1# GPU Development 2 3Runtime selection of libCEED backends allows users to use CPU backends for easier debugging. 4Code that produces correct results with CPU backends will produce correct results on GPU backends, provided that JiT and memory access assumptions of the libCEED API are respected. 5 6## JiT Compilation 7 8The filepath to the user source code is passed in {c:func}`CeedQFunctionCreateInterior` as the `source` argument. 9This filepath should typically be an absolute path to ensure the JiT compilation can locate the source file. 10The filepath may also be a relative path with respect to a root directory set with {c:func}`CeedAddJitSourceRoot`. 11The {c:macro}`CEED_QFUNCTION` macro automatically creates a string with the absolute path, for example a {c:type}`CeedQFunctionUser` called `user` would have this string stored in the variable `user_loc`. 12 13The entire contents of this source file and all included files are used during JiT compilation for GPU backends. 14Include statements for system headers that are required for CPU compilation but are not available in GPU compilation environments should be guarded with `#ifdef CEED_RUNNING_JIT_PASS`. 15Any function definitions in these system headers must still be available in the GPU compilation environments, such as the contents of `<math.h>`. 16These source file must only contain syntax constructs supported by C99 and all targeted backends (i.e. CUDA for `/gpu/cuda`, OpenCL/SYCL for `/gpu/sycl`, etc.). 17 18All source files must be at the provided filepath at runtime for JiT to function. 19 20## Memory Access 21 22GPU backends require stricter adherence to memory access assumptions, but CPU backends may occasionally report correct results despite violations of memory access assumptions. 23Both `CeedVector` and `CeedQFunctionContext` have read-only and read-write accessors, and `CeedVector` allow write-only access. 24Read-only access of `CeedVector` and `CeedQFunctionContext` memory spaces must be respected for to ensure proper GPU behavior. 25Write-only access of `CeedVector` memory spaces asserts that all data in the `CeedVector` is invalid until overwritten. 26 27`CeedQFunction` assume that all input arrays are read-only and all output arrays are write-only and the {c:type}`CeedQFunctionUser` must adhere to these assumptions, only reading data in the input arrays and fully overwriting all entries in the output arrays. 28Additionally, {c:type}`CeedQFunctionUser` have read-write access for `CeedQFunctionContext` data, unless {c:func}`CeedQFunctionSetContextWritable` was used to indicate that read-only access is sufficient. 29 30The `/cpu/self/memcheck` backends explicitly verify read-only and write-only memory access assumptions. 31 32