Re: [PATCH] powerpc/64: Drop ppc_inst_as_str()

[Jordan Niethe <jniethe5@gmail.com>]

On Thu, Jun 2, 2022 at 6:49 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Thu, Jun 02, 2022 at 01:01:04PM +1000, Jordan Niethe wrote:
> > > What about the more fundamental thing?  Have the order of the two halves
> > > of a prefixed insn as ulong not depend on endianness?  It really is two
> > > opcodes, and the prefixed one is first, always, even in LE.
> > The reason would be the value of as ulong is then used to write a
> > prefixed instruction to
> > memory with std.
> > If both endiannesses had the halves the same one of them would store
> > the suffix in front of the prefix.
>
> You cannot do such a (possibly) unaligned access from C though, not
> without invoking undefined behaviour.  The compiler usually lets you get
> away with it, but there are no guarantees.  You can make sure you only
> ever do such an access from assembler code of course.

Would using inline assembly to do it be ok?

>
> Swapping the two halves of a register costs at most one insn.  It is
> harmful premature optimisation to make this single cycle advantage
> override more important consideration (almost everything else :-) )

I'm not sure I follow. We are not doing this as an optimisation, but
out of the necessity of writing
the prefixed instruction to memory in a single instruction so that we
don't end up with half an
instruction in the kernel image.

>
>
> Segher