Re: [PATCH 1/3] Rework 128-bit complex multiply and divide, PR target/107299

[Michael Meissner <meissner@linux.ibm.com>]

On Mon, Dec 12, 2022 at 06:20:14PM +0800, Kewen.Lin wrote:
> Without or with patch #1, the below ICE in libgcc exists, the ICE should have
> nothing to do with the special handling for building_libgcc in patch #1.  I
> think patch #2 which makes _Float128 and __float128 use the same internal
> type fixes that ICE.
> 
> I still don't get the point why we need the special handling for building_libgcc,
> I also tested on top of patch #1 and #2 w/ and w/o the special handling for
> building_libgcc, both bootstrapped and regress-tested.
> 
> Could you have a double check?

As long as patch #2 and #3 are installed, we don't need the special handling
for building_libgcc.  Good catch.

I will send out a replacement patch for it.

> Since your patch #2 (and #3) fixes ICE and some exposed problems, and _Float128
> is to use the same internal type as __float128, types with attribute((mode(TF)))
> and attribute((mode(TC))) should be correct, I assume that this patch is just
> to make the types explicit be with _Float128 (for better readability and
> maintainance), but not for any correctness issues.

Yes, the patch is mainly for clarity.  The history is the libgcc support went
in before _Float128 went in, and I never went back to use those types when we
could use them.

With _Float128, we can just use _Complex _Float128 and not
bother with trying to get the right KC/TC for the attribute mode stuff.

However, if patches 1-3 aren't put in, just applying the patch to use _Float128
and _Complex _Float128 would fix the immediate problem (of not building GCC on
systems with IEEE 128-bit long double).  However, it is a band-aid that just
works around the problem of building __mulkc3 and __divkc3.  It doesn't fix the
other problems between __float128 and _Float128 that show up in some places
that I would like to get fixed.

So I haven't submitted the patch, because I think it is more important to get
the other issues fixed.

> > Now, this patch fixes the specific problem of not being able to build libgcc
> > (along with patch #1 of the series).  But other things show the differences
> > from time time because we are using different internal types and the middle end
> > doesn't know that these types are really the same bits.
> > 
> > It is better long term (IMHO) if we have the two types (__float128 and
> > _Float128) use the same internal type (which is what is done in patches #2 and
> > #3).  This fixes the other issues that show up, such as creating signaling NaNs
> > for one internal type, and converting it to the other internal type, loses that
> > the NaN is signalling.
> > 
> 
> I see, nice!
> 
> BR,
> Kewen

-- 
Michael Meissner, IBM
PO Box 98, Ayer, Massachusetts, USA, 01432
email: meissner@linux.ibm.com