[patch 2/2] Disable epilogue unwinders on recent GCCs [Re: Regression: Re: [PATCH] Fix some i386 unwinder inconcistencies]

[Jan Kratochvil]

On Mon, 13 Jun 2011 12:49:11 +0200, Jan Kratochvil wrote:
> On all the tested platforms Fedora-{13,14,15,Rawhide} for {i686,x86_64-m32}
> (but not for x86_64):
> -PASS: gdb.base/watchpoint-cond-gone.exp: Catch the no longer valid watchpoint
> +FAIL: gdb.base/watchpoint-cond-gone.exp: Catch the no longer valid watchpoint
> -XFAIL: gdb.mi/mi-watch.exp: sw: watchpoint trigger (stopped at wrong place)
> +XFAIL: gdb.mi/mi-watch.exp: sw: watchpoint trigger (unknown output after running)
> -XFAIL: gdb.mi/mi2-watch.exp: sw: watchpoint trigger (stopped at wrong place)
> +XFAIL: gdb.mi/mi2-watch.exp: sw: watchpoint trigger (unknown output after running)

On recent GCCs:
	http://gcc.gnu.org/gcc-4.5/changes.html
	GCC now generates unwind info also for epilogues.

The epilogue unwinders are therefore no longer useful in common cases.
Moreover as they precede the DWARF unwinder they prevent
archer-jankratochvil-entryval (to be submitted these days) functionality at
those return instructions as entryval can provide more constructed parameters
debug info even at those PCs.  It is a bit offtopic now but it causes:


(gdb) bt
#0  d (i=70) at tailcall.c:6
       ^^^^ value provided by archer-jankratochvil-entryval
#1  0x00000000004004ba in c (i=7) at tailcall.c:8
    ^^^^^^^^^^^^^^^^^^^^^^^ frame provided by archer-jankratochvil-entryval
#2  0x00000000004004d8 in b (i=5) at tailcall.c:12
    ^^^^^^^^^^^^^^^^^^^^^^^ frame provided by archer-jankratochvil-entryval
#3  0x0000000000400384 in main () at tailcall.c:15
(gdb) disass
Dump of assembler code for function d:
   0x0000000000400490 <+0>:	callq  0x400480 <e>
   0x0000000000400495 <+5>:	mov    0x20046d(%rip),%edi        # 0x600908 <v>
   0x000000000040049b <+11>:	callq  0x400480 <e>
=> 0x00000000004004a0 <+16>:	xor    %eax,%eax
   0x00000000004004a2 <+18>:	retq   
End of assembler dump.
(gdb) bt
#0  d (i=70) at tailcall.c:6
#1  0x00000000004004ba in c (i=7) at tailcall.c:8
#2  0x00000000004004d8 in b (i=5) at tailcall.c:12
#3  0x0000000000400384 in main () at tailcall.c:15

with epilogue unwinder (=without this patch):

(gdb) stepi
0x00000000004004a2 in d (i=<optimized out>) at tailcall.c:6
6	/* line  6 */             return 0; }
(gdb) bt
#0  0x00000000004004a2 in d (i=<optimized out>) at tailcall.c:6
#1  0x0000000000400384 in main () at tailcall.c:15

without epilogue unwinder (=with this patch):

(gdb) stepi
0x00000000004004a2	6	/* line  6 */             return 0; }
(gdb) bt
#0  0x00000000004004a2 in d (i=70) at tailcall.c:6
#1  0x00000000004004ba in c (i=7) at tailcall.c:8
#2  0x00000000004004d8 in b (i=5) at tailcall.c:12
#3  0x0000000000400384 in main () at tailcall.c:15


I find questionable this detection uses DWARF DW_AT_producer.  Therefore on
binaries with .eh_frame but no (DWARF) debug info the detection fails and
epilogue unwinders will get into affect.  But they cause no harm in such case,
archer-jankratochvil-entryval cannot work without DWARF and the erroring code

1441      /* This restriction could be lifted if other unwinders are known to
1442         compute the frame base in a way compatible with the DWARF
1443         unwinder.  */
1444      if (! frame_unwinder_is (this_frame, &dwarf2_frame_unwind))
1445        error (_("can't compute CFA for this frame"));

is also not relevent when no DWARF is available.

No regressions on {x86_64,x86_64-m32,i686}-fedora15-linux-gnu, it fixes the
regression reported above.  I will check it in some time if no comments
appear.


Thanks,
Jan


gdb/
2011-06-26  Jan Kratochvil  <jan.kratochvil@redhat.com>

	Disable epilogue unwinders on recent GCCs.
	* amd64-tdep.c (amd64_in_function_epilogue_p): New variable symtab,
	initialize it, return 0 on EPILOGUE_UNWIND_VALID.
	* dwarf2read.c (process_full_comp_unit): Initialize
	EPILOGUE_UNWIND_VALID.
	* i386-tdep.c (i386_in_function_epilogue_p): New variable symtab,
	initialize it, return 0 on EPILOGUE_UNWIND_VALID.
	* symtab.h (struct symtab): New field epilogue_unwind_valid.

--- a/gdb/amd64-tdep.c
+++ b/gdb/amd64-tdep.c
@@ -2219,6 +2219,11 @@ static int
 amd64_in_function_epilogue_p (struct gdbarch *gdbarch, CORE_ADDR pc)
 {
   gdb_byte insn;
+  struct symtab *symtab;
+
+  symtab = find_pc_symtab (pc);
+  if (symtab && symtab->epilogue_unwind_valid)
+    return 0;
 
   if (target_read_memory (pc, &insn, 1))
     return 0;   /* Can't read memory at pc.  */
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -4751,6 +4751,9 @@ process_full_comp_unit (struct dwarf2_per_cu_data *per_cu)
 	 */ 
       if (cu->has_loclist && gcc_4_minor >= 0)
 	symtab->locations_valid = 1;
+
+      if (gcc_4_minor >= 5)
+	symtab->epilogue_unwind_valid = 1;
     }
 
   if (dwarf2_per_objfile->using_index)
--- a/gdb/i386-tdep.c
+++ b/gdb/i386-tdep.c
@@ -1885,6 +1885,11 @@ static int
 i386_in_function_epilogue_p (struct gdbarch *gdbarch, CORE_ADDR pc)
 {
   gdb_byte insn;
+  struct symtab *symtab;
+
+  symtab = find_pc_symtab (pc);
+  if (symtab && symtab->epilogue_unwind_valid)
+    return 0;
 
   if (target_read_memory (pc, &insn, 1))
     return 0;	/* Can't read memory at pc.  */
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -779,6 +779,11 @@ struct symtab
 
   unsigned int locations_valid : 1;
 
+  /* DWARF unwinder for this CU is valid even for epilogues (PC at the return
+     instruction).  This is supported by GCC since 4.5.0.  */
+
+  unsigned int epilogue_unwind_valid : 1;
+
   /* The macro table for this symtab.  Like the blockvector, this
      may be shared between different symtabs --- and normally is for
      all the symtabs in a given compilation unit.  */