V4 [PATCH 2/4] i386: Align branches within a fixed boundary
H.J. Lu
hjl.tools@gmail.com
Thu Dec 12 16:23:00 GMT 2019
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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