Re: [PATCH] [PowerPC] VLE update

[Alan Modra <amodra at gmail dot com>]

On Mon, Aug 28, 2017 at 05:20:45PM +0300, Alexander Fedotov wrote:
> > You could reuse the SEC_MEP_VLIW bit, I guess.   The section flags
> > weren't really supposed to be used for target-specific purposes, but
> > it seems pointless trying to be a purist now.
> 
> Okay, so I will set
> +#define SEC_PPC_VLE SEC_MEP_VLIW

If you split off the part dealing with SEC_PPC_VLE that can be
committed separately.

> > That doesn't look elegant at all.  So you are using R_PPC_PLTREL24 and
> > R_PPC_LOCAL24PC relocations in VLE code but giving them a new meaning.
> > Is that documented in the ABI?
> >
> > I think it would be better to define new R_PPC_VLE_PLTREL24 and
> > R_PPC_VLE_LOCAL24 relocs, and make the assembler generate them when
> > assembling VLE code.
> 
> Nope, they are not defined by ABI at all. I agree it would be better
> to rework. I will remove it from current patch set.

I should note that I'm not greatly concerned about adding relocations
that aren't described in an ABI document.  It's not ideal, but the
document can always be modified later.  I was more concerned that
someone may have already described R_PPC_PLTREL24 as behaving
differently for VLE, and the problem that causes for compatibility if
the GNU toolchain then goes off and implements something else.

> > As I understand it, SHF_PPC_VLE and PF_PPC_VLE are set on sections or
> > segments containing instructions that can only be executed by a
> > processor in VLE mode.  PPC_OPCODE_VLE is set for such opcodes too.
> > So, do the LSP instructions only execute in VLE mode?  I suspect not,
> > from their encoding.
> >
> > Do you have the LSP instructions in the correct opcode table?
> 
> I see that LSP instructions are available only on the cores with VLE ISA only.

OK, but if there was a processor that could operate in both VLE and
non-VLE mode (controlled by MAS2 bit VLE), would these instructions be
available in both modes?  If only VLE, then SHF_PPC_VLE ought to be
set for any section containing them.  See tc-ppc.c:3513.

However, from the fact that the "Lightweight Signal Processing APU
(LSP APU) Reference Manual" does not mention VLE, and the encoding of
these instructions, my guess is that these instruction are not VLE and
thus should not be in vle_opcodes.

-- 
Alan Modra
Australia Development Lab, IBM