Re: [PATCH v2 01/11] s390: Remove duplicate checks for cached gdbarch@init

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

Philipp Rudo wrote:

> On Tue, 05 Dec 2017 16:16:07 +0000
> Yao Qi <qiyaoltc@gmail.com> wrote:
> > Is it possible that we have two instances of gdbarch, with the same
> > target description, but different vector_abi?  Two different executables
> > compiled with different vector abis, and GDB can debug them together
> > (multi-process debugging.  If we consider multi-target debugging in the
> > future, these two executable can from different targets).
> 
> For s390 there only is one vector abi (or non at all) at the time.  If you were
> debugging two different executables at the same time you would have two
> inferiors each with its own gdbarch (same would be for multi-target debugging).
> So I don't think those are the reasons.

Actually, I think Yao is right here.  As you say, we can have two executables,
one using the vector ABI and one not.  These will require two different gdbarch
structures.  But with the patch you propose, when trying to allocate the second
of those two, GDB would see the first one that was already created earlier,
and incorrectly assume that it can simply be reused.

Basically, the problem is that there *can* be different gdbarchs that share
the *same* tdesc, but differ in vector ABI.  Therefore *only* checking for
tdesc does not suffice to correctly identify cached gdbarch structures.

I agree that it is redundant to again check differences (e.g. in register set)
that would already have led to a different tdesc; but the vector ABI at least
is not one of those.

Bye,
Ulrich

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