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interesting unwind problem with noreturn functions
- From: Randolph Chung <randolph at tausq dot org>
- To: gdb at sources dot redhat dot com
- Date: Thu, 02 Mar 2006 23:13:27 +0800
- Subject: interesting unwind problem with noreturn functions
i ran across this when debugging gdb on hppa-linux.....
value_from_double calls error() in the case that I was debugging, but if
i break in error(), i get a corrupted backtrace.
(top-gdb) bt
#0 error (
string=0x23d89c "Unexpected type encountered for floating constant.")
at ../../gdb-cvs/gdb/utils.c:645
#1 0x000b848c in value_from_double (type=0x31d9d0,
num=4.9406564584124654e-324) at ../../gdb-cvs/gdb/value.c:1570
#2 0x000b848c in value_from_double (type=0x31d9d0,
num=4.9406564584124654e-324) at ../../gdb-cvs/gdb/value.c:1570
Previous frame identical to this frame (corrupt stack?)
looking into this, it looks like gcc is being a bit too clever for gdb....
000b83dc <value_from_double>:
b83dc: 6b c2 3f d9 stw rp,-14(sp)
b83e0: 6f c3 01 00 stw,ma r3,80(sp)
[...]
b8484: e8 40 08 bc b,l b98e8 <_initialize_values+0xf3c>,rp
b8488: 08 1c 02 5a copy ret0,r26
000b848c <coerce_ref>:
b848c: 6b c2 3f d9 stw rp,-14(sp)
b8490: 6f c4 01 00 stw,ma r4,80(sp)
b8484 is the call to error (through a long branch stub, so the name
looks funny). The return pointer of the call points past
value_from_double into the next function (hppa has a delay slot for
branches, and rp points to .+8 for a branch insn). Since error() is
defined as a noreturn function, I suppose this is a valid thing for gcc
to do, even if it looks a bit weird.
So when you unwind from error(), the hppa frame unwinder tries to unwind
from rp=b848c, which is in a different function from the actual caller,
and the unwinder gets all confused....
I suppose this is not a hppa specific thing, other architectures might
also encounter the same problem. How is it fixed there? Should the
unwinder be storing the PC as RP-offset? (This seems rather ugly, if not
wrong.)
randolph