i386: Align branches within a fixed boundary

Add 3 command-line options to align branches within a fixed boundary
with segment prefixes or NOPs:

1. -malign-branch-boundary=NUM aligns branches within NUM byte boundary.
2. -malign-branch=TYPE[+TYPE...] specifies types of branches to align.
The supported branches are:
  a. Conditional jump.
  b. Fused conditional jump.
  c. Unconditional jump.
  d. Call.
  e. Ret.
  f. Indirect jump and call.
3. -malign-branch-prefix-size=NUM aligns branches with NUM segment
prefixes per instruction.

3 new rs_machine_dependent frag types are added:

1. BRANCH_PADDING.  The variable size frag to insert NOP before branch.
2. BRANCH_PREFIX.  The variable size frag to insert segment prefixes to
an instruction.  The choices of prefixes are:
   a. Use the existing segment prefix if there is one.
   b. Use CS segment prefix in 64-bit mode.
   c. In 32-bit mode, use SS segment prefix with ESP/EBP base register
   and use DS segment prefix without ESP/EBP base register.
3. FUSED_JCC_PADDING.  The variable size frag to insert NOP before fused
conditional jump.

The new rs_machine_dependent frags aren't inserted if the previous item
is a prefix or a constant directive, which may be used to hardcode an
instruction, since there is no clear instruction boundary.  Segment
prefixes and NOP padding are disabled before relaxable TLS relocations
and tls_get_addr calls to keep TLS instruction sequence unchanged.

md_estimate_size_before_relax() and i386_generic_table_relax_frag() are
used to handled BRANCH_PADDING, BRANCH_PREFIX and FUSED_JCC_PADDING frags.
i386_generic_table_relax_frag() grows or shrinks sizes of segment prefix
and NOP to align the next branch frag:

1. First try to add segment prefixes to instructions before a branch.
2. If there is no sufficient room to add segment prefixes, NOP will be
inserted before a branch.

	* config/tc-i386.c (_i386_insn): Add has_gotpc_tls_reloc.
	(tls_get_addr): New.
	(last_insn): New.
	(align_branch_power): New.
	(align_branch_kind): New.
	(align_branch_bit): New.
	(align_branch): New.
	(MAX_FUSED_JCC_PADDING_SIZE): New.
	(align_branch_prefix_size): New.
	(BRANCH_PADDING): New.
	(BRANCH_PREFIX): New.
	(FUSED_JCC_PADDING): New.
	(i386_generate_nops): Support BRANCH_PADDING and FUSED_JCC_PADDING.
	(md_begin): Abort if align_branch_prefix_size <
	MAX_FUSED_JCC_PADDING_SIZE.
	(md_assemble): Set last_insn.
	(maybe_fused_with_jcc_p): New.
	(add_fused_jcc_padding_frag_p): New.
	(add_branch_prefix_frag_p): New.
	(add_branch_padding_frag_p): New.
	(output_insn): Generate a BRANCH_PADDING, FUSED_JCC_PADDING or
	BRANCH_PREFIX frag and terminate each frag to align branches.
	(output_disp): Set i.has_gotpc_tls_reloc to TRUE for GOTPC and
	relaxable TLS relocations.
	(output_imm): Likewise.
	(i386_next_non_empty_frag): New.
	(i386_next_jcc_frag): New.
	(i386_classify_machine_dependent_frag): New.
	(i386_branch_padding_size): New.
	(i386_generic_table_relax_frag): New.
	(md_estimate_size_before_relax): Handle COND_JUMP_PADDING,
	FUSED_JCC_PADDING and COND_JUMP_PREFIX frags.
	(md_convert_frag): Handle BRANCH_PADDING, BRANCH_PREFIX and
	FUSED_JCC_PADDING frags.
	(OPTION_MALIGN_BRANCH_BOUNDARY): New.
	(OPTION_MALIGN_BRANCH_PREFIX_SIZE): New.
	(OPTION_MALIGN_BRANCH): New.
	(md_longopts): Add -malign-branch-boundary=,
	-malign-branch-prefix-size= and -malign-branch=.
	(md_parse_option): Handle -malign-branch-boundary=,
	-malign-branch-prefix-size= and -malign-branch=.
	(md_show_usage): Display -malign-branch-boundary=,
	-malign-branch-prefix-size= and -malign-branch=.
	(i386_target_format): Set tls_get_addr.
	(i386_cons_align): New.
	* config/tc-i386.h (i386_cons_align): New.
	(md_cons_align): New.
	(i386_generic_table_relax_frag): New.
	(md_generic_table_relax_frag): New.
	(i386_tc_frag_data): Add u, padding_address, length,
	max_prefix_length, prefix_length, default_prefix, cmp_size,
	classified and branch_type.
	(TC_FRAG_INIT): Initialize u, padding_address, length,
	max_prefix_length, prefix_length, default_prefix, cmp_size,
	classified and branch_type.
	* doc/c-i386.texi: Document -malign-branch-boundary=,
	-malign-branch= and -malign-branch-prefix-size=.

1 files changed, 26 insertions, 0 deletions

diff --git a/gas/doc/c-i386.texi b/gas/doc/c-i386.texi
index 589b426..74296e6 100644
--- a/gas/doc/c-i386.texi
+++ b/gas/doc/c-i386.texi
@@ -423,6 +423,32 @@ R_X86_64_REX_GOTPCRELX, in 64-bit mode.
 relocations.  The default can be controlled by a configure option
 @option{--enable-x86-relax-relocations}.
 
+@cindex @samp{-malign-branch-boundary=} option, i386
+@cindex @samp{-malign-branch-boundary=} option, x86-64
+@item -malign-branch-boundary=@var{NUM}
+This option controls how the assembler should align branches with segment
+prefixes or NOP.  @var{NUM} must be a power of 2.  It should be 0 or
+no less than 16.  Branches will be aligned within @var{NUM} byte
+boundary.  @option{-malign-branch-boundary=0}, which is the default,
+doesn't align branches.
+
+@cindex @samp{-malign-branch=} option, i386
+@cindex @samp{-malign-branch=} option, x86-64
+@item -malign-branch=@var{TYPE}[+@var{TYPE}...]
+This option specifies types of branches to align. @var{TYPE} is
+combination of @samp{jcc}, which aligns conditional jumps,
+@samp{fused}, which aligns fused conditional jumps, @samp{jmp},
+which aligns unconditional jumps, @samp{call} which aligns calls,
+@samp{ret}, which aligns rets, @samp{indirect}, which aligns indirect
+jumps and calls.  The default is @option{-malign-branch=jcc+fused+jmp}.
+
+@cindex @samp{-malign-branch-prefix-size=} option, i386
+@cindex @samp{-malign-branch-prefix-size=} option, x86-64
+@item -malign-branch-prefix-size=@var{NUM}
+This option specifies the maximum number of prefixes on an instruction
+to align branches.  @var{NUM} should be between 0 and 5.  The default
+@var{NUM} is 5.
+
 @cindex @samp{-mx86-used-note=} option, i386
 @cindex @samp{-mx86-used-note=} option, x86-64
 @item -mx86-used-note=@var{no}