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Diffstat (limited to 'docs/Beignet/Backend/flat_address_space.mdwn')
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diff --git a/docs/Beignet/Backend/flat_address_space.mdwn b/docs/Beignet/Backend/flat_address_space.mdwn deleted file mode 100644 index 3018a295..00000000 --- a/docs/Beignet/Backend/flat_address_space.mdwn +++ /dev/null @@ -1,98 +0,0 @@ -Flat Address Space -================== - -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. - -No support for stateless in required messages ---------------------------------------------- - -Furthermore, in IVB, we are going four types of messages to implement the loads -and the stores - -- Byte scattered reads. They are used to read bytes/shorts/integers that are not -aligned on 4 bytes. This is a gather message i.e. the user provides up to 16 -addresses - -- Byte scattered writes. They are used to write bytes/shorts/integers that are not -aligned on 4 bytes. This is a scatter message i.e. the user provides up to 16 -addresses - -- Untyped reads. They allow to read from 1 to 4 double words (i.e 4 bytes) per -lane. This is also a gather message i.e. up to 16 address are provided per -message. - -- Untyped writes. They are the counter part of the untyped reads - -Problem is that IVB does not support stateless accesses for these messages. So -surfaces are required. Secondly, stateless messages are not that interesting -since all of them require a header which is still slow to assemble. - -Implemented solution --------------------- - -The solution is actually quite simple. Even with no stateless support, it is -actually possible to simulate it with a surface. As one may see in the run-time -code in `intel/intel_gpgpu.c`, we simply create a surface: - -- 2GB big - -- Which starts at offset 0 - -Surprisingly, this surface can actually map the complete GTT address space which -is 2GB big. One may look at `flat_address_space` unit test in the run-time code -that creates and copies buffers in such a way that the complete GTT address -space is traversed. - -This solution brings a pretty simple implementation in the compiler side. -Basically, there is nothing to do when translating from LLVM to Gen ISA. A -pointer to `__global` or `__constant` memory is simply a 32 bits offset in that -surface. - -Related problems ----------------- - -There is one drawback for this approach. Since we use a 2GB surface that maps -the complete GTT space, there is no protection at all. Each write can therefore -potentially modify any buffer including the command buffer, the frame buffer or -the kernel code. There is *no* protection at all in the hardware to prevent -that. |