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)

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