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

[Rich Felker <dalias@libc.org>]

On Fri, Jun 23, 2017 at 07:46:38PM -0500, A. Wilcox wrote:
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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,

Thanks for the feedback. If it ends up being problematic, but gcc has
a way to tell if -march is for a model with or without it, please feel
free to submit a patch to make the use conditional.

Rich