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Diffstat (limited to 'docs/Beignet/Backend/mixed_buffer_pointer.mdwn')
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diff --git a/docs/Beignet/Backend/mixed_buffer_pointer.mdwn b/docs/Beignet/Backend/mixed_buffer_pointer.mdwn new file mode 100644 index 00000000..f43ab7ed --- /dev/null +++ b/docs/Beignet/Backend/mixed_buffer_pointer.mdwn @@ -0,0 +1,46 @@ +Mixed Buffer Pointer +-------------------- + +Segmented address space... +-------------------------- + +The first challenge with OpenCL is its very liberal use of pointers. The memory +is segment into several address spaces: + +- private. This is the memory for each work item + +- global. These are buffers in memory shared by all work items and work groups + +- constant. These are constant buffers in memory shared by all work items and +work groups as well + +- local. These is a memory shared by all work items in the *same* work group + +... But with no restriction inside each address space +----------------------------------------------------- + +The challenge is that there is no restriction in OpenCL inside each address +space i.e. the full C semantic applies in particular regarding pointer +arithmetic. + +Therefore the following code is valid: + +<code> +\_\_kernel void example(\_\_global int *dst, \_\_global int *src0, \_\_global int *src1)<br/> +{<br/> + \_\_global int *from;<br/> + if (get\_global\_id(0) % 2)<br/> + from = src0;<br/> + else<br/> + from = src1;<br/> + dst[get\_global\_id(0)] = from[get\_global\_id(0)];<br/> +} +</code> + +As one may see, the load done in the last line actually mixes pointers from both +source src0 and src1. This typically makes the use of binding table indices +pretty hard. In we use binding table 0 for dst, 1 for src0 and 2 for src1 (for +example), we are not able to express the load in the last line with one send +only. The pointer "from" in the last line is so called a mixed buffer pointer. + +(To be updated) |