Introduce threshold for NEON floating-point SOFT_RELU activation

Large inputs produce incorrect results of Soft ReLu activations where
the output saturates at around 88.72283. This is due to the
approximation algorithm used for the logarithm.

For this reason, we introduce a threshold such that with x > threshold,
Soft ReLu activation will return the value x itself.

SVE does not seem to suffer from the same issue, hence NEON kernels only
are modified.

Resolves COMPMID-4091

Change-Id: I357883deed4e4aba571a1d3163267772096c0412
Signed-off-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/4865
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Giorgio Arena <giorgio.arena@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>

1 files changed, 9 insertions, 6 deletions

diff --git a/src/core/cpu/kernels/activation/NEON/fp32.cpp b/src/core/cpu/kernels/activation/NEON/fp32.cpp
index 0687646be7..c76035b5d2 100644
--- a/src/core/cpu/kernels/activation/NEON/fp32.cpp
+++ b/src/core/cpu/kernels/activation/NEON/fp32.cpp
@@ -44,7 +44,7 @@ inline float32x4_t mask_float_vector(const float32x4_t &in, const uint32x4_t &ma
     auto int_in = vreinterpretq_u32_f32(in);
     return vreinterpretq_f32_u32(wrapper::vand(int_in, mask));
 }
-#endif /* __arch64__ */
+#endif /* __aarch64__ */
 } // namespace
 
 void fp32_neon_activation(const ITensor *src, ITensor *dst, const ActivationLayerInfo &act_info, const Window &window)
@@ -68,13 +68,16 @@ void fp32_neon_activation(const ITensor *src, ITensor *dst, const ActivationLaye
     // In case of aarh64, we call vsqrt directly, so we don't use delta.
 #ifndef __aarch64__
     const auto delta = wrapper::vdup_n(static_cast<float>(1e-24), ExactTagType {});
-#endif /* __aarch64 */
+#endif /* __aarch64__ */
     const auto const_1     = wrapper::vdup_n(static_cast<float>(1.f), ExactTagType {});
     const auto const_0     = wrapper::vdup_n(static_cast<float>(0.f), ExactTagType{});
     const auto const_6     = wrapper::vdup_n(static_cast<float>(6.f), ExactTagType{});
     const auto const_3     = wrapper::vdup_n(static_cast<float>(3.f), ExactTagType{});
     const auto const_inv_6 = wrapper::vdup_n(static_cast<float>(0.166666667f), ExactTagType{});
 
+    constexpr float soft_relu_thresh  = 12.f;
+    const auto      vsoft_relu_thresh = wrapper::vdup_n(static_cast<float>(soft_relu_thresh), ExactTagType{});
+
     const auto va = wrapper::vdup_n(static_cast<float>(act_info.a()), ExactTagType{});
     const auto vb = wrapper::vdup_n(static_cast<float>(act_info.b()), ExactTagType{});
     const auto a  = static_cast<float>(act_info.a());
@@ -115,7 +118,7 @@ void fp32_neon_activation(const ITensor *src, ITensor *dst, const ActivationLaye
                     tmp = wrapper::vbsl(wrapper::vcgt(vin, const_0), vin, wrapper::vmul(va, vin));
                     break;
                 case ActivationLayerInfo::ActivationFunction::SOFT_RELU:
-                    tmp = wrapper::vlog(wrapper::vadd(const_1, wrapper::vexpq(vin)));
+                    tmp = wrapper::vbsl(wrapper::vcgt(vin, vsoft_relu_thresh), vin, wrapper::vlog(wrapper::vadd(const_1, wrapper::vexpq(vin))));
                     break;
                 case ActivationLayerInfo::ActivationFunction::ELU:
                     tmp = wrapper::vbsl(wrapper::vcge(vin, const_0), vin, wrapper::vmul(va, wrapper::vsub(wrapper::vexpq(vin), const_1)));
@@ -123,13 +126,13 @@ void fp32_neon_activation(const ITensor *src, ITensor *dst, const ActivationLaye
                 case ActivationLayerInfo::ActivationFunction::SQRT:
 #ifdef __aarch64__
                     tmp = wrapper::vsqrt(vin);
-#else  /* aarch64 */
+#else  /* __aarch64__ */
                     {
                         const auto bitmask = wrapper::vceq(vin, wrapper::vdup_n(0.f, ExactTagType{}));
                         tmp                 = wrapper::vinv(wrapper::vinvsqrt(wrapper::vadd(vin, mask_float_vector(delta, bitmask))));
                         tmp                 = mask_float_vector(tmp, wrapper::vnot(bitmask));
                     }
-#endif /* aarch64 */
+#endif /* __aarch64__ */
                     break;
                 case ActivationLayerInfo::ActivationFunction::SQUARE:
                     tmp = wrapper::vmul(vin, vin);
@@ -178,7 +181,7 @@ void fp32_neon_activation(const ITensor *src, ITensor *dst, const ActivationLaye
                     tmp = (in > 0) ? in : a * in;
                     break;
                 case ActivationLayerInfo::ActivationFunction::SOFT_RELU:
-                    tmp = std::log(static_cast<float>(1) + std::exp(in));
+                    tmp = (in > soft_relu_thresh) ? in : std::log(static_cast<float>(1) + std::exp(in));
                     break;
                 case ActivationLayerInfo::ActivationFunction::ELU:
                     tmp = (in >= 0) ? in : a * (std::exp(in) - 1);