[Mesa-dev] Numeric precision problem with highp tanh

[Jason Ekstrand]

On Thu, Dec 8, 2016 at 3:31 PM, Haixia Shi <hshi at chromium.org> wrote:

> We're encountering failures in the latest version of dEQP (specifically,
> dEQP-GLES3.functional.shaders.builtin_functions.precision.tanh.highp_*)
> on all Intel devices.
>
> The problem is that when the abs value of input is too large (say 80), the
> function should return +/-1, but the actual shader output is 0.
>
> The following patch in glsl compiler fixes the problem, but I'm not really
> sure if a fix in the compiler frontend is the right place. On the other
> hand, once the intermediate code is emitted, it's not really feasible for
> drivers to handle precision problems in backend. Any ideas?
>

I think this is the correct way to handle this.  It isn't really a back-end
precision problem.  The problem is that, once x gets large enough, exp(x)
tends towards infinity and you may end up with inf/inf.


> Here's the patch
>
> diff --git a/src/compiler/glsl/builtin_functions.cpp
> b/src/compiler/glsl/builtin_functions.cpp
> index 3e4bcbb..6cfeb1b 100644
> --- a/src/compiler/glsl/builtin_functions.cpp
> +++ b/src/compiler/glsl/builtin_functions.cpp
> @@ -3563,9 +3563,13 @@ builtin_builder::_tanh(const glsl_type *type)
>     ir_variable *x = in_var(type, "x");
>     MAKE_SIG(type, v130, 1, x);
>
> +   /* clamp x to [-20, +20] to avoid numeric problems */
> +   ir_variable *t = body.make_temp(type, "tmp");
> +   body.emit(assign(t, min2(max2(x, imm(-20.0f)), imm(20.0f))));
>

I think I would feel more comfortable if we clamped to +- 10 instead.  I'm
a bit more comfortable with the stability of the calculation at lower
values.  Also, when x > 10, e^(-x) is so small relative to e^x that it
won't actually count in the floating-point calculation.  In particular
e^(-10)/e^(10) = e^(-20) = 2^-27.182 and you  only have 23 mantissa bits.

Also, would you mind fixing spirv_to_nir while you're at it?  It should be
in vtn_glsl450.c.  I can port it myself if you'd prefer.


> +
>     /* (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;
>  }
>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/mesa-dev/attachments/20161208/c74b0f67/attachment.html>