Re: RFC: change needs_frame_tls_address

Pedro Alves
2010-07-24T01:04:30Z

> For this we get a warning from value_static_field, then things go
> downhill and we hit an internal_error. From the PR:

(...)

> After tracing through the TLS code for a bit, I have concluded that TLS
> does not really need a frame, at least not in the gdb sense. Instead, I
> think it only needs registers -- a funny sort of distinction to make,
> but nevertheless...

Given that if you have registers, you always have a frame, I don't
think that's a good check.

> (gdb) p A::num
> $1 = 1
> (gdb) kill
> Kill the program being debugged? (y or n) y
> (gdb) p A::num
> Cannot access memory at address 0xb7fdb6d8

Is that useful? It seems to be trying to print a bogus
address. Actually, I'm surprised you even got that memory error
instead of "Cannot find thread-local variables on this target", which
is what I get on x86-64 (both -m64/-m32) with your change applied.
Clearly gdb didn't try reading any register, otherwise, you would see
some other error. Do you have other changes in your tree perhaps?

> If it is not acceptable, I would appreciate some enlightenment as to
> what other approach I should take.

The warning seems bogus to me:

> (gdb) print A::num
> warning: static field's value depends on the current frame - bad debug info?
> findvar.c:427: internal-error: read_var_value: Assertion `frame' failed.
> A problem internal to GDB has been detected,
> further debugging may prove unreliable.
> Quit this debugging session? (y or n)

I think we should throw an error instead, just like what you get when
you try to print a non class static global TLS variable:

__thread int global_num = 1;

Both cases are the same in the user's perpective, so should behave
equal. Trying to print `global_num' with no process/core throws an
error in value_of_variable. I guess we should make value_static_field
use value_of_variable instead of read_var_value directly too (or a factored
out variant that doesn't throw if read_var_value returns NULL).

--
Pedro Alves