[PATCH 0/6] correct and further utilize x86'es last-insn tracking

[PATCH 0/6] correct and further utilize x86'es last-insn tracking

[Jan Beulich]

..., involving corrections to common code as well. Note however that
this doesn't address all shortcomings (yet).

1: x86: last-insn recording should be per-section
2: x86: suppress optimization after potential non-insn
3: gas: no md_cons_align() for .nop{,s}
4: x86: i386_cons_align() badly affects diagnostics
5: x86: adjust NOP generation after potential non-insn
6: gas: drop unused fields from struct segment_info_struct

Jan

[PATCH 1/6] x86: last-insn recording should be per-section

[Jan Beulich]

Otherwise intermediate section switches result in inconsistent behavior.
Note, however, that intermediate sub-section switches will continue to
result in inconsistent or even inappropriate behavior.

While there also add recording of state to s_insn().
---
i386_cons_align() qualifying the recording on SEC_CODE seems suspicious,
too: md_assemble() isn't constrained to emitting instructions to only
text sections.

For sub-sections the best I can think of that we could do is warn when
now_subseg is nonzero wherever we already warn based on last_insn state
(and similarly bail rather than inserting any code). That would have
not really intended effects in subsequent patches, though: We'd then
generally suppress optimizations outside of subseg 0, and we'd also
prefix plain old NOPs there (in i386_generate_nops()).

For .insn I'm not entirely decided: We might also assume that only
proper insns are constructed this way, and hence record last_insn_other
in s_insn().

--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -178,7 +178,7 @@ static int check_word_reg (void);
 static int finalize_imm (void);
 static int process_operands (void);
 static const reg_entry *build_modrm_byte (void);
-static void output_insn (void);
+static void output_insn (const struct last_insn *);
 static void output_imm (fragS *, offsetT);
 static void output_disp (fragS *, offsetT);
 #ifndef I386COFF
@@ -677,21 +677,6 @@ static enum lfence_before_ret_kind
   }
 lfence_before_ret;
 
-/* Types of previous instruction is .byte or prefix.  */
-static struct
-  {
-    segT seg;
-    const char *file;
-    const char *name;
-    unsigned int line;
-    enum last_insn_kind
-      {
-	last_insn_other = 0,
-	last_insn_directive,
-	last_insn_prefix
-      } kind;
-  } last_insn;
-
 /* 1 if the assembler should generate relax relocations.  */
 
 static int generate_relax_relocations
@@ -4971,7 +4956,7 @@ insert_lfence_after (void)
 /* Output lfence, 0xfaee8, before instruction.  */
 
 static void
-insert_lfence_before (void)
+insert_lfence_before (const struct last_insn *last_insn)
 {
   char *p;
 
@@ -5007,12 +4992,11 @@ insert_lfence_before (void)
       else
 	return;
 
-      if (last_insn.kind != last_insn_other
-	  && last_insn.seg == now_seg)
+      if (last_insn->kind != last_insn_other)
 	{
-	  as_warn_where (last_insn.file, last_insn.line,
+	  as_warn_where (last_insn->file, last_insn->line,
 			 _("`%s` skips -mlfence-before-indirect-branch on `%s`"),
-			 last_insn.name, insn_name (&i.tm));
+			 last_insn->name, insn_name (&i.tm));
 	  return;
 	}
 
@@ -5027,12 +5011,11 @@ insert_lfence_before (void)
   if (lfence_before_ret != lfence_before_ret_none
       && (i.tm.base_opcode | 1) == 0xc3)
     {
-      if (last_insn.kind != last_insn_other
-	  && last_insn.seg == now_seg)
+      if (last_insn->kind != last_insn_other)
 	{
-	  as_warn_where (last_insn.file, last_insn.line,
+	  as_warn_where (last_insn->file, last_insn->line,
 			 _("`%s` skips -mlfence-before-ret on `%s`"),
-			 last_insn.name, insn_name (&i.tm));
+			 last_insn->name, insn_name (&i.tm));
 	  return;
 	}
 
