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-rw-r--r--src/compiler/glsl/builtin_functions.cpp18
1 files changed, 10 insertions, 8 deletions
diff --git a/src/compiler/glsl/builtin_functions.cpp b/src/compiler/glsl/builtin_functions.cpp
index 3dead1af00f..17f03a3fa27 100644
--- a/src/compiler/glsl/builtin_functions.cpp
+++ b/src/compiler/glsl/builtin_functions.cpp
@@ -3563,17 +3563,19 @@ 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.
- * 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.
+ /* tanh(x) := (0.5 * (e^x - e^(-x))) / (0.5 * (e^x + e^(-x)))
+ *
+ * With a little algebra this reduces to (e^2x - 1) / (e^2x + 1)
+ *
+ * Clamp x to (-inf, +10] to avoid precision problems. When x > 10, e^2x
+ * is so much larger than 1.0 that 1.0 gets flushed to zero in the
+ * computation e^2x +/- 1 so it can be ignored.
*/
ir_variable *t = body.make_temp(type, "tmp");
- body.emit(assign(t, min2(max2(x, imm(-10.0f)), imm(10.0f))));
+ body.emit(assign(t, min2(x, imm(10.0f))));
- /* (e^x - e^(-x)) / (e^x + e^(-x)) */
- body.emit(ret(div(sub(exp(t), exp(neg(t))),
- add(exp(t), exp(neg(t))))));
+ body.emit(ret(div(sub(exp(mul(t, imm(2.0f))), imm(1.0f)),
+ add(exp(mul(t, imm(2.0f))), imm(1.0f)))));
return sig;
}