On Thu, Dec 8, 2016 at 3:31 PM, Haixia Shi <h...@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; > } > > > >
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