Re: [PATCH] rs6000: Fix up __SIZEOF_{FLOAT,IBM}128__ defines [PR99708]

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

On Mon, Mar 07, 2022 at 03:37:18PM -0600, Segher Boessenkool wrote:
> Hi!
> 
> On Sat, Mar 05, 2022 at 09:21:51AM +0100, Jakub Jelinek wrote:
> > As mentioned in the PR, right now on powerpc* __SIZEOF_{FLOAT,IBM}128__
> > macros are predefined unconditionally, because {ieee,ibm}128_float_type_node
> > is always non-NULL, doesn't reflect whether __ieee128 or __ibm128 are
> > actually supported or not.
> > 
> > The following patch:
> > 1) makes those {ieee,ibm}128_float_type_node trees NULL if we don't
> >    really support them instead of equal to long_double_type_node
> > 2) adjusts the builtins code to use
> >    ibm128_float_type_node ? ibm128_float_type_node : long_double_type_node
> >    for the 2 builtins, so that we don't ICE during builtins creation
> >    if ibm128_float_type_node is NULL (using error_mark_node instead of
> >    long_double_type_node sounds more risky to me)
> 
> I feel the opposite way: (potentially) using the wrong thing is just a
> ticking time bomb, never "safer".
> 
> > 3) in rs6000_type_string will not match NULL as __ibm128, and adds
> >    a __ieee128 case
> > 4) removes the clearly unused ptr_{ieee,ibm}128_float_type_node trees;
> >    if something needs them in the future, they can be easily added back,
> >    but wasting GTY just in case...
> > 5) actually syncs __SIZEOF_FLOAT128__ with the __float128 macro being
> >    defined in addition to __ieee128 being usable
> > 
> > Now, in the PR Segher said he doesn't like 5), but I think it is better
> > to match the reality and get this PR fixed and if we want to rethink
> > how __float128 is defined (whether it is a macro, or perhaps another
> > builtin type like __ieee128 which could be easily done by
> >        lang_hooks.types.register_builtin_type (ieee128_float_type_node,
> >                                               "__ieee128");
> >        lang_hooks.types.register_builtin_type (ieee128_float_type_node,
> >                                               "__float128");
> > perhaps under some conditions, rethink if the -mfloat128 option exists
> > and what it does etc., it can be done incrementally and the rs6000-c.cc
> > hunks in the patch can be easily reverted (appart from the formatting
> > fix).
> 
> There needs to be a __SIZEOF_IEEE128__ as well, if you like reality :-)
> Sorry I did not pick up on that earlier.

No, no, no.

The '__ieee128' keyword was used as a way to define the type but not enable the
'__float128' keyword.  Then rs6000-c.cc defines __float128 to be __ieee128,
similar to defining 'vector' and '__vector' to be
__attribute__((altivec(vector__))'.

Unfortunately, there is no way to remove a keyword after the creation (or at
least there wasn't in the GCC 8 time frame when I wrote the code), and you need
to create the types at GCC startup to set up the built-ins.

No one should be using '__ieee128'.  The official keywords are '__float128' and
for C (not C++) '_Float128'.

Perhaps in GCC 13 it is time to just remove it and always just define
'__float128' instead.

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