@@ -5129,6 +5112,8 @@ md_assemble (char *line)
   const char *end, *pass1_mnem = NULL;
   enum i386_error pass1_err = 0;
   const insn_template *t;
+  struct last_insn *last_insn
+    = &seg_info(now_seg)->tc_segment_info_data.last_insn;
 
   /* Initialize globals.  */
   current_templates.end = current_templates.start = NULL;
@@ -5682,23 +5667,21 @@ md_assemble (char *line)
   if (i.rex != 0)
     add_prefix (REX_OPCODE | i.rex);
 
-  insert_lfence_before ();
+  insert_lfence_before (last_insn);
 
   /* We are ready to output the insn.  */
-  output_insn ();
+  output_insn (last_insn);
 
   insert_lfence_after ();
 
-  last_insn.seg = now_seg;
-
   if (i.tm.opcode_modifier.isprefix)
     {
-      last_insn.kind = last_insn_prefix;
-      last_insn.name = insn_name (&i.tm);
-      last_insn.file = as_where (&last_insn.line);
+      last_insn->kind = last_insn_prefix;
+      last_insn->name = insn_name (&i.tm);
+      last_insn->file = as_where (&last_insn->line);
     }
   else
-    last_insn.kind = last_insn_other;
+    last_insn->kind = last_insn_other;
 }
 
 /* The Q suffix is generally valid only in 64-bit mode, with very few
@@ -9667,7 +9650,8 @@ maybe_fused_with_jcc_p (enum mf_cmp_kind
 /* Return 1 if a FUSED_JCC_PADDING frag should be generated.  */
 
 static int
-add_fused_jcc_padding_frag_p (enum mf_cmp_kind* mf_cmp_p)
+add_fused_jcc_padding_frag_p (enum mf_cmp_kind *mf_cmp_p,
+			      const struct last_insn *last_insn)
 {
   /* NB: Don't work with COND_JUMP86 without i386.  */
   if (!align_branch_power
@@ -9678,13 +9662,12 @@ add_fused_jcc_padding_frag_p (enum mf_cm
 
   if (maybe_fused_with_jcc_p (mf_cmp_p))
     {
-      if (last_insn.kind == last_insn_other
-	  || last_insn.seg != now_seg)
+      if (last_insn->kind == last_insn_other)
 	return 1;
       if (flag_debug)
-	as_warn_where (last_insn.file, last_insn.line,
+	as_warn_where (last_insn->file, last_insn->line,
 		       _("`%s` skips -malign-branch-boundary on `%s`"),
-		       last_insn.name, insn_name (&i.tm));
+		       last_insn->name, insn_name (&i.tm));
     }
 
   return 0;
@@ -9693,7 +9676,7 @@ add_fused_jcc_padding_frag_p (enum mf_cm
 /* Return 1 if a BRANCH_PREFIX frag should be generated.  */
 
 static int
-add_branch_prefix_frag_p (void)
+add_branch_prefix_frag_p (const struct last_insn *last_insn)
 {
   /* NB: Don't work with COND_JUMP86 without i386.  Don't add prefix
      to PadLock instructions since they include prefixes in opcode.  */
@@ -9709,14 +9692,13 @@ add_branch_prefix_frag_p (void)
   if (!i.operands || i.tm.opcode_modifier.isprefix)
     return 0;
 
-  if (last_insn.kind == last_insn_other
-      || last_insn.seg != now_seg)
+  if (last_insn->kind == last_insn_other)
     return 1;
 
   if (flag_debug)
-    as_warn_where (last_insn.file, last_insn.line,
+    as_warn_where (last_insn->file, last_insn->line,
 		   _("`%s` skips -malign-branch-boundary on `%s`"),
-		   last_insn.name, insn_name (&i.tm));
+		   last_insn->name, insn_name (&i.tm));
 
   return 0;
 }
@@ -9725,7 +9707,8 @@ add_branch_prefix_frag_p (void)
 
 static int
 add_branch_padding_frag_p (enum align_branch_kind *branch_p,
-			   enum mf_jcc_kind *mf_jcc_p)
+			   enum mf_jcc_kind *mf_jcc_p,
+			   const struct last_insn *last_insn)
 {
   int add_padding;
 
