RE: [PATCH v2 1/4] x86: zap value-less Disp8MemShift from non-EVEX templates

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

> On 24.04.2024 08:49, Cui, Lili wrote:
> >> In order to allow to continue to use templatized SSE2AVX templates
> >> when enhancing those to also cover eGPR usage, Disp8MemShift wants
> >> using to deviate from what general template attributes supply. That
> >> requires using Disp8MemShift in a way also affecting non-EVEX
> >> templates, yet having this attribute set would so far implicitly mean EVEX
> encoding.
> >> Recognize the case and instead zap the attribute if no other
> >> attribute indicates EVEX encoding.
> >>
> >
> > I'm confused about this patch, is it related to the movsd template? You
> removed the "Masking" for it and only left Disp8MemShift, but I thought it
> still belongs to EVEX template.
> >
> > +movsd, 0xf210, AVX512F,
> > +D|Modrm|EVexLIG|Space0F|VexW1|Disp8MemShift=3|NoSuf|SSE2AVX,
> {
> > +Qword|Unspecified|BaseIndex, RegXMM }
> 
> There's no "masking" in an SSE2AVX template. Use of masking in a legacy-
> form input instruction is simply wrong, and wants rejecting.
> 

Is this patch intended to fix the following situation? If not, could you give an example?

<sse41:cpu:attr:scal:vvvv, $avx:AVX:Vex128|VexW0|SSE2AVX:VexLIG|VexW0|SSE2AVX:VexVVVV, $sse:SSE4_1:::>

+<SSE41D:cpu:attr:scal:vvvv, +
+    $avx:AVX|AVX512VL:Vex128|EVex128|VexW0|Disp8MemShift=4|SSE2AVX:VexLIG|EVexLIG|VexW0|Disp8MemShift=2|SSE2AVX:VexVVVV, +
+    $sse:SSE4_1:::>

-insertps<sse41>, 0x660f3a21, <sse41:cpu>, Modrm|<sse41:attr>|<sse41:vvvv>|NoSuf, { Imm8, Dword|Unspecified|BaseIndex|RegXMM, RegXMM }
+insertps<SSE41D>, 0x660f3a21, <SSE41D:cpu>, Modrm|<SSE41D:attr>|<SSE41D:vvvv>|Disp8MemShift|NoSuf, { Imm8, Dword|Unspecified|BaseIndex|RegXMM, RegXMM }

I'm confused why the Disp8MemShift is added here.

> >> --- a/opcodes/i386-gen.c
> >> +++ b/opcodes/i386-gen.c
> >> @@ -1126,6 +1126,7 @@ process_i386_opcode_modifier (FILE *tabl
> >>  			      char **opnd, int lineno, bool rex2_disallowed)  {
> >>    char *str, *next, *last;
> >> +  bool disp8_shift_derived = false;
> >>    bitfield modifiers [ARRAY_SIZE (opcode_modifiers)];
> >>    static const char *const spaces[] = {  #define SPACE(n)
> >> [SPACE_##n] = #n @@ -1190,7 +1191,10 @@
> process_i386_opcode_modifier (FILE *tabl
> >>  	      if (strcasecmp(str, "Broadcast") == 0)
> >>  		val = get_element_size (opnd, lineno) + BYTE_BROADCAST;
> >>  	      else if (strcasecmp(str, "Disp8MemShift") == 0)
> >> -		val = get_element_size (opnd, lineno);
> >> +		{
> >> +		  val = get_element_size (opnd, lineno);
> >> +		  disp8_shift_derived = true;
> >> +		}
> >>
> >>  	      set_bitfield (str, modifiers, val, ARRAY_SIZE (modifiers),
> >>  			    lineno);
> >> @@ -1243,13 +1247,21 @@ process_i386_opcode_modifier (FILE *tabl
> >>
> >>    /* Rather than evaluating multiple conditions at runtime to determine
> >>       whether an EVEX encoding is being dealt with, derive that information
> >> -     right here.  A missing EVex attribute means "dynamic".  */
> >> -  if (!modifiers[EVex].value
> >> -      && (modifiers[Disp8MemShift].value
> >> -	  || modifiers[Broadcast].value
> >> +     right here.  A missing EVex attribute means "dynamic".  There's one
> >> +     exception though: A value-less Disp8MemShift needs zapping rather
> than
> >> +     respecting if no other attribute indicates EVEX encoding.  This is for
> >> +     certain SSE2AVX templatized templates to work reasonably.  */
> >> + if
> >> + (!modifiers[EVex].value)
> >> +    {
> >> +      if (modifiers[Broadcast].value
> >>  	  || modifiers[Masking].value
> >> -	  || modifiers[SAE].value))
> >> -    modifiers[EVex].value = EVEXDYN;
> >> +	  || modifiers[SAE].value)
> >> +	modifiers[EVex].value = EVEXDYN;
> >> +      else if (disp8_shift_derived)
> >> +	modifiers[Disp8MemShift].value = 0;
> >> +      else if (modifiers[Disp8MemShift].value)
> >> +	modifiers[EVex].value = EVEXDYN;
> >> +    }
> >>
> >
> > Why not just delete the Disp8MemShift? Maybe I missed something.
> >
> >   if (!modifiers[EVex].value
> >       && (modifiers[Broadcast].value
> >           || modifiers[Masking].value
> >           || modifiers[SAE].value))
> >     modifiers[EVex].value = EVEXDYN;
> 
> There are templates where Disp8MemShift is the only indication of EVEX
> encoding, e.g. VMOVNTDQ and VMOVNTDQA. Those could all gain EVexDYN
> (I think), but that would be a somewhat more intrusive change then. I'm
> willing to be convinced of going that route (assuming it actually would work
> out).
> 
Oh, got it, they still need Disp8MemShift for EVEX, that makes sense.

Lili.