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

[Jason Ekstrand]

On Thu, Dec 22, 2016 at 1:37 PM, Ian Romanick <idr at freedesktop.org> wrote:

> On 12/22/2016 01:14 PM, Jason Ekstrand wrote:
> > On Thu, Dec 22, 2016 at 1:13 PM, Ian Romanick <idr at freedesktop.org
> > <mailto: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!
>
> As far as I'm aware, (!(a == b)) is always the same as (a !== b).
> According to
> https://en.wikipedia.org/wiki/IEEE_754-1985#Recommended_
> functions_and_predicates,
>  a != a is how you can implement isnan().
>

Bah!  I just wrote a small C program to confirm this using your example and
you are 100% correct (according to gcc).  I'll drop fne from the list.


> > --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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