RE: FW: [PATCH 3/8] Add tests for APX GPR32 with extend evex prefix

["Cui, Lili" <lili.cui@intel.com>]

> Subject: Re: FW: [PATCH 3/8] Add tests for APX GPR32 with extend evex prefix
> 
> On 18.10.2023 13:26, Cui, Lili wrote:
> >> From: Jan Beulich <jbeulich@suse.com>
> >> Sent: Wednesday, October 18, 2023 4:06 PM
> >>
> >> On 18.10.2023 09:16, Cui, Lili wrote:
> >>>> On 18.10.2023 04:32, Cui, Lili wrote:
> >>>>>>>>> --- a/gas/testsuite/gas/i386/x86-64-inval-movbe.s
> >>>>>>>>> +++ b/gas/testsuite/gas/i386/x86-64-inval-movbe.s
> >>>>>>>>> @@ -1,5 +1,6 @@
> >>>>>>>>>  # Check illegal movbe in 64bit mode.
> >>>>>>>>>  	.text
> >>>>>>>>> +	.arch .noapx_f
> >>>>>>>>>  foo:
> >>>>>>>>>  	movbe	(%rcx),%bl
> >>>>>>>>>  	movbe	%ecx,%ebx
> >>>>>>>>
> >>>>>>>> I don't understand the need for this addition (and hence for
> >>>>>>>> the need to change the test's expecations). Like was mentioned
> >>>>>>>> on the original
> >>>>>>>> AVX10 series, tests like this shall not need modification, or
> >>>>>>>> else it indicates people's code also may need ".arch .noapx_f"
> >>>>>>>> additions, which I'm sure you agree may not be required.
> >>>>>>>> Finally, if testcase expecations like the above would be needed
> >>>>>>>> anywhere, please generalize them such that a similar mere
> >>>>>>>> addition of a line doesn't require the entire test to be
> >>>>>>>> touched. Here this means that while for the diagnostics you of
> >>>>>>>> course want exact line number matches, for the actual listing
> >>>>>>>> line numbers don't don't need matching
> >>>> individually.
> >>>>>>>>
> >>>>>>>
> >>>>>>> Agree with you, but movbe is special, movbe didn't support reg
> >>>>>>> to reg
> >>>>>> before, but APX enable it. so I added .arch .noapx_f for this invalid
> test.
> >>>>>>
> >>>>>> To be honest I didn't really notice this difference so far.
> >>>>>> That's somewhat ugly, because people mistakenly using a reg->reg
> >>>>>> form would suddenly get an EVEX encoding rather than an error.
> >>>>>> This will need some further thought; please mention the "anomaly"
> >>>>>> explicitly in the
> >>>> description of the respective patch.
> >>>>>>
> >>>>> SPEC:
> >>>>> Note: The promoted versions of MOVBE will be extended to include
> >>>>> the “MOVBE reg1, reg2” form (namely, the ModRM.Mod = 3 case) for
> >>>>> both opcodes 0xF0 and 0xF1. This extension makes the promotion of
> >>>>> BSWAP for
> >>>> NDD support unnecessary.
> >>>>>
> >>>>> I'll add a description of movbe to the patch 2/8 changelog.
> >>>>
> >>>> Thanks. I did deduce the connection to BSWAP on the way home
> yesterday.
> >>>> Which made me come up with a (spec) question: Wouldn't it make
> >>>> sense to name these
> >>>> reg->reg forms BSWAP, not MOVBE? It doesn't really matter that they
> >>>> reg->use the
> >>>> same encoding as the MOVBE (then mem-only) forms. That would then
> >>>> eliminate the concern I raised.
> >>>>
> >>>
> >>> If we extend BSWAP, it needs to add the operation of reading
> >>> ModRM.Mod. I
> >> think this is the benefit of this solution now.
> >>
> >> I'm afraid I don't follow: I don't propose extending the original BSWAP.
> >> I merely propose to use the BSWAP mnemonic with the reg->reg form of
> >> what's presently called MOVBE. (As an aside, in principle no ModR/M
> >> byte would be needed to extend the original BSWAP: One operand is
> >> encoded in the base opcode, and the other could be encoded in VVVV.
> >> But that would of course needlessly consume an entire row in the
> >> opcode table. Hence why I fully understand that the opcode wants
> >> sharing with MOVBE.)
> >>
> >
> > I think it's a trade-off between functionality closer to BSWAP and opcode
> closer to MOVBE. Since the spec was published and gcc has committed the
> patch to the community, I think it would be hard to push it to change.
> 
> In which case I wonder how one could influence such decisions, when the first
> publication of such a spec already means things are set in stone. I was hoping
> that only inclusion in the SDM would mean no (easy) changes anymore.
> 

We feedback this issue to the author, he said : mobve has the same operation as bswap, while bswap only has 1 operand. Movbe has src and dest. The reason for introducing movbe is that we don't want to promote bswap.

Lili.