Re: [PATCH,V2] gas: x86: ginsn: adjust ginsns for certain lea ops

[Jan Beulich <jbeulich@suse.com>]

On 24.01.2024 07:40, Indu Bhagat wrote:
> @@ -5664,76 +5666,64 @@ x86_ginsn_lea (const symbolS *insn_end_sym)
>  {
>    offsetT src_disp = 0;
>    ginsnS *ginsn = NULL;
> -  unsigned int base_reg;
> -  unsigned int index_reg;
> +  unsigned int src1_reg;
> +  const reg_entry *src1;
>    offsetT index_scale;
>    unsigned int dst_reg;
>  
>    if (!i.index_reg && !i.base_reg)
>      {
> -      /* lea symbol, %rN.  */
> -      dst_reg = ginsn_dw2_regnum (i.op[1].regs);
> -      /* TBD_GINSN_INFO_LOSS - Skip encoding information about the symbol.  */
> -      ginsn = ginsn_new_mov (insn_end_sym, false,
> -			     GINSN_SRC_IMM, 0xf /* arbitrary const.  */, 0,
> -			     GINSN_DST_REG, dst_reg, 0);
> +      if (i.disp_operands && i.op[0].disps->X_op == O_constant)
> +	{
> +	  /* lea const, %rN.  */
> +	  src_disp = i.op[0].disps->X_add_number;
> +	  dst_reg = ginsn_dw2_regnum (i.op[1].regs);
> +	  ginsn = ginsn_new_mov (insn_end_sym, false,
> +				 GINSN_SRC_IMM, 0, src_disp,
> +				 GINSN_DST_REG, dst_reg, 0);
> +	  ginsn_set_where (ginsn);
> +	}

Since earlier on you've been mentioning that you primarily target insn
forms actually in use in existing code, I wonder whether you've ever
seen any use of this. The same is better (shorter) expressed by MOV,
and hence I'd expect people to prefer that form. IOW the question here
is: Is there much value in having this code, rather than simply
penalizing people bogusly using such by having this case end at the
common x86_ginsn_unhandled path as well.

> +      /* Skip handling lea symbol, %rN here.  Deal with it in the
> +	 x86_ginsn_unhandled code path.  TBD_GINSN_GEN_NOT_SCFI.  */
>      }
> -  else if (i.base_reg && !i.index_reg)
> +  else if ((i.base_reg && !i.index_reg)
> +	   || (!i.base_reg && i.index_reg))

I frequently see conditionals like this written this way. Maybe it's
indeed clearer to a majority; personally I'd prefer the shorter

  else if (!i.base_reg != !i.index_reg)

However, considering the earlier if() this is an else-if to, even

  else if (!i.base_reg || !i.index_reg)

would also suffice (but as per above that if() may want to go away).

>      {
> -      /* lea    -0x2(%base),%dst.  */
> -      base_reg = ginsn_dw2_regnum (i.base_reg);
> -      dst_reg = ginsn_dw2_regnum (i.op[1].regs);
> +      /* lea disp(%base) %dst    or    lea disp(,%index,imm) %dst.  */

Would be nice if the missing commas were added here.

> -      if (i.disp_operands)
> -	src_disp = i.op[0].disps->X_add_number;
> -
> -      if (src_disp)
> -	/* Generate an ADD ginsn.  */
> -	ginsn = ginsn_new_add (insn_end_sym, true,
> -			       GINSN_SRC_REG, base_reg, 0,
> -			       GINSN_SRC_IMM, 0, src_disp,
> -			       GINSN_DST_REG, dst_reg, 0);
> -      else
> -	/* Generate a MOV ginsn.  */
> -	ginsn = ginsn_new_mov (insn_end_sym, true,
> -			       GINSN_SRC_REG, base_reg, 0,
> -			       GINSN_DST_REG, dst_reg, 0);
> -    }
> -  else if (!i.base_reg && i.index_reg)
> -    {
> -      /* lea (,%index,imm), %dst.  */
> -      /* TBD_GINSN_INFO_LOSS - There is no explicit ginsn multiply operation,
> -	 instead use GINSN_TYPE_OTHER.  Also, note that info about displacement
> -	 is not carried forward either.  But this is fine because
> -	 GINSN_TYPE_OTHER will cause SCFI pass to bail out any which way if
> -	 dest reg is interesting.  */
>        index_scale = i.log2_scale_factor;
> -      index_reg = ginsn_dw2_regnum (i.index_reg);
> +      src1 = (i.base_reg) ? i.base_reg : i.index_reg;
> +      src1_reg = ginsn_dw2_regnum (src1);

Since I can't spot any other use of src1, why not simply

      src1_reg = ginsn_dw2_regnum (i.base_reg ? i.base_reg : i.index_reg);

? Otherwise at the very least please omit the pointless parentheses in
the conditional expression.

>        dst_reg = ginsn_dw2_regnum (i.op[1].regs);
> -      ginsn = ginsn_new_other (insn_end_sym, true,
> -			       GINSN_SRC_REG, index_reg,
> -			       GINSN_SRC_IMM, index_scale,
> -			       GINSN_DST_REG, dst_reg);
> -      /* FIXME - It seems to make sense to represent a scale factor of 1
> -	 correctly here (i.e. not as "other", but rather similar to the
> -	 base-without- index case above)?  */
> -    }
> -  else
> -    {
> -      /* lea disp(%base,%index,imm) %dst.  */
> -      /* TBD_GINSN_INFO_LOSS - Skip adding information about the disp and imm
> -	 for index reg.  */
> -      base_reg = ginsn_dw2_regnum (i.base_reg);
> -      index_reg = ginsn_dw2_regnum (i.index_reg);
> -      dst_reg = ginsn_dw2_regnum (i.op[1].regs);
> -      /* Generate an GINSN_TYPE_OTHER ginsn.  */
> -      ginsn = ginsn_new_other (insn_end_sym, true,
> -			       GINSN_SRC_REG, base_reg,
> -			       GINSN_SRC_REG, index_reg,
> -			       GINSN_DST_REG, dst_reg);
> -    }
> +      /* It makes sense to represent a scale factor of 1 precisely here
> +	 (i.e., not using GINSN_TYPE_OTHER, but rather similar to the
> +	 base-without-index case).  Ignore the case when disp has a symbol
> +	 instead.  */
> +      if (!index_scale
> +	  && (!i.disp_operands
> +	      || (i.disp_operands && i.op[0].disps->X_op == O_constant)))

This is functionally identical to the shorter

      if (!index_scale
	  && (!i.disp_operands || i.op[0].disps->X_op == O_constant))

But: What about any of

	lea	(%rax,%riz),%rbp
	lea	(%rax,4),%rbp
	lea	(%rax,%riz,4),%rbp

?

Jan