I also forgot to mention PA6T, who also has Altivec.

> 
> > To support everything you would ideally need 3 versions (or 3
> > libraries):
> > - no vector instructions (basic Power4)
> > - Altivec (G5, P5, P6)
> > - VSX (P7 and later)
> 
> We don't want to support anything newer than P5 really. POWER4
> and POWER5 is enough.
> 
> > However, the big jump for performance is likely provided by Altivec.
> 
> Yeah, I agree.
> 
> > Do you really want to support base Power4 (no Altivec) or not?
> > 
> > If not, a single version (Altivec and no VSX) is probably the simplest.
> 
> The problem is that some people want to run the ppc64 port on hardware
> which doesn't support Altivec like those embedded E5550 CPUs. There was
> actually a large archive rebuild to lower the baseline for that reason
> (the discussion is in the mailing list archives for this mailing list).
> 
> We could maybe have a poll which baseline users prefer.

Most of the code is in libraries, isn't multilib the mechanism used to
choose at runtime which library is loaded when you have two copies of
the same library in different paths?

I have vague memories of separate sse2 directories for libraries in my
computers before switching to 64 bit (to avoid using the messy x87 stack
based FPU).

Non-Altivec multimedia engines are going to be slow. I'd vote for
Altivec as the baseline (even if I might soon get a P9 workstation).

	Regards,
	Gabriel