Re: [PATCH][v3] Support APX NDD optimized encoding.

[Jan Beulich <jbeulich@suse.com>]

On 15.11.2023 03:59, Hu, Lin1 wrote:
> --- a/gas/config/tc-i386.c
> +++ b/gas/config/tc-i386.c
> @@ -7208,6 +7208,43 @@ check_EgprOperands (const insn_template *t)
>    return 0;
>  }
>  
> +/* Optimize APX NDD insns to legacy insns.  */
> +static bool
> +convert_NDD_to_REX2 (const insn_template *t)
> +{
> +  if (t->opcode_modifier.vexvvvv == VexVVVV_DST
> +      && t->opcode_space == SPACE_EVEXMAP4
> +      && !i.has_nf
> +      && i.reg_operands >= 2)
> +    {
> +      unsigned int readonly_var = ~0;
> +      unsigned int dest = i.operands - 1;
> +      unsigned int src1 = i.operands - 2;
> +      unsigned int src2 = (i.operands > 3) ? i.operands - 3 : 0;
> +
> +      if (i.types[src1].bitfield.class == Reg
> +	  && i.op[src1].regs == i.op[dest].regs)
> +	readonly_var = src2;
> +      /* adcx, adox and imul can't support to swap the source operands.  */
> +      else if (i.types[src2].bitfield.class == Reg
> +	       && i.op[src2].regs == i.op[dest].regs
> +	       && optimize > 1
> +	       && t->opcode_modifier.commutative)

Comment and code still aren't in line: "support to swap the source operands"
really is the D attribute in the opcode table, whereas
t->opcode_modifier.commutative is related to the C attribute (and all three
insns named really are commutative). It looks to me that the code is
correct, so it would then be the comment that may need updating. But it may
also be better to additionally check .d here (making the code robust against
C being added to the truly commutative yet not eligible to be optimized
insns). In which case the comment might say "adcx, adox, and imul, while
commutative, don't support to swap the source operands".

> +	readonly_var = src1;
> +      if (readonly_var != (unsigned int) ~0)
> +	{
> +	  if (readonly_var != src2)
> +	    swap_2_operands (readonly_var, src2);
> +
> +	  --i.operands;
> +	  --i.reg_operands;
> +
> +	  return true;
> +	}
> +    }
> +  return false;
> +}
> +
>  /* Helper function for the progress() macro in match_template().  */
>  static INLINE enum i386_error progress (enum i386_error new,
>  					enum i386_error last,
> @@ -7728,6 +7765,21 @@ match_template (char mnem_suffix)
>  	  i.memshift = memshift;
>  	}
>  
> +      /* If we can optimize a NDD insn to non-NDD insn, like

The terminology here wants to match the function name below, i.e. (as
indicated elsewhere for the name, in reply to your question) "legacy"
instead of "non-NDD" (assuming the function name is changed as well,
in line with that).

> +	 add %r16, %r8, %r8 -> add %r16, %r8,
> +	 add  %r8, %r16, %r8 -> add %r16, %r8, then rematch template.  
> +	 Note that the semantics have not been changed.  */
> +      if (optimize
> +	  && !i.no_optimize
> +	  && i.vec_encoding != vex_encoding_evex
> +	  && t + 1 < current_templates->end
> +	  && !t[1].opcode_modifier.evex

This is more fragile than it needs to be; it would imo be better to indeed
go from opcode space of the supposed alternative encoding. Perhaps that's
going to mean checking both.

Jan