[PATCH 04/10] Support Intel CMPccXADD

[Jiang, Haochen]

> -----Original Message-----
> From: Jan Beulich <jbeulich@suse.com>
> Sent: Tuesday, October 25, 2022 2:53 PM
> To: Jiang, Haochen <haochen.jiang@intel.com>
> Cc: hjl.tools@gmail.com; binutils@sourceware.org
> Subject: Re: [PATCH 04/10] Support Intel CMPccXADD
> 
> On 24.10.2022 07:55, Jiang, Haochen wrote:
> >> -----Original Message-----
> >> From: Jan Beulich <jbeulich@suse.com>
> >> Sent: Friday, October 14, 2022 9:47 PM
> >>
> >> On 14.10.2022 11:12, Haochen Jiang wrote:
> >> And finally - is SwapSources really appropriate to use here? There's only
> one
> >> pure source operand, the other two are also serving as destinations.
> >> I wonder whether an attribute is necessary here in the first place: Vex-
> encoded
> >> insns with a memory destination never have two further register
> operands, so
> >> that property should suffice for identifying the case in
> build_modrm_byte().
> >> Alternatively you could also simply use the CPU flag.
> >
> > We may need a special identifier for CMPccXADD since we have VVVV at
> > operand 3, where it is always at operand 2 for all other insts which
> > have VVVV. That is the reason we reuse SwapSources. It might be not
> > that same as the original meaning. But we want to avoid adding a bit
> > for this very rare case. Do we need to change that?
> 
> Re-using existing attributes is certainly preferred. But the question here
> was whether _any_ special attribute is needed. Did you try out my
> suggestion,
> and it didn't work out for some reason? Avoiding the (ab)use of an
> inappropriately (for the purpose here) named attribute would imo be
> preferable.

Actually we have some similar instructions. For example, vmaskmovps/d.
It could also take one memory operand as dest and two register operands
as source.

However, cmp<cc>xadd has a different encoding pattern with it. The default
behavior for previous insts like vmaskmovps/d in Intel syntax is to encode
first register operand as vvvv and second register operand as modrm:reg.
In cmp<cc>xadd, it is swapped, with first register operand as modrm:reg and
second register operand as vvvv. I suppose it makes sense to use SwapSource
and it is quite hard or might be dirty to just use the number of registers to
identify them in build_modrm_byte().

Haochen

> 
> Jan