Merge pull request #564 from EvgeniiG/Unity-2017.3

Manually optimize exp()

ScriptableRenderPipeline/Core/ShaderLibrary/Common.hlsl

 #define HALF_PI     1.57079632679
 #define INV_HALF_PI 0.636619772367
 #define INFINITY    asfloat(0x7F800000)
+#define LOG2_E      1.44269504089
 
 #define FLT_EPSILON 1.192092896e-07 // Smallest positive number, such that 1.0 + FLT_EPSILON != 1.0
 #define FLT_MIN     1.175494351e-38 // Minimum representable positive floating-point number

ScriptableRenderPipeline/Core/ShaderLibrary/CommonMaterial.hlsl

 // Evaluate Int{0, inf}{2 * Pi * r * R(sqrt(r^2 + d^2))}, where R is the diffusion profile.
 // Note: 'volumeAlbedo' should be premultiplied by 0.25.
 // Ref: Approximate Reflectance Profiles for Efficient Subsurface Scattering by Pixar (BSSRDF only).
-float3 ComputeTransmittance(float3 S, float3 volumeAlbedo, float thickness, float radiusScale)
+float3 ComputeTransmittanceDisney(float3 S, float3 volumeAlbedo, float thickness, float radiusScale)
+#if 0
+#else
+    // Help the compiler.
+    float  k = (-1.0 / 3.0) * LOG2_E;
+    float3 p = (k * thickness) * S;
+    float3 expOneThird = exp2(p);
+#endif
 
     // Premultiply & optimize: T = (1/4 * A) * (e^(-t * S) + 3 * e^(-1/3 * t * S))
     return volumeAlbedo * (expOneThird * expOneThird * expOneThird + 3 * expOneThird);

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

 
         if (_UseDisneySSS)
         {
-            bsdfData.transmittance = ComputeTransmittance(_ShapeParams[subsurfaceProfile].rgb,
-                                                          _TransmissionTints[subsurfaceProfile].rgb,
-                                                          bsdfData.thickness, bsdfData.subsurfaceRadius);
+            bsdfData.transmittance = ComputeTransmittanceDisney(_ShapeParams[subsurfaceProfile].rgb,
+                                                                _TransmissionTints[subsurfaceProfile].rgb,
+                                                                bsdfData.thickness, bsdfData.subsurfaceRadius);
         }
         else
         {

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Resources/SubsurfaceScattering.compute

 // We can drop the constant (s / (8 * Pi)) due to the subsequent weight renormalization.
 float3 DisneyProfilePolar(float r, float3 S)
 {
+#if 0
+#else
+    // Help the compiler.
+    float  k = (-1.0 / 3.0) * LOG2_E;
+    float3 p = (k * r) * S;
+    float3 expOneThird = exp2(p);
+#endif
     return expOneThird + expOneThird * expOneThird * expOneThird;
 }
 

ScriptableRenderPipeline/HDRenderPipeline/SceneSettings/Resources/DrawTransmittanceGraph.shader

 
                 if (_UseDisneySSS)
                 {
-                    T = ComputeTransmittance(_ShapeParam.rgb, float3(0.25, 0.25, 0.25), d, 1);
+                    T = ComputeTransmittanceDisney(_ShapeParam.rgb, float3(0.25, 0.25, 0.25), d, 1);
                 }
                 else
                 {