Re: [PATCH] powerpc64le: Add single instruction math functions

["A. Wilcox" <awilfox@adelielinux.org>]

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On 23/06/17 14:53, David Edelsohn wrote:
> On Fri, Jun 23, 2017 at 3:35 PM, Rich Felker <dalias@libc.org> 
> wrote:
>> On Wed, Jun 21, 2017 at 10:53:13AM -0400, David Edelsohn wrote:
>>> The following two patches are a start at single instruction 
>>> math functions for PowerPC64 architecture.  Although PPC64LE 
>>> Linux and ELFv2 ABI require Power8 as the minimum
>>> architecture, I have added guards that fallback to C code for
>>> earlier architectures.
>> 
>> Indeed, musl uses the ELFv2 ABI (minus its gratuitous mandate of
>>  minimum ISA level) for both little and big endian powerpc64,
>> and I think we have users of both (people running it on old 
>> powerbooks, etc.).
>> 
>> Am I reading correctly that sqrt, fma, and fabs are available 
>> even in the lowest powerpc64 ISA, and don't need preprocessor 
>> conditionals?
> 
> fabs and fma are part of the base ISA for Power processors that 
> include floating point support.  fsqrt originally was optional 
> feature in the distant past (General Purpose group of optional 
> instructions), but is required in the ISA for Power processors.
> 
> Thanks, David
> 

Chiming in as one of the heavy users/developers involved with musl/ppc.

I can confirm FSQRT exists on the oldest PPC64 chip I have - a 970FX
from 2003 - but it is indeed optional. (I didn't bother checking the
actual IBM chips because they're all on the higher end.) The Linux
kernel actually has support for emulating the instruction on PowerPC
chips where it wasn't implemented. See arch/powerpc/math-emu/fsqrt.c
in the kernel tree.

It depends on CONFIG_MATH_EMULATION_HW_UNIMPLEMENTED (or
CONFIG_MATH_EMULATION_FULL), but most (all?) distros that ship PPC
kernels have that knob turned on, as far as I can tell.

So this should be safe. The worst case scenario is that distros would
need to twiddle a config knob in the kernel.

All the best,
- --arw

- -- 
A. Wilcox (awilfox)
Project Lead, Adélie Linux
http://adelielinux.org
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