Re: [PATCH] fortran, libgfortran, v2: Avoid using libquadmath for glibc 2.26+

[Harald Anlauf <anlauf@gmx.de>]

Hi Jakub,

Am 24.06.22 um 12:29 schrieb Jakub Jelinek via Gcc-patches:
> On Thu, Jun 23, 2022 at 11:17:50PM +0200, Jakub Jelinek via Gcc-patches wrote:
>> We currently use
>> %rename lib liborig
>> *lib: %{static-libgfortran:--as-needed} -lquadmath %{static-libgfortran:--no-as-needed} -lm %(libgcc) %(liborig)
>> in libgfortran.spec (on targets where we do configure in libquadmath).
>> So, I believe the patch as is is an ABI change for *.o files if they use
>> real(kind=16) math functions (one needs to recompile them), but not
>> for linked shared libraries or executables, because the above aranges
>> for them to be linked with -lquadmath or for -static-libgfortran with
>> --as-needed -lquadmath --no-as-needed.  The former adds libquadmath.so.0
>> automatically to anything gfortran links, the latter to anything that
>> actually needs it (i.e. has undefined math/complex *q symbols).
>>
>> Note, libgfortran.so.5 itself is ABI compatible, just no longer has
>> DT_NEEDED libquadmath.so.0 and uses the *f128 APIs + str{to,from}f128
>> instead of *q APIs + strtoflt128 and quadmath_snprintf.
>>
>> Now, what we could do if we'd want to also preserve *.o file compatibility,
>> would be for gcc configured on glibc 2.26+ change libgfortran.spec to
>> *lib: --as-needed -lquadmath --no-as-needed -lm %(libgcc) %(liborig)
>> so that we even without -static-libgfortran link in libquadmath.so.0
>> only if it is needed.  So, if there will be any gcc <= 12 compiled
>> *.o files or *.o files compiled by gcc 13 if it was configured against
>> glibc 2.25 or older, we'd link -lquadmath in, but if there are just
>> *.o files referencing *f128 symbols, we wouldn't.
>>
>>> Am I right in assuming that this also effectively fixes PR46539?
>>> (Add -static-libquadmath).
>>
>> That was one of the intents of the patch.
>> But sure, it doesn't introduce -static-libquadmath, nor arranges for
>> -static-libgfortran to link libquadmath statically, just in some cases
>> (gcc configured on glibc 2.26 or later) and when everything that calls
>> real(kind=16) math functions has been recompiled arranges for libquadmath
>> not to be used at all.
>
> Here is an updated patch that does that.

very good!

>>>> --- gcc/fortran/trans-intrinsic.cc.jj	2022-05-16 11:14:57.587427707 +0200
>>>> +++ gcc/fortran/trans-intrinsic.cc	2022-06-23 11:42:03.355899271 +0200
>>>> @@ -155,7 +155,7 @@ builtin_decl_for_precision (enum built_i
>>>>      else if (precision == TYPE_PRECISION (double_type_node))
>>>>        i = m->double_built_in;
>>>>      else if (precision == TYPE_PRECISION (long_double_type_node)
>>>> -	   && (!gfc_real16_is_float128
>>>> +	   && ((!gfc_real16_is_float128 & !gfc_real16_is__Float128)
>>>
>>> Shouldn't this be && instead of & ?
>>
>> You're right, will fix.
>
> And this too.
>
> So I believe it should now be fully ABI compatible.

Great, thanks!

Harald