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I've been looking through the blender source code and noticed that nearly all cycles shaders are programmed in OSL.
However, the Wiki mentions the following:
It is also possible to create your own nodes using Open Shading Language (OSL). Note that these nodes will only work for CPU rendering; there is no support for running OSL code on the GPU. (source)
Does this imply that most of the Cycles shader nodes only support CPU rendering?
Cycles implements the shaders for both CPU and GPU through the shader virtual machine which can be found in /intern/cycles/kernel/svm. The same nodes are also separately implemented in the Open Shading Language (OSL) and can be found in /intern/cycles/kernel/shaders. For instance you will see that the brick texture node exists both as svm_brick.h and node_brick_texture.osl. OSL only runs on the CPU and integrating a OSO to SVM compiler was previously discussed but abandoned due to maintenance and performance concerns. Therefore your own OSL scripts have the same limitation and will only be executed on the CPU.
Note that using OSL is entirely optional and it's not enabled by default. OSL can be enabled in the Render Properties.
Additional information about Cycles can be found in the wiki. The articles may not always be fully polished, but they should be a good starting point to dig deeper into the code. The structure of Cycles source code is documented in the Source Code Layout page.
