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

[Kenneth Graunke]

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>

> +    * 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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