[PATCH, rs6000] Restrict reload use of FLOAT_REGS

[David Edelsohn]

On Fri, Feb 28, 2014 at 7:11 PM, Bill Schmidt
<wschmidt@linux.vnet.ibm.com> wrote:
> Hi,
>
> We've encountered a rare bug that occurs when attempting to reload for
> an unaligned store in DImode.  For an unaligned store, using stfd gets
> preference over std since stfd doesn't have an alignment restriction and
> therefore the "m" constraint matches.  However, when there is not a
> register available for the REG to be stored, register elimination can
> replace the REG with its REQ_EQUIV.  When this is a PLUS, we end up with
> an attempt to compute an integer add into a floating-point register, and
> things rapidly go downhill.
>
> We had some internal discussion and determined the best way to fix this
> is to avoid ever using FLOAT_REGS for a PLUS in
> rs6000_preferred_reload_class, similar to what's currently done to avoid
> loading constants into FLOAT_REGS.  Uli Weigand pointed out that this
> existing test is actually a bit too strong, as rclass could be ALL_REGS
> and this prevents us from using GENERAL_REGS in that case.  So I've
> relaxed that test to only look for superclasses of FLOAT_REGS.  (If you
> feel this is too risky, I can avoid that change.)
>
> The patch below fixes the one case where we've observed this bug in the
> wild (it occurred for a particular snapshot of code for an internal
> build that doesn't match any public branch).  Because it's dependent on
> register spill, it is very difficult to try to produce a test case that
> isn't too fragile, so I haven't tried to add one.
>
> Bootstrapped and tested on powerpc64le-unknown-linux-gnu
> (--with-cpu=power8) and powerpc64-unknown-linux-gnu (--with-cpu=power7)
> with no regressions.  Is this ok for trunk?
>
> Thanks,
> Bill
>
>
> 2014-02-28  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>
>
>         * config/rs6000/rs6000.c (rs6000_preferred_reload_class): Disallow
>         PLUS rtx's from reloading into a superset of FLOAT_REGS; relax
>         constraint on constants to only prevent them from being reloaded
>         into a superset of FLOAT_REGS.

This is okay with me. Uli is the best one to comment if this is the right test.

Thanks, David