RE: [PATCH 2/3] x86: Drop SwapSources

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

> On 24.04.2024 09:23, Cui, Lili wrote:
> > --- a/gas/config/tc-i386.c
> > +++ b/gas/config/tc-i386.c
> > @@ -10434,6 +10434,14 @@ build_modrm_byte (void)
> >
> >    switch (i.tm.opcode_modifier.vexvvvv)
> >      {
> > +    case VexVVVV_SRC2:
> > +      if (source != op)
> > +	{
> > +	  v = source++;
> > +	  break;
> > +	}
> > +      /* For XOP: vpshl* and vpsha*.  */
> > +      /* Fall through.  */
> >      case VexVVVV_SRC1:
> 
> This falling-through is odd and hence needs a better comment (then also
> covering vprot*, which afaict is similarly affected). The reason for this is the
> XOP.W-controlled operand swapping, if I'm not mistaken? In which case
> perhaps instead of the fall-through here the logic swapping the operands
> should replace VexVVVV_SRC2 by VexVVVV_SRC1?
> 

Yes, vprot* should be included, and it is related to XOP.W-controlled operand swapping, the comments says " /* Only the first two register operands need reversing, alongside flipping VEX.W.  */ ",  But there is actually a memory operand, not two register operands.

I think VexVVVV_SRC2 makes more sense here, it matches the actual situation, we want to use vvvv to encode the first operand.

Opcode table:
vprot<xop>, 0x90 | <xop:opc>, XOP, D|Modrm|Vex128|SpaceXOP09|VexVVVV_Src2|VexW0|NoSuf, { RegXMM, RegXMM|Unspecified|BaseIndex, RegXMM }

testcase:
vprotb (%rax),%xmm12,%xmm15
vprotb %xmm15,(%r12),%xmm0

Thanks,
Lili.