[gdb/tdep] Don't use i386 unwinder for amd64

For i386 we have these unwinders:
...
$ gdb -q -batch -ex "set arch i386" -ex "maint info frame-unwinders"
The target architecture is set to "i386".
dummy                   DUMMY_FRAME
dwarf2 tailcall         TAILCALL_FRAME
inline                  INLINE_FRAME
i386 epilogue           NORMAL_FRAME
dwarf2                  NORMAL_FRAME
dwarf2 signal           SIGTRAMP_FRAME
i386 stack tramp        NORMAL_FRAME
i386 sigtramp           SIGTRAMP_FRAME
i386 prologue           NORMAL_FRAME
...
and for amd64:
...
$ gdb -q -batch -ex "set arch i386:x86-64" -ex "maint info frame-unwinders"
The target architecture is set to "i386:x86-64".
dummy                   DUMMY_FRAME
dwarf2 tailcall         TAILCALL_FRAME
inline                  INLINE_FRAME
python                  NORMAL_FRAME
amd64 epilogue          NORMAL_FRAME
i386 epilogue           NORMAL_FRAME
dwarf2                  NORMAL_FRAME
dwarf2 signal           SIGTRAMP_FRAME
amd64 sigtramp          SIGTRAMP_FRAME
amd64 prologue          NORMAL_FRAME
i386 stack tramp        NORMAL_FRAME
i386 sigtramp           SIGTRAMP_FRAME
i386 prologue           NORMAL_FRAME
...

ISTM me there's no reason for the i386 unwinders to be there for amd64.

Furthermore, there's a generic need to play around with enabling and disabling
unwinders, see PR8434.  Currently, that's only available for both the dwarf2
unwinders at once using "maint set dwarf unwinders on/off".

If I manually disable the "amd64 epilogue" unwinder, the "i386 epilogue"
unwinder becomes active and gives the wrong answer, while I'm actually
interested in the result of the dwarf2 unwinder.  Of course I can also
manually disable the "i386 epilogue", but I take the fact that I have to do
that as evidence that on amd64, the "i386 epilogue" is not only unnecessary,
but in the way.

Fix this by only adding the i386 unwinders if
"info.bfd_arch_info->bits_per_word == 32".

Note that the x32 abi (x86_64/-mx32):
- has the same unwinder list as amd64 (x86_64/-m64) before this commit,
- has info.bfd_arch_info->bits_per_word == 64, the same as amd64, and
  consequently,
- has the same unwinder list as amd64 after this commit.

Tested on x86_64-linux, -m64 and -m32.  Not tested with -mx32.

Reviewed-By: John Baldwin <jhb@freebsd.org>

PR tdep/30102
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30102

1 files changed, 8 insertions, 4 deletions

diff --git a/gdb/i386-tdep.c b/gdb/i386-tdep.c
index 580664d..8dccae6 100644
--- a/gdb/i386-tdep.c
+++ b/gdb/i386-tdep.c
@@ -8613,7 +8613,8 @@ i386_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
      appended to the list first, so that it supercedes the DWARF
      unwinder in function epilogues (where the DWARF unwinder
      currently fails).  */
-  frame_unwind_append_unwinder (gdbarch, &i386_epilogue_frame_unwind);
+  if (info.bfd_arch_info->bits_per_word == 32)
+    frame_unwind_append_unwinder (gdbarch, &i386_epilogue_frame_unwind);
 
   /* Hook in the DWARF CFI frame unwinder.  This unwinder is appended
      to the list before the prologue-based unwinders, so that DWARF
@@ -8792,9 +8793,12 @@ i386_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
     tdep-> bnd0_regnum = -1;
 
   /* Hook in the legacy prologue-based unwinders last (fallback).  */
-  frame_unwind_append_unwinder (gdbarch, &i386_stack_tramp_frame_unwind);
-  frame_unwind_append_unwinder (gdbarch, &i386_sigtramp_frame_unwind);
-  frame_unwind_append_unwinder (gdbarch, &i386_frame_unwind);
+  if (info.bfd_arch_info->bits_per_word == 32)
+    {
+      frame_unwind_append_unwinder (gdbarch, &i386_stack_tramp_frame_unwind);
+      frame_unwind_append_unwinder (gdbarch, &i386_sigtramp_frame_unwind);
+      frame_unwind_append_unwinder (gdbarch, &i386_frame_unwind);
+    }
 
   /* If we have a register mapping, enable the generic core file
      support, unless it has already been enabled.  */