@@ -9799,13 +9782,12 @@ add_branch_padding_frag_p (enum align_br
     }
 
   if (add_padding
-      && last_insn.kind != last_insn_other
-      && last_insn.seg == now_seg)
+      && last_insn->kind != last_insn_other)
     {
       if (flag_debug)
-	as_warn_where (last_insn.file, last_insn.line,
+	as_warn_where (last_insn->file, last_insn->line,
 		       _("`%s` skips -malign-branch-boundary on `%s`"),
-		       last_insn.name, insn_name (&i.tm));
+		       last_insn->name, insn_name (&i.tm));
       return 0;
     }
 
@@ -9813,7 +9795,7 @@ add_branch_padding_frag_p (enum align_br
 }
 
 static void
-output_insn (void)
+output_insn (const struct last_insn *last_insn)
 {
   fragS *insn_start_frag;
   offsetT insn_start_off;
@@ -9943,7 +9925,7 @@ output_insn (void)
   insn_start_frag = frag_now;
   insn_start_off = frag_now_fix ();
 
-  if (add_branch_padding_frag_p (&branch, &mf_jcc))
+  if (add_branch_padding_frag_p (&branch, &mf_jcc, last_insn))
     {
       char *p;
       /* Branch can be 8 bytes.  Leave some room for prefixes.  */
@@ -10029,7 +10011,7 @@ output_insn (void)
       if (branch)
 	/* Skip if this is a branch.  */
 	;
-      else if (add_fused_jcc_padding_frag_p (&mf_cmp))
+      else if (add_fused_jcc_padding_frag_p (&mf_cmp, last_insn))
 	{
 	  /* Make room for padding.  */
 	  frag_grow (MAX_FUSED_JCC_PADDING_SIZE);
@@ -10045,7 +10027,7 @@ output_insn (void)
 	  fragP->tc_frag_data.branch_type = align_branch_fused;
 	  fragP->tc_frag_data.max_bytes = MAX_FUSED_JCC_PADDING_SIZE;
 	}
-      else if (add_branch_prefix_frag_p ())
+      else if (add_branch_prefix_frag_p (last_insn))
 	{
 	  unsigned int max_prefix_size = align_branch_prefix_size;
 
@@ -10948,6 +10930,7 @@ s_insn (int dummy ATTRIBUTE_UNUSED)
   unsigned int j;
   valueT val;
   bool vex = false, xop = false, evex = false;
+  struct last_insn *last_insn;
 
   init_globals ();
 
@@ -11701,7 +11684,11 @@ s_insn (int dummy ATTRIBUTE_UNUSED)
   else if (i.rex != 0)
     add_prefix (REX_OPCODE | i.rex);
 
-  output_insn ();
+  last_insn = &seg_info(now_seg)->tc_segment_info_data.last_insn;
+  output_insn (last_insn);
+  last_insn->kind = last_insn_directive;
+  last_insn->name = ".insn directive";
+  last_insn->file = as_where (&last_insn->line);
 
