V4 [PATCH 2/4] i386: Align branches within a fixed boundary

[Jan Beulich]

On 10.12.2019 17:21,  H.J. Lu  wrote:
> @@ -8473,9 +8815,105 @@ output_insn (void)
>  	  if (j > 15)
>  	    as_warn (_("instruction length of %u bytes exceeds the limit of 15"),
>  		     j);
> +	  else if (fragP)
> +	    {
> +	      /* NB: Don't add prefix with GOTPC relocation since
> +		 output_disp() above depends on the fixed encoding
> +		 length.  Can't add prefix with TLS relocation since
> +		 it breaks TLS linker optimization.  */
> +	      unsigned int max = i.has_gotpc_tls_reloc ? 0 : 15 - j;
> +	      /* Prefix count on the current instruction.  */
> +	      unsigned int count = i.vex.length;
> +	      unsigned int k;
> +	      for (k = 0; k < ARRAY_SIZE (i.prefix); k++)
> +		/* REX byte is encoded in VEX/EVEX prefix.  */
> +		if (i.prefix[k] && (k != REX_PREFIX || !i.vex.length))
> +		  count++;
> +
> +	      /* Count prefixes for extended opcode maps.  */
> +	      if (!i.vex.length)
> +		switch (i.tm.opcode_length)
> +		  {
> +		  case 3:
> +		    if (((i.tm.base_opcode >> 16) & 0xff) == 0xf)
> +		      {
> +			count++;
> +			switch ((i.tm.base_opcode >> 8) & 0xff)
> +			  {
> +			  case 0x38:
> +			  case 0x3a:
> +			    count++;
> +			    break;
> +			  default:
> +			    break;
> +			  }
> +		      }
> +		    break;
> +		  case 2:
> +		    if (((i.tm.base_opcode >> 8) & 0xff) == 0xf)
> +		      count++;
> +		    break;
> +		  case 1:
> +		    break;
> +		  default:
> +		    abort ();
> +		  }
> +
> +	      if (TYPE_FROM_RELAX_STATE (fragP->fr_subtype)
> +		  == BRANCH_PREFIX)
> +		{
> +		  /* Set the maximum prefix size in BRANCH_PREFIX
> +		     frag.  */
> +		  if (fragP->tc_frag_data.max_bytes > max)
> +		    fragP->tc_frag_data.max_bytes = max;
> +		  if (fragP->tc_frag_data.max_bytes > count)
> +		    fragP->tc_frag_data.max_bytes -= count;
> +		  else
> +		    fragP->tc_frag_data.max_bytes = 0;
> +		}
> +	      else
> +		{
> +		  /* Remember the maximum prefix size in FUSED_JCC_PADDING
> +		     frag.  */
> +		  unsigned int max_prefix_size;
> +		  if (align_branch_prefix_size > max)
> +		    max_prefix_size = max;
> +		  else
> +		    max_prefix_size = align_branch_prefix_size;
> +		  if (max_prefix_size > count)
> +		    fragP->tc_frag_data.max_prefix_length
> +		      = max_prefix_size - count;
> +		}
> +
> +	      /* Use existing segment prefix if possible.  Use CS
> +		 segment prefix in 64-bit mode.  In 32-bit mode, use SS
> +		 segment prefix with ESP/EBP base register and use DS
> +		 segment prefix without ESP/EBP base register.  */
> +	      if (i.prefix[SEG_PREFIX])
> +		fragP->tc_frag_data.default_prefix = i.prefix[SEG_PREFIX];
> +	      else if (flag_code == CODE_64BIT)
> +		fragP->tc_frag_data.default_prefix = CS_PREFIX_OPCODE;
> +	      else if (i.base_reg
> +		       && (i.base_reg->reg_num == 4
> +			   || i.base_reg->reg_num == 5))
> +		fragP->tc_frag_data.default_prefix = SS_PREFIX_OPCODE;
> +	      else
> +		fragP->tc_frag_data.default_prefix = DS_PREFIX_OPCODE;
> +	    }

Could you point me at where it is being excluded that any of
these prefixes would get added to insns where they would have
the meaning of a hint (taken / not taken or no-track)?

Jan