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

Jan Kratochvil
2011-06-26T08:42:01Z

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 -PASS: gdb.base/watchpoint-cond-gone.exp: Catch the no longer valid watchpoint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 +FAIL: gdb.base/watchpoint-cond-gone.exp: Catch the no longer valid watchpoint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. */