  done:
   *saved_ilp = saved_char;
@@ -15496,13 +15483,15 @@ s_bss (int ignore ATTRIBUTE_UNUSED)
 void
 i386_cons_align (int ignore ATTRIBUTE_UNUSED)
 {
-  if (last_insn.kind != last_insn_directive
+  struct last_insn *last_insn
+    = &seg_info(now_seg)->tc_segment_info_data.last_insn;
+
+  if (last_insn->kind != last_insn_directive
       && (bfd_section_flags (now_seg) & SEC_CODE))
     {
-      last_insn.seg = now_seg;
-      last_insn.kind = last_insn_directive;
-      last_insn.name = "constant directive";
-      last_insn.file = as_where (&last_insn.line);
+      last_insn->kind = last_insn_directive;
+      last_insn->name = "constant directive";
+      last_insn->file = as_where (&last_insn->line);
       if (lfence_before_ret != lfence_before_ret_none)
 	{
 	  if (lfence_before_indirect_branch != lfence_branch_none)
--- a/gas/config/tc-i386.h
+++ b/gas/config/tc-i386.h
@@ -281,6 +281,23 @@ extern enum i386_flag_code {
 	CODE_64BIT
 } i386_flag_code;
 
+struct i386_segment_info {
+  /* Type of previous "instruction", e.g. .byte or stand-alone prefix.  */
+  struct last_insn {
+    const char *file;
+    const char *name;
+    unsigned int line;
+    enum last_insn_kind
+      {
+	last_insn_other = 0,
+	last_insn_directive,
+	last_insn_prefix
+      } kind;
+  } last_insn;
+};
+
+#define TC_SEGMENT_INFO_TYPE struct i386_segment_info
+
 struct i386_tc_frag_data
 {
   union
--- /dev/null
+++ b/gas/testsuite/gas/i386/lfence-section.d
@@ -0,0 +1,19 @@
+#as: -mlfence-before-indirect-branch=all
+#warning_output: lfence-section.e
+#objdump: -dw
+#name: -mlfence-before-indirect-branch=all w/ section switches
+
+.*: +file format .*
+
+
+Disassembly of section .text:
+
+0+ <_start>:
+ +[a-f0-9]+:	f3 ff d0             	repz call \*%eax
+ +[a-f0-9]+:	f3 c3                	repz ret
+ +[a-f0-9]+:	cc                   	int3
+ +[a-f0-9]+:	cc                   	int3
+ +[a-f0-9]+:	cc                   	int3
+
+Disassembly of section \.text2:
+#pass
--- /dev/null
+++ b/gas/testsuite/gas/i386/lfence-section.e
@@ -0,0 +1,3 @@
+.*: Assembler messages:
+.*:3: Warning: `rep` skips -mlfence-before-indirect-branch on `call`
+.*:11: Warning: `rep` skips -mlfence-before-ret on `ret`
--- /dev/null
+++ b/gas/testsuite/gas/i386/lfence-section.s
@@ -0,0 +1,19 @@
+	.text
+_start:
+	rep
+
+	.section .text2, "ax"
+aux1:
+	nop
+
+	.text
+	call *%eax
+	rep
+
+	.section .text2, "ax"
+aux2:
+	nop
+
+	.text
+	ret
+	.p2align 2, 0xcc
--- a/gas/testsuite/gas/i386/i386.exp
+++ b/gas/testsuite/gas/i386/i386.exp
@@ -642,6 +642,17 @@ if [gas_32_check] then {
     run_dump_test "lfence-ret-c"
     run_dump_test "lfence-ret-d"
     run_dump_test "lfence-byte"
+    # This test uses the .section directive, which only ELF and COFF/PE support.
+    if {[is_elf_format]
+	|| [istarget "*-*-vxworks*"]
+	|| [istarget "*-*-coff*"]
+	|| [istarget "*-*-pe*"]
+	|| [istarget "*-*-cygwin*"]
+	|| [istarget "*-*-mingw*"]
+    } then {
+	run_dump_test "lfence-section"
+    }
+
     run_dump_test "branch"
 
     # These tests require support for 8 and 16 bit relocs,

[PATCH 2/6] x86: suppress optimization after potential non-insn

[Jan Beulich]

Just like avoiding to do other transformations potentially affected by
stand-alone prefixes or direct data emission, also avoid optimization
on the following insn.

