diff --git a/intern/cycles/kernel/kernel_types.h b/intern/cycles/kernel/kernel_types.h
index aeb63b4a65e..c26769341f0 100644
--- a/intern/cycles/kernel/kernel_types.h
+++ b/intern/cycles/kernel/kernel_types.h
@@ -347,12 +347,20 @@ enum PathRayFlag {
 
 	PATH_RAY_ALL_VISIBILITY = ((1 << 14)-1),
 
-	PATH_RAY_MIS_SKIP               = (1 << 15),
-	PATH_RAY_DIFFUSE_ANCESTOR       = (1 << 16),
-	PATH_RAY_SINGLE_PASS_DONE       = (1 << 17),
-	PATH_RAY_SHADOW_CATCHER         = (1 << 18),
-	PATH_RAY_STORE_SHADOW_INFO      = (1 << 19),
-	PATH_RAY_TRANSPARENT_BACKGROUND = (1 << 20),
+	/* Don't apply multiple importance sampling weights to emission from
+	 * lamp or surface hits, because they were not direct light sampled. */
+	PATH_RAY_MIS_SKIP                    = (1 << 14),
+	/* Diffuse bounce earlier in the path, skip SSS to improve performance
+	 * and avoid branching twice with disk sampling SSS. */
+	PATH_RAY_DIFFUSE_ANCESTOR            = (1 << 15),
+	/* Single pass has been written. */
+	PATH_RAY_SINGLE_PASS_DONE            = (1 << 16),
+	/* Ray is behind a shadow catcher .*/
+	PATH_RAY_SHADOW_CATCHER              = (1 << 17),
+	/* Store shadow data for shadow catcher or denoising. */
+	PATH_RAY_STORE_SHADOW_INFO           = (1 << 18),
+	/* Zero background alpha, for camera or transparent glass rays. */
+	PATH_RAY_TRANSPARENT_BACKGROUND      = (1 << 19),
 };
 
 /* Closure Label */
diff --git a/intern/cycles/render/osl.cpp b/intern/cycles/render/osl.cpp
index 5c5ac6e2be9..9c107274305 100644
--- a/intern/cycles/render/osl.cpp
+++ b/intern/cycles/render/osl.cpp
@@ -233,18 +233,22 @@ void OSLShaderManager::shading_system_init()
 			"glossy",			/* PATH_RAY_GLOSSY */
 			"singular",			/* PATH_RAY_SINGULAR */
 			"transparent",		/* PATH_RAY_TRANSPARENT */
+
 			"shadow",			/* PATH_RAY_SHADOW_OPAQUE_NON_CATCHER */
 			"shadow",			/* PATH_RAY_SHADOW_OPAQUE_CATCHER */
 			"shadow",			/* PATH_RAY_SHADOW_TRANSPARENT_NON_CATCHER */
 			"shadow",			/* PATH_RAY_SHADOW_TRANSPARENT_CATCHER */
 
 			"__unused__",
+			"volume_scatter",	/* PATH_RAY_VOLUME_SCATTER */
+			"__unused__",
+
 			"__unused__",
 			"diffuse_ancestor",	/* PATH_RAY_DIFFUSE_ANCESTOR */
 			"__unused__",
 			"__unused__",
-			"__unused__",		/* PATH_RAY_SINGLE_PASS_DONE */
-			"volume_scatter",	/* PATH_RAY_VOLUME_SCATTER */
+			"__unused__",
+			"__unused__",
 		};
 
 		const int nraytypes = sizeof(raytypes)/sizeof(raytypes[0]);