Re: [RFC/PATCH] Remove the workaround for _Float128 precision [PR107299]

["Kewen.Lin" <linkw@linux.ibm.com>]

Hi Segher,

on 2022/12/22 05:24, Segher Boessenkool wrote:
> Hi!
> 
> On Wed, Dec 21, 2022 at 05:02:17PM +0800, Kewen.Lin wrote:
>> This a different attempt from Mike's approach[1][2] to fix the
>> issue in PR107299.
> 
> Ke Wen, Mike: so iiuc with this patch applied all three of Mike's
> patches are unnecessary?

I think the 1st patch is still needed, it's to fix a latent issue
as the associated test cases {mul,div}ic3-1.c show.

> Does it fix the new testcases in Mike's series as well?

Yeah, it doesn't suffer the issue exposed by float128-hw4.c, so
it doesn't need that adjustment on float128-hw4.c.  It can also
make newly added float128-hw{12,13}.c pass.

>> As above, I wonder if we can consider this approach which
>> makes type _Float128 have the same precision as MODE_PRECISION
>> of its mode, it keeps the previous implementation to make type
>> long double compatible with _Float128.  Since the REAL_MODE_FORMAT
>> of the mode still holds the information, even if some place which
>> isn't covered in the above testing need the actual precision, we
>> can still retrieve the actual precision with that.
> 
> "Precision" does not have a useful meaning for all floating point
> formats.  It does not have one for double-double for example.  The way
> precision is defined in IEEE FP means double-double has 2048 bits of
> precision (or is it 2047), not useful.  Taking the precision of the
> format instead to be the minimum over all values in that format gives
> double-double the same precision as IEEE DP, not useful in any practical
> way either :-/

OK, I think that's why we don't see any regressions with this work
around removal,   :)

> 
>> --- a/gcc/tree.cc
>> +++ b/gcc/tree.cc
>> @@ -9442,15 +9442,6 @@ build_common_tree_nodes (bool signed_char)
>>        if (!targetm.floatn_mode (n, extended).exists (&mode))
>>  	continue;
>>        int precision = GET_MODE_PRECISION (mode);
>> -      /* Work around the rs6000 KFmode having precision 113 not
>> -	 128.  */
> 
> It has precision 126 now fwiw.

Yes, with -mabi=ibmlongdouble, it uses KFmode so 126, while with
-mabi=ieeelongdouble, it uses TFmode so 127.

BR,
Kewen