Re: assertion failure in regcache.c

Kris Warkentin
2003-05-23T20:29:29Z

Interesting. If I go through sh-tdep.c and comment out all the
'set_gdbarch_register_byte(blah)' calls, it works. Are there any potential
negative implications of this or can we just trust regcache to do it's job?

cheers,

Kris

----- Original Message -----
From: "Andrew Cagney" <ac131313@redhat.com>
To: "Elena Zannoni" <ezannoni@redhat.com>; "Kris Warkentin"
<kewarken@qnx.com>
Cc: "Gdb@Sources.Redhat.Com" <gdb@sources.redhat.com>
Sent: Friday, May 23, 2003 3:46 PM
Subject: Re: assertion failure in regcache.c

> > Kris Warkentin writes:
> > > > > I'd start with the obvious thing - a simple tipo in the SH4
register
> > > > > byte function. The code was written long before these sanity
checks
> > > > > were added and ``the old way'' makes it very hard to notice that
the
> > > > > values are skewed.
> > > > >
> > > > > Andrew
> > > > >
> > > >
> > > >
> > > > yes, look at sh_sh4_register_byte. Maybe FV0_REGNUM or
FV_LAST_REGNUM
> > > > are not set correctly or fv_reg_base_num does something wrong.
These
> > > > registers with (*1) are pseudo registers, so it's easy that the
> > > > calculations could have been screwed up.
> > >
> > > Well, I found the disagreement. It looks to me like
> > > regcache->descr->register_offset[] is pointing to an upwardly growing
list
> > > of registers including the pseudo-registers. So you get something
like dr5
> > > being 260 in the register_offset array but sh4_register_byte will
return 124
> > > which would be the offset of fr10 (taking into account that dr0 is
overlaid
> > > on top of the fr regs). I'm inclined to think that the regcache way
is
> > > wrong since someone who updates dr0 and then reads fr0 will get
conflicting
> > > values. We shouldn't be storing extra copies of the same register.
>
> Plan B. Try not setting [deprecated_]register_byte.
>
> > Looking at regcache.c I see that the long term goal is to not allocate
> > space in the regcache for the PSEUDOs. But in the meantime,
> > descr->register_offset[i] = REGISTER_BYTE (i);
> > in the legacy init function, while
> > descr->sizeof_register[i] = TYPE_LENGTH (descr->register_type[i]);
> > descr->register_offset[i] = offset;
> > offset += descr->sizeof_register[i];
> > in the new version of the function.
> >
> > So the mismatch seems to come from the TYPE_LENGTH() on the type of a
> > pseudo, because that's always a positive quantity, while the
> > REGISTER_BYTE points 'backwards'. Maybe we should be using the legacy
> > version of the regcache init function? Is that doable?
>
> Are we all sitting comfortably? I'll try to explain the history to this
> ... :-)
>
> Long ago, the code for reading/writing register values looked like (this
> was burried in read_register_bytes):
>
> extract_integer (&registers[value->address], value->length);
>
> That is, given $dr5, gdb would create a value the location of which was
> described by:
>
> location = lval_register;
> regnum = $dr5 regnum
> address = REGISTER_BYTE ($dr5 regnum)
>
> and then, assuming that the register cache was one big byte array, and
> that the value was already in that array, and that they array only held
> raw registers, use the ADDRESS and not the REGNUM to extract the value's
> raw bytes.
>
> This forced the SH (and a few other architectures) to directly map
> pseudo registers onto the raw register range (to guarentee that the
> value read was always valid).
>
> Fortunatly, since then problems like this have (hopefully) all been
> identified and fixed. It uses a read / modify / write to access the
values:
>
> - map the [value->address, value->length) back to a sequence of
> registers (identified by their register number). Here, they would be
> pseudo registers
> - read, using regcache_cooked_read, those registers and contatenate
> them into a large buffer
> - ``modify'' the bytes implied by [value->address, value->length)
> - write, using regcache_cooked_write, each of the registers back into
> the regcache
>
> The procedural interface means that something like $dr5 is always
> constructed / updated on the fly (using the relevant raw registers), and
> potential problems with out-dated register values are avoided.
>
> In the end, GDB doesn't store multiple values of the same register, but
> it can end up caching the various values. That doesn't matter, GDB
> flushes all such cached values after any target modify.
>
> enjoy,
> Andrew
>
>
>