Re: [PATCH] mips: add single-instruction math functions

[Rich Felker <dalias@libc.org>]

On Fri, Sep 13, 2019 at 03:12:11PM -0400, James Y Knight wrote:
> I've no idea about the issues with MIPS' instruction set, but yes, to be
> correct, abs does need to affect only the sign bit, leaving everything else
> the same.
> 
> It's specified as such in IEEE 754-2008: 5.5.1 Sign bit operations --
> "Implementations shall provide the following homogeneous
> quiet-computational sign bit operations for all supported arithmetic
> formats; they only affect the sign bit. The operations treat floating-point
> numbers and NaNs alike, and signal no exception. These operations may
> propagate non-canonical encodings."
> ....
> "abs(x) copies a floating-point operand x to a destination in the same
> format, setting the sign bit to 0 (positive)."
> 
> (That section also defines the copy, negate, and copySign operations)

OK, the text I was missing was:

"The fabs functions in <math.h> provide the abs function recommended
in the Appendix to IEC 60559."

from F.3 ¶1.

Rich

> On Fri, Sep 13, 2019 at 2:57 PM Rich Felker <dalias@libc.org> wrote:
> 
> > On Fri, Sep 13, 2019 at 02:31:23PM -0400, Rich Felker wrote:
> > > On Wed, Sep 11, 2019 at 01:05:04PM +0300, info@mobile-stream.com wrote:
> > > >
> > > > non-commit text:
> > > > gcc puts annoying nop into the delay slot for these functions, e.g.:
> > > >     abs.d   $f0,$f12
> > > >     jr      $ra
> > > >      nop
> > > > is there any way to get rid of this without using pure .S?
> > > >
> > > >
> > > >
> > > > SQRT.fmt exists on MIPS II+ (float), MIPS III+ (double).
> > > >
> > > > ABS.fmt exists on MIPS I+ but only cores with ABS2008 flag in FCSR
> > > > implement the required behaviour.
> > > > ---
> > > >  src/math/mips/fabs.c  | 16 ++++++++++++++++
> > > >  src/math/mips/fabsf.c | 16 ++++++++++++++++
> > > >  src/math/mips/sqrt.c  | 16 ++++++++++++++++
> > > >  src/math/mips/sqrtf.c | 16 ++++++++++++++++
> > > >  4 files changed, 64 insertions(+)
> > > >  create mode 100644 src/math/mips/fabs.c
> > > >  create mode 100644 src/math/mips/fabsf.c
> > > >  create mode 100644 src/math/mips/sqrt.c
> > > >  create mode 100644 src/math/mips/sqrtf.c
> > > >
> > > > diff --git a/src/math/mips/fabs.c b/src/math/mips/fabs.c
> > > > new file mode 100644
> > > > index 00000000..0a5aa3b1
> > > > --- /dev/null
> > > > +++ b/src/math/mips/fabs.c
> > > > @@ -0,0 +1,16 @@
> > > > +#if !defined(__mips_soft_float) && defined(__mips_abs2008)
> > >
> > > Why is this dependent on __mips_abs2008?
> >
> > OK, I see. The macro isn't well-documented, but it corresponds to the
> > gcc -mabs=2008 option indicating an ISA variant that treats the
> > abs.[sd] instruction as bitwise rather than arithmetic. I'm somewhat
> > unclear on whether we should do anything like this; it's one of the
> > places where MIPS blurred the distinction between ISA levels and
> > incompatible ABIs, and seems to be able to produce code that would
> > silently run on the wrong type of cpu and produce the wrong result (or
> > maybe this is a runtime-switchable cpu mode?)
> >
> > I'm also unclear though whether fabs() actually needs to behave as
> > bitwise. Neither C nor Annex F nor POSIX seems to specify its behavior
> > for NANs except that a NAN is returned. Am I missing something? If
> > not, it seems the 'legacy' abs.[sd] instructions would also be fine
> > for implementing this, and then no conditional is needed.
> >
> > Rich
> >