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

[Jan Beulich]

On 11.12.2019 17:58, H.J. Lu wrote:
> On Wed, Dec 11, 2019 at 12:05 AM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> 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)?
> 
> taken / not taken or no-track prefixes are for branches.  We never
> add prefixes to branches.

But this was my question - could you point me at the code making
sure this won't happen? I in particular can't spot anything to this
effect in add_branch_prefix_frag_p().

Jan