--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -5120,6 +5120,11 @@ md_assemble (char *line)
  retry:
   init_globals ();
 
+  /* Suppress optimization when the last thing we saw may not have been
+     a proper instruction (e.g. a stand-alone prefix or .byte).  */
+  if (last_insn->kind != last_insn_other)
+    i.no_optimize = true;
+
   /* First parse an instruction mnemonic & call i386_operand for the operands.
      We assume that the scrubber has arranged it so that line[0] is the valid
      start of a (possibly prefixed) mnemonic.  */

[PATCH 3/6] gas: no md_cons_align() for .nop{,s}

[Jan Beulich]

.nop and .nops generate code, not data. Hence them invoking
md_cons_align() is at best inappropriate. In fact it actually gets in
the of x86'es state maintenance involving i386_cons_align().
---
I also consider it at least a little suspicious that all common code
invocations of md_cons_align() have an argument of 1 passed to it:
What's the point of that argument then? Target specific code invoking
their own target hook can surely deal with passing appropriate arguments
other than 1.

--- a/gas/read.c
+++ b/gas/read.c
@@ -3466,10 +3466,6 @@ s_nop (int ignore ATTRIBUTE_UNUSED)
   md_flush_pending_output ();
 #endif
 
-#ifdef md_cons_align
-  md_cons_align (1);
-#endif
-
   SKIP_WHITESPACE ();
   expression (&exp);
   demand_empty_rest_of_line ();
@@ -3519,10 +3515,6 @@ s_nops (int ignore ATTRIBUTE_UNUSED)
   md_flush_pending_output ();
 #endif
 
-#ifdef md_cons_align
-  md_cons_align (1);
-#endif
-
   SKIP_WHITESPACE ();
   expression (&exp);
   /* Note - this expression is tested for an absolute value in
--- a/gas/testsuite/gas/i386/align-branch-6.e
+++ b/gas/testsuite/gas/i386/align-branch-6.e
@@ -1,2 +1,2 @@
 .*: Assembler messages:
-.*:4: Warning: `constant directive` skips -malign-branch-boundary on `jnc`
+.*:5: Warning: `constant directive` skips -malign-branch-boundary on `jnc`
--- a/gas/testsuite/gas/i386/lfence-byte.d
+++ b/gas/testsuite/gas/i386/lfence-byte.d
@@ -27,4 +27,13 @@ Disassembly of section .text:
  +[a-f0-9]+:	f3 c3                	repz ret
  +[a-f0-9]+:	c3                   	ret
  +[a-f0-9]+:	f3 ff d0             	repz call \*%eax
+
+[a-f0-9]+ <directive>:
+ +[a-f0-9]+:	90                   	nop
+ +[a-f0-9]+:	0f ae e8             	lfence
+ +[a-f0-9]+:	ff d0                	call   \*%eax
+ +[a-f0-9]+:	8d 76 00             	lea    (0x)?0\(%esi\),%esi
+ +[a-f0-9]+:	83 0c 24 00          	orl    \$0x0,\(%esp\)
+ +[a-f0-9]+:	0f ae e8             	lfence
+ +[a-f0-9]+:	c3                   	ret
 #pass
--- a/gas/testsuite/gas/i386/lfence-byte.s
+++ b/gas/testsuite/gas/i386/lfence-byte.s
@@ -21,3 +21,10 @@ _start:
 	call *%eax
 	.data
 	.byte 0
+
+	.text
+directive:
+	.nop
+	call	*%eax
+	.nops 3
+	ret

[PATCH 4/6] x86: i386_cons_align() badly affects diagnostics

[Jan Beulich]

Warning without knowing what's going to follow isn't useful, the more
that appropriate warnings are emitted elsewhere in all cases. Not
updating state (file/line in particular) also isn't helpful, as it's
always the last directive ahead of a construct potentially needing
fiddling with that's "guilty" in that fiddling being suppressed.
---
Maybe the stray diagnostics are the reason why there is that
questionable SEC_CODE check in the function (which for now I'm retaining
there)?

