[Mesa-dev] [PATCH 1/2] nir: add lowering for ffloor

[Roland Scheidegger]

Am 12.11.18 um 20:40 schrieb Jason Ekstrand:
> On Mon, Nov 12, 2018 at 1:29 PM Christian Gmeiner
> <christian.gmeiner at gmail.com <mailto:christian.gmeiner at gmail.com>> wrote:
> 
>     Hi Jason
> 
>     Am Sa., 1. Sep. 2018 um 21:23 Uhr schrieb Jason Ekstrand
>     <jason at jlekstrand.net <mailto:jason at jlekstrand.net>>:
>     >
>     > I don't think either of these work for negative numbers
>     >
> 
>     I would like to land this patch - can you provide some details why
>     this does not work for
>     negative numbers?
> 
> 
> No, this is correct.  It's GLSL's definition of fract(x) that's dumb. 
> GLSL defines fract(x) as x - floor(x) (exactly what your formula says. 
> This means that
> 
> fract(-1.4) = -1.4 - floor(-1.4) = -1.4 - (-2.0) = -1.4 + 2.0 = 0.6
> 
> so GLSL fract() doesn't give you the fractional part at all.  *sigh*
Can you elaborate why you think this result to be wrong? In fact it's
the only definition I've ever heard of. But yes, wikipedia says fract
for negative numbers is open for debate (with 3 possible solutions). In
the context of graphics shading languages, it is however most definitely
the agreed-upon formula for fractional parts (and certainly the same
formula was used pre-glsl or hlsl).

Roland


> 
> Reviewed-by: Jason Ekstrand <jason at jlekstrand.net
> <mailto:jason at jlekstrand.net>>
>  
> 
>     > On September 1, 2018 14:16:11 Christian Gmeiner
>     > <christian.gmeiner at gmail.com <mailto:christian.gmeiner at gmail.com>>
>     wrote:
>     >
>     > > Signed-off-by: Christian Gmeiner <christian.gmeiner at gmail.com
>     <mailto:christian.gmeiner at gmail.com>>
>     > > ---
>     > > src/compiler/nir/nir.h                | 3 +++
>     > > src/compiler/nir/nir_opt_algebraic.py | 1 +
>     > > 2 files changed, 4 insertions(+)
>     > >
>     > > diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
>     > > index 169fa1fa20..d81eefc032 100644
>     > > --- a/src/compiler/nir/nir.h
>     > > +++ b/src/compiler/nir/nir.h
>     > > @@ -2054,6 +2054,9 @@ typedef struct nir_shader_compiler_options {
>     > >     */
>     > >    bool fdot_replicates;
>     > >
>     > > +   /** lowers ffloor to fsub+ffract: */
>     > > +   bool lower_ffloor;
>     > > +
>     > >    /** lowers ffract to fsub+ffloor: */
>     > >    bool lower_ffract;
>     > >
>     > > diff --git a/src/compiler/nir/nir_opt_algebraic.py
>     > > b/src/compiler/nir/nir_opt_algebraic.py
>     > > index ae1261f874..3d2b861a42 100644
>     > > --- a/src/compiler/nir/nir_opt_algebraic.py
>     > > +++ b/src/compiler/nir/nir_opt_algebraic.py
>     > > @@ -118,6 +118,7 @@ optimizations = [
>     > >    (('~flrp', a, 0.0, c), ('fadd', ('fmul', ('fneg', a), c), a)),
>     > >    (('flrp at 32', a, b, c), ('fadd', ('fmul', c, ('fsub', b, a)), a),
>     > >    'options->lower_flrp32'),
>     > >    (('flrp at 64', a, b, c), ('fadd', ('fmul', c, ('fsub', b, a)), a),
>     > >    'options->lower_flrp64'),
>     > > +   (('ffloor', a), ('fsub', a, ('ffract', a)),
>     'options->lower_ffloor'),
>     > >    (('ffract', a), ('fsub', a, ('ffloor', a)),
>     'options->lower_ffract'),
>     > >    (('~fadd', ('fmul', a, ('fadd', 1.0, ('fneg', ('b2f', c)))),
>     ('fmul', b,
>     > >    ('b2f', c))), ('bcsel', c, b, a), 'options->lower_flrp32'),
>     > >    (('~fadd at 32', ('fmul', a, ('fadd', 1.0, ('fneg',         c
>     ))), ('fmul', b,
>     > >            c )), ('flrp', a, b, c), '!options->lower_flrp32'),
>     > > --
>     > > 2.17.1
>     > >
>     > > _______________________________________________
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>     >
> 
>     -- 
>     greets
>     --
>     Christian Gmeiner, MSc
> 
>     https://christian-gmeiner.info
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