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

[Ian Romanick]

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.

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