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

[Haixia Shi]

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.

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 | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/compiler/glsl/builtin_functions.cpp b/src/compiler/glsl/builtin_functions.cpp
index 3e4bcbb..4ede925 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 [-10, +10] to avoid precision problems */
+   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;
 }
-- 
2.8.0.rc3.226.g39d4020