[Mesa-dev] [PATCH v3] compiler/glsl: fix precision problem of tanh

[Jason Ekstrand]

On Thu, Dec 8, 2016 at 5:50 PM, Kenneth Graunke <kenneth at whitecape.org>
wrote:

> On Thursday, December 8, 2016 5:41:02 PM PST Haixia Shi wrote:
> > Clamp input scalar value to range [-10, +10] to avoid precision problems
> > when the absolute value of input is too large.
> >
> > Fixes dEQP-GLES3.functional.shaders.builtin_functions.precision.tanh.*
> test
> > failures.
> >
> > v2: added more explanation in the comment.
> > v3: fixed a typo in the comment.
> >
> > Signed-off-by: Haixia Shi <hshi at chromium.org>
> > Cc: Jason Ekstrand <jason at jlekstrand.net>,
> > Cc: Stéphane Marchesin <marcheu at chromium.org>,
> > Cc: Kenneth Graunke <kenneth at whitecape.org>
> >
> > Change-Id: I324c948b3323ff8107127c42934f14459e124b95
> > ---
> >  src/compiler/glsl/builtin_functions.cpp | 13 +++++++++++--
> >  1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/src/compiler/glsl/builtin_functions.cpp
> b/src/compiler/glsl/builtin_functions.cpp
> > index 3e4bcbb..0bacffb 100644
> > --- a/src/compiler/glsl/builtin_functions.cpp
> > +++ b/src/compiler/glsl/builtin_functions.cpp
> > @@ -3563,9 +3563,18 @@ builtin_builder::_tanh(const glsl_type *type)
> >     ir_variable *x = in_var(type, "x");
> >     MAKE_SIG(type, v130, 1, x);
> >
> > +   /*
>
> For future reference, /* doesn't go on its own line in Mesa.
> (We can fix that when pushing, no big deal.)
>
> Thanks for fixing this.  The explanation makes sense.
>
> Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
>

I just pushed this with my and ken's reviews and the comment change Ken
suggested.


> > +    * Clamp x to [-10, +10] to avoid precision problems.
> > +    * When x > 10, e^(-x) is so small relative to e^x that it gets
> flushed to
> > +    * zero in the computation e^x + e^(-x). The same happens in the
> other
> > +    * direction when x < -10.
> > +    */
> > +   ir_variable *t = body.make_temp(type, "tmp");
> > +   body.emit(assign(t, min2(max2(x, imm(-10.0f)), imm(10.0f))));
> > +
> >     /* (e^x - e^(-x)) / (e^x + e^(-x)) */
> > -   body.emit(ret(div(sub(exp(x), exp(neg(x))),
> > -                     add(exp(x), exp(neg(x))))));
> > +   body.emit(ret(div(sub(exp(t), exp(neg(t))),
> > +                     add(exp(t), exp(neg(t))))));
> >
> >     return sig;
> >  }
> >
>
>
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