RFR: 8216392: Enable cmovP_mem and cmovP_memU instructions

[B. Blaser]

On Thu, 10 Jan 2019 at 10:19, Andrew Haley <aph at redhat.com> wrote:
>
> On 1/9/19 12:13 PM, Roman Kennke wrote:
> > I cannot say if if this has performance implication. I suspect not. If
> > it has, it's probably miniscule improvement. I can't see how it could be
> > worse though.
>
> I can. x86 can have some very weird performance characteristics. It'd be
> helpful to do some measurement.

I'm not sure we are really able to conclude anything from performance
measurement on highly implementation-dependent instructions unless we
make an average on a significant number of different x86_64 processors
which might well change with future generations...

Shouldn't we follow a more pragmatic direction considering that less
instructions/registers and a better/smaller encoding is generally
preferable, as Roman suggested, which is the purpose of complex
instruction sets?

Bernard