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Cycles Kernel Language


The kernel contains the core rendering routines, where most of the render time is spent. The code here can be compiled as a subset of C++14, CUDA and HIP. In order to support this, we must be careful to only use language features that are supported for all targets, with a few preprocessor macros to smooth over the language differences.

In short, this currently means:

  • C++14 syntax
  • Vector types like float3 or uchar4, with common operators
  • Constant memory for small amount of fixed parameters
  • Global memory for most read-only data
  • Texture handles for GPU devices that natively support image textures
  • No call stack, no recursive functions, function pointers or virtual functions
  • No dynamic memory allocation
  • No doubles, only floats and half floats

Vector Types

The vector types are the same as CUDA: float2, float3, float4, and similar for int, uint, uchar. Common operators like add, multiply, etc work as expected. For construction, use the make_*() functions, for example:

float3 v = make_float3(0.0f, 1.0f, 0.0f);

We define the necessary classes and operator overloading to implement them for all supported platforms


  • ccl_device: for functions, all kernel functions should use this
  • ccl_global: for pointers to global memory (mostly function parameters)
  • ccl_local: explicitly places stack variables in local memory
  • ccl_shared: shared memory
  • ccl_constant: constant memory

These are defined as macros for each target. Some targets may define some qualifiers as empty, or may accept them only in particular contexts, so it's best to test compiling all. ccl can be considered an abbreviation for cycles or cycles compute language, whichever you prefer.

Constant Memory and Textures

Small, fixed size data is stored in constant memory. KernelData kernel_data contains all constant memory, and is available as a global variable everywhere.

All large read-only data is stored in a small number of arrays in global memory, and texture handles for image textures. These global memory arrays are accessible as global variables in the kernel, and must be accessed through the kernel_tex_* functions.

For historical reasons this is still called "textures", as old GPU architectures had to use texture memory for good performance. This will be renamed at some point.