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


On Thu, Dec 12, 2019 at 8:10 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 12.12.2019 16:54, H.J. Lu wrote:
> > On Thu, Dec 12, 2019 at 1:55 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> 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().
> >>
> >
> > There are
> >
> >       if (branch)
> >         /* Skip if this is a branch.  */
> >         ;
> >       else if (add_fused_jcc_padding_frag_p ())
> > ...
> >        else if (add_branch_prefix_frag_p ())
> >
> > add_branch_prefix_frag_p is never called on branch nor fused jcc.
>
> Ah, I see. Of course add_branch_padding_frag_p() does neither deal
> with all kinds of branches, nor does it set the caller's branch
> variable to other than align_branch_none. This may be only a
> latent issue, but I assume you're aware of the SDM stating "use
> with any branch instruction is reserved" for every single segment
> override prefix (which, possibly wrongly, includes far branches as
> well as JCXZ/LOOP et al).
>

There are

  if (add_branch_padding_frag_p (&branch))
     {
     }

  /* Output jumps.  */
  if (i.tm.opcode_modifier.jump == JUMP)
    output_branch ();
  else if (i.tm.opcode_modifier.jump == JUMP_BYTE
           || i.tm.opcode_modifier.jump == JUMP_DWORD)
    output_jump ();
  else if (i.tm.opcode_modifier.jump == JUMP_INTERSEGMENT)
    output_interseg_jump ();
  else
    {

We don't add prefix to any branches.


-- 
H.J.


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