Re: [PATCH] target/s390x: Fix CC set by CONVERT TO FIXED/LOGICAL

[Ulrich Weigand <uweigand@de.ibm.com>]

On Mon, Jun 28, 2021 at 06:26:52AM -0700, Richard Henderson wrote:
> On 6/28/21 5:58 AM, Ulrich Weigand wrote:
> >>>helper.h:DEF_HELPER_FLAGS_3(clgdb, TCG_CALL_NO_WG, i64, env, i64, i32)
> >>
> >>This won't work reliably.  You're writing to a tcg global inside of
> >>a function that says that it won't.
> >
> >I missed that, sorry.  That problem can be fixed by changing the above
> >line to something like:
> >DEF_HELPER_3(clgdb, i64, env, i64, i32)
> >right?
> 
> Yes.

OK, I'll send a v2 including that change shortly; maybe that is an
acceptable fix for the immediate bug, at least for now.

> >In any case, the current implementation already has two helpers, and
> >I initially tried to keep that, by using a different second part to
> >correctly compute CC.  But this ran into the problem that I didn't
> >see any way to detect the fact that the conversion operation had run
> >into one of the special cases in the second helper, without re-doing
> >the whole conversion a second time.  Is there any way to pass
> >information between the two helpers (without running again into the
> >same qemu global state updating problem)?
> 
> Don't clear out env->fpu_status.float_exception_flags in
> handle_exceptions.  Wait until we're actually done with the data.

I don't really know much about qemu internals, but this is really
confusing me, sorry.  Aren't env->fpu_status and env->cc_op two
elements of the same global state?  Why it is OK to use one of
these fields to pass information to the next helper, but not the
other?  I guess I must be missing something here ...

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU/Linux compilers and toolchain
  Ulrich.Weigand@de.ibm.com