Workaround for some PowerVR devices

The PowerVR compiler systematically crashes on some devices when
`gl_Position` is written twice in the vertex shader.


fixes #5118, b/190221124

NEW_RELEASE_NOTES.md

@@ -8,3 +8,4 @@ appropriate header in [RELEASE_NOTES.md](./RELEASE_NOTES.md).
 
 ## Release notes for next branch cut
 - backend: fix #6997 : picking can fail on Adreno [⚠️ **New Material Version**]
+- backend: A partial workaround for PowerVR devices (#5118, b/190221124) [⚠️ **Recompile Materials**]

shaders/src/main.vs

@@ -154,28 +154,29 @@ void main() {
 
 #endif // !defined(USE_OPTIMIZED_DEPTH_VERTEX_SHADER)
 
+    vec4 position;
 
 #if defined(VERTEX_DOMAIN_DEVICE)
     // The other vertex domains are handled in initMaterialVertex()->computeWorldPosition()
-    gl_Position = getPosition();
+    position = getPosition();
 
 #if !defined(USE_OPTIMIZED_DEPTH_VERTEX_SHADER)
 #if defined(MATERIAL_HAS_CLIP_SPACE_TRANSFORM)
-    gl_Position = getMaterialClipSpaceTransform(material) * gl_Position;
+    position = getMaterialClipSpaceTransform(material) * position;
 #endif
 #endif // !USE_OPTIMIZED_DEPTH_VERTEX_SHADER
 
 #if defined(MATERIAL_HAS_VERTEX_DOMAIN_DEVICE_JITTERED)
     // Apply the clip-space transform which is normally part of the projection
-    gl_Position.xy = gl_Position.xy * frameUniforms.clipTransform.xy + (gl_Position.w * frameUniforms.clipTransform.zw);
+    position.xy = position.xy * frameUniforms.clipTransform.xy + (position.w * frameUniforms.clipTransform.zw);
 #endif
 #else
-    gl_Position = getClipFromWorldMatrix() * getWorldPosition(material);
+    position = getClipFromWorldMatrix() * getWorldPosition(material);
 #endif
 
 #if defined(VERTEX_DOMAIN_DEVICE)
     // GL convention to inverted DX convention (must happen after clipSpaceTransform)
-    gl_Position.z = gl_Position.z * -0.5 + 0.5;
+    position.z = position.z * -0.5 + 0.5;
 #endif
 
 #if defined(VARIANT_HAS_VSM)
@@ -197,15 +198,20 @@ void main() {
 #endif
 
     // this must happen before we compensate for vulkan below
-    vertex_position = gl_Position;
+    vertex_position = position;
 
 #if defined(TARGET_VULKAN_ENVIRONMENT)
     // In Vulkan, clip space is Y-down. In OpenGL and Metal, clip space is Y-up.
-    gl_Position.y = -gl_Position.y;
+    position.y = -position.y;
 #endif
 
 #if !defined(TARGET_VULKAN_ENVIRONMENT) && !defined(TARGET_METAL_ENVIRONMENT)
     // This is not needed in Vulkan or Metal because clipControl is always (1, 0)
-    gl_Position.z = dot(gl_Position.zw, frameUniforms.clipControl);
+    // (We don't use a dot() here because it workaround a spirv-opt optimization that in turn
+    //  causes a crash on PowerVR, see #5118)
+    position.z = position.z * frameUniforms.clipControl.x + position.w * frameUniforms.clipControl.y;
 #endif
+
+    // some PowerVR drivers crash when gl_Position is written more than once
+    gl_Position = position;
 }

shaders/src/post_process.vs

@@ -5,14 +5,17 @@ void main() {
 
     inputs.normalizedUV = position.xy * 0.5 + 0.5;
 
-    gl_Position = getPosition();
+    vec4 position = getPosition();
+
     // GL convention to inverted DX convention
-    gl_Position.z = gl_Position.z * -0.5 + 0.5;
+    position.z = position.z * -0.5 + 0.5;
 
     // Adjust clip-space
 #if !defined(TARGET_VULKAN_ENVIRONMENT) && !defined(TARGET_METAL_ENVIRONMENT)
     // This is not needed in Vulkan or Metal because clipControl is always (1, 0)
-    gl_Position.z = dot(gl_Position.zw, frameUniforms.clipControl);
+    // (We don't use a dot() here because it workaround a spirv-opt optimization that in turn
+    //  causes a crash on PowerVR, see #5118)
+    position.z = position.z * frameUniforms.clipControl.x + position.w * frameUniforms.clipControl.y;
 #endif
 
     // Invoke user code
@@ -31,4 +34,7 @@ void main() {
 #if defined(VARIABLE_CUSTOM3)
     VARIABLE_CUSTOM_AT3 = inputs.VARIABLE_CUSTOM3;
 #endif
+
+    // some PowerVR drivers crash when gl_Position is written more than once
+    gl_Position = position;
 }