--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -15491,22 +15491,11 @@ i386_cons_align (int ignore ATTRIBUTE_UN
   struct last_insn *last_insn
     = &seg_info(now_seg)->tc_segment_info_data.last_insn;
 
-  if (last_insn->kind != last_insn_directive
-      && (bfd_section_flags (now_seg) & SEC_CODE))
+  if (bfd_section_flags (now_seg) & SEC_CODE)
     {
       last_insn->kind = last_insn_directive;
       last_insn->name = "constant directive";
       last_insn->file = as_where (&last_insn->line);
-      if (lfence_before_ret != lfence_before_ret_none)
-	{
-	  if (lfence_before_indirect_branch != lfence_branch_none)
-	    as_warn (_("constant directive skips -mlfence-before-ret "
-		       "and -mlfence-before-indirect-branch"));
-	  else
-	    as_warn (_("constant directive skips -mlfence-before-ret"));
-	}
-      else if (lfence_before_indirect_branch != lfence_branch_none)
-	as_warn (_("constant directive skips -mlfence-before-indirect-branch"));
     }
 }
 
--- a/gas/testsuite/gas/i386/lfence-byte.e
+++ b/gas/testsuite/gas/i386/lfence-byte.e
@@ -1,9 +1,6 @@
 .*: Assembler messages:
 .*:5: Warning: `rep` skips -mlfence-before-ret on `ret`
 .*:7: Warning: `rep` skips -mlfence-before-ret on `ret`
-.*:10: Warning: constant directive skips -mlfence-before-ret and -mlfence-before-indirect-branch
 .*:13: Warning: `rep` skips -mlfence-before-ret on `ret`
-.*:17: Warning: constant directive skips -mlfence-before-ret and -mlfence-before-indirect-branch
-.*:17: Warning: `constant directive` skips -mlfence-before-ret on `ret`
-.*:20: Warning: constant directive skips -mlfence-before-ret and -mlfence-before-indirect-branch
+.*:18: Warning: `constant directive` skips -mlfence-before-ret on `ret`
 .*:20: Warning: `constant directive` skips -mlfence-before-indirect-branch on `call`
--- a/gas/testsuite/gas/i386/x86-64-lfence-byte.e
+++ b/gas/testsuite/gas/i386/x86-64-lfence-byte.e
@@ -1,9 +1,6 @@
 .*: Assembler messages:
 .*:5: Warning: `rep` skips -mlfence-before-ret on `ret`
 .*:7: Warning: `rep` skips -mlfence-before-ret on `ret`
-.*:10: Warning: constant directive skips -mlfence-before-ret and -mlfence-before-indirect-branch
 .*:13: Warning: `rep` skips -mlfence-before-ret on `ret`
-.*:17: Warning: constant directive skips -mlfence-before-ret and -mlfence-before-indirect-branch
-.*:17: Warning: `constant directive` skips -mlfence-before-ret on `ret`
-.*:20: Warning: constant directive skips -mlfence-before-ret and -mlfence-before-indirect-branch
+.*:18: Warning: `constant directive` skips -mlfence-before-ret on `ret`
 .*:20: Warning: `constant directive` skips -mlfence-before-indirect-branch on `call`

[PATCH 5/6] x86: adjust NOP generation after potential non-insn

[Jan Beulich]

Just like avoiding to do certain transformations potentially affected by
stand-alone prefixes or direct data emission, also avoid emitting
optimized NOPs right afterwards; insert a plain old NOP first in such
cases.

