V4 [PATCH 2/4] i386: Align branches within a fixed boundary
H.J. Lu
hjl.tools@gmail.com
Wed Dec 11 16:59:00 GMT 2019
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.
--
H.J.
More information about the Binutils
mailing list