[Mesa-dev] [PATCH 1/2] nir: add a compiler option for disabling float comparison simplifications

[Jason Ekstrand]

Can you provide some context for this?  Those rules are already flagged
"inexact" (that's what the ~ means) so they won't apply to anything that's
"precise" or "invariant".

On Thu, Nov 29, 2018 at 9:18 AM Samuel Pitoiset <samuel.pitoiset at gmail.com>
wrote:

> It's correct in GLSL because the behaviour is undefined in
> presence of NaNs. But this seems incorrect in Vulkan.
>
> Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
> ---
>  src/compiler/nir/nir.h                | 6 ++++++
>  src/compiler/nir/nir_opt_algebraic.py | 8 ++++----
>  2 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> index db935c8496b..4107c293962 100644
> --- a/src/compiler/nir/nir.h
> +++ b/src/compiler/nir/nir.h
> @@ -2188,6 +2188,12 @@ typedef struct nir_shader_compiler_options {
>     /* Set if nir_lower_wpos_ytransform() should also invert
> gl_PointCoord. */
>     bool lower_wpos_pntc;
>
> +       /* If false, lower ~inot(flt(a,b)) -> fge(a,b) and variants.
> +        * In presence of NaNs, this is correct in GLSL because the
> behaviour is
> +        * undefined. In Vulkan, doing these transformations is incorrect.
> +        */
> +       bool exact_float_comparisons;
> +
>     /**
>      * Should nir_lower_io() create load_interpolated_input intrinsics?
>      *
> diff --git a/src/compiler/nir/nir_opt_algebraic.py
> b/src/compiler/nir/nir_opt_algebraic.py
> index f2a7be0c403..3750874407b 100644
> --- a/src/compiler/nir/nir_opt_algebraic.py
> +++ b/src/compiler/nir/nir_opt_algebraic.py
> @@ -154,10 +154,10 @@ optimizations = [
>     (('ishl', ('imul', a, '#b'), '#c'), ('imul', a, ('ishl', b, c))),
>
>     # Comparison simplifications
> -   (('~inot', ('flt', a, b)), ('fge', a, b)),
> -   (('~inot', ('fge', a, b)), ('flt', a, b)),
> -   (('~inot', ('feq', a, b)), ('fne', a, b)),
> -   (('~inot', ('fne', a, b)), ('feq', a, b)),
> +   (('~inot', ('flt', a, b)), ('fge', a, b),
> '!options->exact_float_comparisons'),
> +   (('~inot', ('fge', a, b)), ('flt', a, b),
> '!options->exact_float_comparisons'),
> +   (('~inot', ('feq', a, b)), ('fne', a, b),
> '!options->exact_float_comparisons'),
> +   (('~inot', ('fne', a, b)), ('feq', a, b),
> '!options->exact_float_comparisons'),
>

The feq/fne one is actually completely safe.  fne is defined to be !feq
even when NaN is considered.

--Jasoan


>     (('inot', ('ilt', a, b)), ('ige', a, b)),
>     (('inot', ('ult', a, b)), ('uge', a, b)),
>     (('inot', ('ige', a, b)), ('ilt', a, b)),
> --
> 2.19.2
>
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