--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -1529,6 +1529,14 @@ i386_generate_nops (fragS *fragP, char *
   else if (fragP->fr_type != rs_machine_dependent)
     fragP->fr_var = count;
 
+  /* Emit a plain NOP first when the last thing we saw may not have been
+     a proper instruction (e.g. a stand-alone prefix or .byte).  */
+  if (!fragP->tc_frag_data.last_insn_normal)
+    {
+      *where++ = 0x90;
+      --count;
+    }
+
   if ((count / max_single_nop_size) > max_number_of_nops)
     {
       /* Generate jump over NOPs.  */
--- a/gas/config/tc-i386.h
+++ b/gas/config/tc-i386.h
@@ -321,6 +321,7 @@ struct i386_tc_frag_data
   unsigned int branch_type : 3;
   unsigned int cpunop : 1;
   unsigned int isanop : 1;
+  unsigned int last_insn_normal : 1;
 };
 
 /* We need to emit the right NOP pattern in .align frags.  This is
@@ -347,7 +348,10 @@ struct i386_tc_frag_data
      (FRAGP)->tc_frag_data.cmp_size = 0;			\
      (FRAGP)->tc_frag_data.classified = 0;			\
      (FRAGP)->tc_frag_data.branch_type = 0;			\
-     (FRAGP)->tc_frag_data.mf_type = 0;			\
+     (FRAGP)->tc_frag_data.mf_type = 0;				\
+     (FRAGP)->tc_frag_data.last_insn_normal			\
+	= (seg_info(now_seg)->tc_segment_info_data.last_insn.kind \
+	   == last_insn_other);					\
    }								\
  while (0)
 

[PATCH 6/6] gas: drop unused fields from struct segment_info_struct

[Jan Beulich]

user_stuff, dot, and lineno_list_{head,tail} have no users (left), while
bfd_section was only ever written.

--- a/gas/subsegs.c
+++ b/gas/subsegs.c
@@ -61,7 +61,6 @@ alloc_seginfo (segT seg)
 
   seginfo = obstack_alloc (&notes, sizeof (*seginfo));
   memset (seginfo, 0, sizeof (*seginfo));
-  seginfo->bfd_section = seg;
   bfd_set_section_userdata (seg, seginfo);
 }
 /*
--- a/gas/subsegs.h
+++ b/gas/subsegs.h
@@ -71,23 +71,13 @@ typedef struct segment_info_struct {
      there are frags.  */
   unsigned int bss : 1;
 
-  int user_stuff;
-
   /* Fixups for this segment.  This is only valid after the frchains
      are run together.  */
   fixS *fix_root;
   fixS *fix_tail;
 
-  symbolS *dot;
-
-  struct lineno_list *lineno_list_head;
-  struct lineno_list *lineno_list_tail;
-
-  /* Which BFD section does this gas segment correspond to?  */
-  asection *bfd_section;
-
   /* NULL, or pointer to the gas symbol that is the section symbol for
-     this section.  sym->bsym and bfd_section->symbol should be the same.  */
+     this section.  */
   symbolS *sym;
 
   /* Used by dwarf2dbg.c for this section's line table entries.  */

Re: [PATCH 1/6] x86: last-insn recording should be per-section

[Jan Beulich]

On 24.11.2023 10:03, Jan Beulich wrote:
> Otherwise intermediate section switches result in inconsistent behavior.
> Note, however, that intermediate sub-section switches will continue to
> result in inconsistent or even inappropriate behavior.
> 
> While there also add recording of state to s_insn().
> ---
> i386_cons_align() qualifying the recording on SEC_CODE seems suspicious,
> too: md_assemble() isn't constrained to emitting instructions to only
> text sections.
> 
> For sub-sections the best I can think of that we could do is warn when
> now_subseg is nonzero wherever we already warn based on last_insn state
> (and similarly bail rather than inserting any code). That would have
> not really intended effects in subsequent patches, though: We'd then
> generally suppress optimizations outside of subseg 0, and we'd also
> prefix plain old NOPs there (in i386_generate_nops()).

With some prereq tweaking of (at least) obj-elf.c, perhaps we could leverage
md_elf_section_change_hook() to deal with subsections. The prereq tweaking
would be needed as from my initial investigations I'm getting the impression
that not all section switches actually properly invoke that hook.

Jan