Re: [PATCH v3 7/7] powerpc64le: Enable float128

[Peter Bergner <bergner at vnet dot ibm dot com>]

On 6/27/17 1:42 PM, Florian Weimer wrote:
> On 06/27/2017 05:00 PM, Peter Bergner wrote:
>> We have a __builtin_cpu_supports() version that can test for float128, so
>> why can't we use the following?  On older systems (ie, older glibcs),
>> the builtin will expand to false, which is conservatively correct.
> 
>> +#define SW_OR_HW(SW, HW) (__builtin_cpu_supports ("ieee128") ? HW : SW)
> 
> How is that implemented?  Does it call into the kernel or glibc, too?

No.  It reads a bit mask stored in the TCB which glibc has initialized
for us long before main() is called:

  https://sourceware.org/ml/libc-alpha/2015-12/msg00041.html

..so all that is generated is a load followed by a masking operation:

bergner@pike:~$ cat float128.c
int
float128 (void)
{
  return __builtin_cpu_supports ("ieee128");
}
bergner@pike:~$ gcc -O2 -S float128.c
bergner@pike:~$ cat float128.s
	[snip]
float128:
	lwz 3,-28776(13)
	rldicl 3,3,42,63
	blr

Peter