On Thu, Dec 22, 2016 at 1:37 PM, Ian Romanick <i...@freedesktop.org> wrote:
> On 12/22/2016 01:14 PM, Jason Ekstrand wrote: > > On Thu, Dec 22, 2016 at 1:13 PM, Ian Romanick <i...@freedesktop.org > > <mailto:i...@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@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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