[Mesa-dev] [PATCH] nir/algebraic: Add optimizations for "a == a && a CMP b"

[Jason Ekstrand]

On Thu, Dec 22, 2016 at 1:13 PM, Ian Romanick <idr at freedesktop.org> wrote:

> On 12/19/2016 09:32 PM, Jason Ekstrand wrote:
> > This sequence shows up The Talos Principal, at least under Vulkan,
> > and prevents loop analysis from properly computing trip counts in a
> > few loops.
> > ---
> >  src/compiler/nir/nir_opt_algebraic.py | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/src/compiler/nir/nir_opt_algebraic.py
> b/src/compiler/nir/nir_opt_algebraic.py
> > index 698ac67..cc70ad5 100644
> > --- a/src/compiler/nir/nir_opt_algebraic.py
> > +++ b/src/compiler/nir/nir_opt_algebraic.py
> > @@ -464,6 +464,14 @@ def bitfield_reverse(u):
> >
> >  optimizations += [(bitfield_reverse('x at 32'), ('bitfield_reverse',
> 'x'))]
> >
> > +# For any comparison operation, "cmp", if you have "a != a && a cmp b"
> then
> > +# the "a != a" is redundant because it's equivalent to "a is not NaN"
> and, if
> > +# a is a NaN then the second comparison will fail anyway.
> > +for op in ['flt', 'fge', 'feq', 'fne']:
> > +   optimizations += [
> > +      (('iand', ('feq', a, a), (op, a, b)), (op, a, b)),
> > +      (('iand', ('feq', a, a), (op, b, a)), (op, b, a)),
> > +   ]
>
> I don't think this is right.  That will change (NaN == NaN) && (NaN !=
> 7) from false to true.  Right?  You should just drop fne from the
> list... I think.  Math is hard.
>

Don't all comparisons with NaN return false?  Wouldn't NaN != 7 be false?
Maybe I don't understand NaN!

--Jason


>
> >  # Add optimizations to handle the case where the result of a ternary is
> >  # compared to a constant.  This way we can take things like
> >
>
>
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