Re: [PATCH v3 3/6] powerpc64le: raise GCC requirement to 7.4 for long double transition

[Paul E Murphy <murphyp@linux.ibm.com>]

On 4/22/20 10:19 AM, Florian Weimer wrote:
> * Paul E. Murphy via Libc-alpha:
> 
>> diff --git a/manual/install.texi b/manual/install.texi
>> index 71bf47cac6..66de8ea59b 100644
>> --- a/manual/install.texi
>> +++ b/manual/install.texi
>> @@ -507,6 +507,11 @@ the newest version of the compiler that is known to work for building
>>   release time, GCC 9.2.1 is the newest compiler verified to work to build
>>   @theglibc{}.
>>   
>> +For PowerPC 64-bits little-endian (powerpc64le), a GCC version with
>> +support for @option{-mno-gnu-attribute} is required.  As of release time,
>> +this means GCC 7.4 (excepting 7.5) or newer. The compiler option is required
>> +for building the GNU C Library with support for IEEE long double.
> 
> Why “excepting 7.5”?  Somewhere, there should be an explanation, maybe
> in a @c comment in the Texinfo sources if it's too confusing to end
> users.

FSF GCC 7.5 does not accept both the -mabi=* and -mlong-double-128 
options being present on the command line.  Ironically, it errors 
instructing you to pass the -mlong-double-128 option.

> Also, missing two spaces at the end of sentences.
> 
> And quick question for my understanding: Could we get the effect of
> -mno-gnu-attribute using strip if we wanted?  (Not that I think this
> change is required, I'm fine with bumping the minimum GCC
> requirement.)

Yes, I think one could post-process each object file to strip the 
.gnu.attributes section.  Though, you do highlight an impedance mismatch 
between the commit message and documentation regarding the -mabi 
requirements.

How about the following instead?

For PowerPC 64-bits little-endian (powerpc64le), a GCC version with 
support for @option{-mno-gnu-attribute}, @option{-mabi=ieeelongdouble}, 
and @option{-mabi=ibmlondouble} is required.  Likewise, the compiler 
must also support passing @options{-mlong-double-128} with the preceding 
options.  As of release, this implies GCC 7.4 and newer (excepting GCC 
7.5.0, see GCC PR94200).  These additional features are required for 
building the GNU C Library with support for IEEE long double.